2014 Chemistry - NEAP Trial With Solutions

HSC Trial Examination 2014

Chemistry Th is pape r m ust be kep t un de r strict security and m ay only be used on or after the m orning of Friday 1 A ug ust, 2014 as specified in the N eap E xam ination Tim etable.

Gen eralInstru ctions

TotalMarks100 Section Pages I 2–20

Reading time – 5 minutes

75marks

Working time – 3 hours Write using blue or black pen Draw diagrams using pencil Board-approved calc ulators may be used A data sheet and Peri odic Table are provided at het back of thispaper

This section has two parts, Part A and Part B Part A – 20 marks Attempt Questions 9–20 Allow about 35 minutes for this part Part B – 55 marks Attempt Questions 21–30 Allow about 1hour and 40minutes o f r thispart Section II Pages 21–26 25marks Attempt ONEquestionfrom Questions31–34 Allow about 45 minutes for this section No Biochemistry of Movement Option is included.

Students areadvised that thissia trial examination only and cannot n any i way guarantee theontent c or the format of the 201 4 HSC Chem istry Exam ination.

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TENCHEM_QA_14.FM

HSC Chemistry Trial Examination

S ection I 75 m arks

Part A –20 marks Attem pt Qu esti ons 1– 20 Allow about5 3 minutes for thi s part Use the multiple-choice answer sheet for Quest ions 1–20.

1.

The structural formula of a compound is shown below.

CH3 H HO

H

CCCH CH3 H

H

The systematic name for this compound is

2.

(A)

2-methyl-2-butanol

(B)

1,1-dimethyl-1-propanol

(C)

2-pentanol

(D)

1,1-dimethyl-2-propanol

A few drops of liquid bromine is added to each of two test tubes: one tube contains cyclohexane (C6H12) and the other 1-hexene (C 6H12). Which one of the following statements best describes the results observed during this experiment?

3.

(A)

Both test tubes remain red-brown when bromine is added.

(B)

Only the test tub e containing cyclohexane changes colour from red -brown to col ourless.

(C)

Both test tubes remain colourless.

(D)

The test tube containing cyclohexane will remain red-brown for much longer than the test tube containing 1-hexene.

The ester 1-butyl ethanoate is prepared from the acid-catalysed reaction between 1-butanol and ethanoic acid: CH3CH2CH2CH2OH + CH3COOH → CH3COOCH 2CH2CH2CH3 + H2O The reaction between 1-butanol and ethanoic acid is an example of

2

(A)

an addition reaction.

(B) (C)

a condensation r eaction. an e limination reaction.

(D)

a redox reaction.

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Copyright © 2014 Neap


4.

The diagram below represents a simple galvanic cell in which two metals are placed into a solution containing an appropriate electrolyte. The two metals are connected by a voltmeter.

V

1

2

Which of the following cells would generate the greatest voltage reading?

(A)

5.

Metal

Metal

silver

copper

Electrolyte solution 0.10 mol L

(B)

iron

lead

0.10 mol L

(C)

iron

copper

0.10 mol L

(D)

zinc

silver

0.10 mol L

–1 –1 –1 –1

H2SO4(aq) H2SO4(aq) H2SO4(aq) H2SO4(aq)

The stability of the isotopes is related to the ratio of neutrons to protons in the nucleus. Unstable isotopes with too many

6.

(A)

neutrons (for the number of electrons present in the nuc leus) emit electrons.

(B)

protons (for the number of neutrons present in the nucleus) emit α-radiation.

(C)

neutrons (for the number of protons present in the nucleus) emit β-radiation.

(D)

valence electrons (for the number of protons present in the nucleus) emit β-radiation.

A student prepared a solution of hydrochloric acid with a pH of 2.00. She added pure water to this solution until the final diluted solution had a pH of 4.00. If the student started with 100 mL of the pH 2.00 solution, the final volume of the pH 4.00 solution is (A)

200 mL

(B)

1000 mL

(C)

2000 mL

(D)

10 000 mL

Copyright © 2 014 Neap

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7.

An unknown chemical was extracted from a plant extract sample and sent to a laboratory for analysis. The pH of a 0.0100 mol L

–1

solution of the extract in water was determ ined using a series of indicator

solutions. The results are shown in the table below.

Colour of plant extract solution with indicator

Indicator methylorangesolution

orange

bromocresolgreen

green

bromothymolblue

yellow

neutral red

red

phenolphthaleins olution

colourless

The individual indicator colour change range for each indicator is shown below.

methyl orange red yellow bromocresol green yellow bromothymol blue

yellow blue blue yellow

red

neutral red

colourless

phenolphthalein 0

2

4

6

red 8

10

12

14

pH How should the laboratory classify this plant extract? (A) The extract is a strong acid.

4

(B)

The extract is a weak acid.

(C)

The extract is a strong base.

(D)

The extract is a weak base.

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8.

The decomposition of dinitrogen tetroxide (N2O4) is represented by the following equilibrium equation: N2O4(g)



2NO2(g)

∆H

= 58 kJ mol

–1

The graph below shows this equilibrium system undergoing a disturbance and shifting to re-establish a new equilibrium.

1 –

) 0.02

N2O4

L l o ( m n o it 0.01 a rt n e c n o c

0.00

NO2

0

2 time (hours)

The disturbance shown in the above graph was caused by

9.

(A)

a decrease in pressure.

(B)

an increase in temperature.

(C)

the addition of some NO 2 gas.

(D)

the removal of some NO 2 gas.

Which of the following correctly matches the name of the scientists with the theory they proposed to explain the chemical nature of acids?

Scientists

10.

Theory of the chemical nature of acids +

(A)

Arrhenius

Acids ionise to produce H

in solution.

(B)

Brønsted–Lowry

Acids contain oxygen.

(C)

Davy

Acidscontainoxygen.

(D)

Lavoisier

Acids ionise to produce H 3O in solution.

+

It is often necessary to conduct reactions at high temperatures, however volatile reactants and products may be lost when reaction mixtures are heated to boiling. What experimental procedure is employed to avoid this problem and to allow heating to be conducted in a safe manner? (A)

atomic absorption spectroscopy (AAS)

(B)

reduction

(C)

reflux

(D)

steam distillation


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11.

Incomplete combustion of petrol may result in the production of undesirable pollutants. Two such pollutants produced from the incomplete combustion of petrol are

12.

(A)

carbon and carbon monoxide.

(B)

carbon and carbon dioxide.

(C)

carbon dioxide and water.

(D)

nitrogen oxides and carbon monoxide.

The formation of ammonia from nitrogen and hydrogen can be summarised by the following equilibrium equation: N2(g) + 3H2(g)



2NH3(g)

∆H

= –92 kJ mol

–1

A catalyst is used in this industrial process to

13.

(A)

increase the equilibrium yield of product.

(B)

allow the reaction to proceed at a higher pressure.

(C)

allow a lower temperature to be used.

(D)

reduce the amount of expensive hydrogen that is used in the reaction.

Boron trichloride reacts with the chloride ion to form the tetrachloroborate ion: BCl3 + Cl

–

→

–

BCl4

Which of the following best describes the type of bond formed in this reaction?

14.

(A)

An ionic bond is formed between boron and chlorine.

(B)

A coordinate covalent bond is formed between boron and chlorine.

(C)

A non-polar covalent bond forms between boron and chlorine.

(D)

A dipole-dipole bond forms between boron and chlorine.

A bright apple-green flame was observed when a flame test was performed on a white solid. Adding a small amount of the white solid to 2.0 M HCl solution produced bubbles of a colourless gas. From these results it is possible to conclude that the unknown solution contained

6

(A)

nickel and chloride ions.

(B)

copper and sulfate ions.

(C)

barium and carbonate ions.

(D)

copper and carbonate ions.

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15.

A schematic diagram representing the processes used at a small sewage treatment plant is shown below.

raw sewage

treated sewage

activated sludge skimming and screening

sedimentation

sedimentation

chlorination

trickle filters anaerobic respiration

anaerobic respiration

aerobic respiration

primarytreatment

secondarytreatment

Two water samples from this sewage plant were tested for the biochemical demand (BOD). The following results were obtained:

Watersample

BOD(mgL

1

1.2

2

2500

–1

)

These results indicate that (A) sample 1 is treated sewage while sample 2 is raw sewage.

16.

(B)

both samples were taken from the primary treatment plant.

(C)

sample 2 has more than 2000 times the dissolved oxygen of sample 1.

(D)

both samples were taken from the secondary treatment plant.

A reaction occurs according to the following equation: CH3CH2OH(l) + 3O 2(g) → 2CO2(g) + 3H2O(l) According to this equation, the maximum mass of carbon dioxide gas produced when 37 L of oxygen reacts with ethanol at 25°C and 100 kPa is

17.

(A)

22g

(B)

25g

(C)

44g

(D)

66g –1

The heat of combustion of methanol is 22.7 kJ g . What mass of water can be heated through temperature change of 28.0°C by the complete combustion of 1.00 g of methanol? (A)

1.00 g

(B)

18.0 g

(C)

194 g

(D)

388 g


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18.

A chemist analysed a sample of soil for lead contamination. A 10.0 gram sample of soil was treated with a mixture of nitric and hydrochloric acids to dissolve all the lead in the soil sample. The resulting solution was diluted to 1.00 litre and the concentration of lead was determined using atomic absorption spectroscopy (AAS). The absorbance of a series of lead standard solutions was measured and the results plotted to give the graph below.

0.600 e c n a b r o s b a

0.400 0.200

0

0.100

0.200

0.300

0.400

concentration of lead (mg/100 mL) The absorbance of the 1.00 L soil sample solution was found to be 0.200. The concentration, in ppm, of lead in the soil sample is

19.

20.

8

(A)

0.130

(B)

13.0

(C)

130

(D)

300

Which of the following equations correctly represents the production of a transuranic element? (A)

238 92U

+ 2He → 94U + 3 0 n

4

239

4

239

1

(B)

238 92U

+ 2He → 94Pu + 3 0 p

(C)

238 238 P U 94 → 92

(D)

1 n 0

1

4

+ 2He

238

88

136

1

+ 92U → 38Sr + 54Xe + 14 0 n

The pH of 5.00 × 10 (A)

2.00

(B)

2.30

(C)

3.00

(D)

5.00

–3

mol L

–1

M H2SO4 is

TENCHEM_QA_14.FM



S ection I (co ntinued)

Part B –55 marks Attem pt Q uesti ons 21 –30 Allow abouthour 1 and 40 m inut es or f thi s part Answer the questions in the spacesprovided. Show all relevant working in quest ions involving calc ulations. Marks

Question 21 (4 marks) Most ethene is produced by the cracking of larger hydrocarbon molecules. (a)

What is meant by the term ‘cracking of larger hydrocarbon molecules’?

1

........................................................................ ........................................................................ ........................................................................ ........................................................................ (b)

The two polymers that are most commonly produced from ethylene (ethene) are HDPE

3

and LDPE. Account for the differences in the properties of HDPE and LDPE in terms of both the polymerisation process used in their production and in their structural properties. ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................


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Marks

Question 22 (5 marks) Iron-53 is a neutron-deficient isotope with a half-life of 8.51 minutes. Cobalt-60 is a neutron-rich radioisotope with a half-life of 5.5 years. (a)

1

Write a nuclear equation for the beta decay of co balt-60. ....... ................................................................. ....... .................................................................

(b)

Discuss the environmental and health impacts associated with the disposal of cobalt-60 and iron-53.

2

....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. (c)

Describe how neutron-rich isotopes can be formed, including an equation for a named example.

2

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................

10

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Marks

Question 23 (6 marks) Many cosmetic and pharmaceutical preparations use ethanol as a solvent, which can be produced by the fermentation of sugars. (a)

With the aid of a st ructural formula, relate the use of ethanol as a solvent to the structure of the ethanol molecule.

3

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ (b)

Explain how measurements of changes in mass can be used to monitor and determine the amount of ethanol that is produced during the fermentation of a sugar solution. A suitable equation should be included in your answer.

3

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................


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Marks

Question 24

(4 marks)

A galvanic cell was constructed using copper, silver and appropriate solutions of their salts. (a)

Write a balanced ionic equation to represent the reaction that will occur.

1

....... ................................................................. ....... ................................................................. (b)

Determine the standard cell voltage for this cell.

1

....... ................................................................. ....... ................................................................. (c)

Complete the cell diagram shown below by filling in the blank spaces to indicate the solution in which the silver electrode is placed, the direction in which electrons move through the wire (either towards Ag or towards Cu), and the direction anions move through the salt bridge (either towards Ag or towards Cu).

2

Direction which electrons move through wire:

V

Cu electrode

Ag electrode Direction which anions t roug sa t r

Solution: CuCl2(aq)

12

Solution:

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Marks

Question 25 (4 marks) Sulfur dioxide is produced when metal sulfide ores are roasted in air and converted to the metal oxide. (a)

What volume of sulfur dioxide will be produced at 25°C and 100 kPa when one tonne (1000 kg) of zinc sulfide ore is roasted?

2

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ (b)

With the aid of a che mical equation, outline the effect of releasing sulfur dioxide on th e pH of natural waterways.

2

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................


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Marks

Question 26 (7 marks) A student was given a quantity of dilute hydrochloric acid and two clear colourless liquids, L1 and L2. He determined the initial pH of L1 and L2 and the subsequent pH of each solution when varying amounts of hydrochloric acid were added to each of these. The results are shown below.

8 7 e r u ti x m f o H p

6 5

L1

4 3 2 1

L2 –7

10

10

–6

–5

10

10

–4

–3

10

–2

10

–1

10

+

moles H added per litre (a)

Describe two ways in which the pH of the liquids could be measured.

3

....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... .................................................................

14

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Marks

(b)

The student suspected that one of the liquids was water and that the other was a buffer solution.

4

Discuss this conclusion and suggest how the student could confirm this. ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................


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Marks

Question 27 (7 marks) As part of your course you had to research how Fritz Haber developed the process for the industrial synthesis of ammonia. (a)

2

Identify and describe TWO industrial uses of ammonia. ....... ................................................................. ....... ................................................................. ....... .................................................................

(b)

3

Describe the conditions under which Haber developed this process. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... .................................................................

(c)

How did y ou ensure that the research you d id was reliable?

2

........................................................................ ........................................................................ ........................................................................ ........................................................................

16

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Marks

Question 28 (5 marks) A student was preparing a standard solution from a solid to use in a titration. Unfortunately, during this process the student dropped his glassware and spilt the standard solution that he was preparing. (a)

Describe how, starting from a solid, a standard solution may be prepared. Include all apparatus used.

3

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ (b)

The student wanted to neutralise the spilt solution, but was unsure as to whether the solution was that of an acid or a base.

2

Name a substance that would be suitable to neutralise either an acid and a base, explaining your choice. ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................


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Marks

Question 29 (7 marks) The diagram below shows a catchment area where concerns have been raised regarding the levels of heavy metals in the river.

mountains

P

dairy farming housing

Q R factories water treatment plant

S

sea

Samples were taken from the river at sites P, Q, R and S and analysed. The results are shown below.

Site tested

(a)

Concentration of lead –1

(g L )

P

0.005

Q

0.008

R

0.012

S

0.028

Identify the most likely source of the lead pollution. Justify your answer.

2

....... ................................................................. ....... ................................................................. ....... ................................................................. (b)

Describe one method that can be u sed to quantitatively analyse levels of lead in water.

3

........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ ........................................................................ 18

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Marks

Chlorine and other chemicals are added to water at the water treatment plant located in the catchment area. (c)

Identify why chlorine, as well as one other named chemical, are added to water.

2

........................................................................ ........................................................................ ........................................................................ ........................................................................ Question 30 (6 marks) Chlorofluorocarbons (CFCs) and halons are present in significant amounts in the atmosphere. The structure of one such CFC is shown below.

F

Cl

Cl

C

C

F

F

Cl

(a)

Draw and name an isomer of the compound shown above.

2

(b)

How did CFCs and halons become part of the atmosphere?

2

........................................................................ ........................................................................ ........................................................................ ........................................................................


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Marks

(c)

Many chemicals have been developed as alternatives to CFCs.

2

For TWO named types of chemicals, evaluate the effectiveness of their use as a replacement for CFCs. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... ................................................................. ....... .................................................................

20

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S ection II

25 m arks Attem pt O NE questi on from Questi ons 1 3–34 Allow abou t 45 minutes for ith s sec tion Answer the question in a writing booklet. Extra writing booklets are available. Show all relevant working in quest ions involving calc ulations.

Pages

Question31 IndustrialChemistry........... ............ ........... ........... ............ ............ ..22 Question 32 Shipwrecks, Corrosion an d Conservation...........................23–24 Question33 The Chemistryof Art.......... ............ ............ .......... ............ ........... ...25 Question34 ForensicChemistry........................................................................26


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Marks

Question 31 — Industrial Chemistry (25 marks) (a)

Compare the structure and uses of soap and cationic detergents, including their effectiveness in hard water.

(b)

The Solvay process is the i ndustrial method for the preparation of sod ium carbonate, and is widely used in countries which do not have deposits of the mineral trona, from which sodium carbonate can also be obtained.

(c)

3

(i)

Describe a safe method that can be used to model one step in the Solvay process in the school laboratory.

3

(ii)

Calculate the mass of Na 2CO3 which could be prepared using 125 kg of NaCl in the Solvay process.

2

(i)

Outline how the chemical industry has addressed increasing de mand for a ( non-fossil fuel) natural resource.

2

N2O4 is formed in the following equilibrium reaction, and the value of K at

3

(ii)

25°C is 3.2. 2NO2(g)

N2O4(g)

A 2 L reaction vessel containing NO 2 and N 2O4 (at 25°C) was sampled and the results are shown in the table below.

Gas

Colour

Number of moles present in 2 L when sampled

NO2

brown

4.6

N2O4

colourless

3.4

Use the data provided to describe the macroscopic and microscopic state of the system. (d)

Sodium hydroxide can be prepared by the e lectrolysis of sodium chloride solution but the products of electrolysis depend on the concentration of chloride ions. (i)

(ii)

(e)

Write chemical equations to show the difference between the electrolysis of dilute and concentrated sodium chloride solution.

2

Compare and co ntrast the diaphragm and the me mbrane processes for sod ium hydroxide production with particular reference to impact on society and purity of product.

3

Describe the production of sulfuric acid from sulfur and assess the importance of the reaction conditions for the rate and yield.

7

End of Question 31

22

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Marks

Question 32 — Shipwrecks, Corrosion and Conservation (25 marks) (a)

Wooden artefacts that a re salvaged from long-submerged wrecks are initially placed in sal t water, then placed in fresh water, before final treatment to stabilise the wooden artefact, often using polyethylene glycol (PEG) solutions.

3

Explain the damage that will occur to a wooden artefact if it is removed from a long-submerged wreck and allowed to dry slowly then put on display without first treating the artefact as described above. (b)

The electrolysis of copper(II) sulfate can be represented by the fo llowing equation: 2CuSO 4(aq) + 2H 2O(l) → 2H2SO4(aq) + 2Cu(s) + O2(g) A student studied the electrolysis of 1.00 M CuSO 4 solution using the experimental set-up with a 1.5 V battery shown below.

+ battery – platinum electrode A

platinum electrode B

CuSO4

(i) (ii)

Write half-equations for the mai n reactions occurring at the a node and at the cathode.

2

Galvani, Volta, Davy and Faraday were pioneers in the development of electrolysis and electrochemical techniques.

4

Discuss Davy’s contribution and the impact his work has had on modern society. (c)

The following table shows the composition of five different alloys:

Alloy

Composition

1

1.3%Cu,2.6%Mg,96.1%Al

2

11.5%Al,5.0%Fe,4.0%Ni,79.5%Cu

3

0.3% C, 99.7% Fe

4 5

4

0.1%C,17.0%Cr,11.6%Ni,71.3%Fe 0.05% C, 16.0% Cr, 9.05% Ni, 2.4% Mo, 1.2% V, 1.2% Mn, 70.1% Fe

Explain, using two examples from the table above, how the composition of a sample of steel determines its properties and uses. Question 32 continues on page 23


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Marks

Question 32 (continued) (d)

You are as ked to experimentally identify the effects of diss olved oxygen and pH on the rat e of corrosion of iron. (i) (ii)

(e)

Describe an appropriate procedure for this experiment.

3

Explain the results you obtained using relevant equations.

3

Corrosion of iron alloys is a ma jor issue that a variety of industries must overcome, and corrosion prevention is costly.

6

Compare the use of sacrificial anodes with painting as methods of corrosion protection in both marine and terrestrial environments. End of Question 32

24

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Marks

Question 33 — The Chemistry of Art (25 marks) (a)

(b)

Describe the contribution of Pauli and Hund to our understanding of the position of electrons in atoms. (i)

Describe the bonding in transition metal complexes, and explain why different colours are observed for [Co(H 2O)6]

(ii) (c)

(ii)

Write the electron configuration of the Co

2+

4

(blue).

ion using subshell notation.

1

Outline a method that could be safely used to observe a transition metal changing oxidation state, and the observations that would be made.

3

Write labelled half-equations associated with the procedure you described in part (c) (i) above.

2

Ancient cultures had many uses for pigments, for example, in the p roduction of artworks. (i)

(ii) (e)

2–

(pink) and [CoCl4]

Some of the colours that transition metal ions exhibit can be observed when they change oxidation state. (i)

(d)

2+

3

Outline one ot her use of pigments by an anci ent culture and a hea lth hazard associated with the use of one specific pigment.

2

Describe how ultraviolet and infrared radiation may be used to help study artworks.

3

Discuss the Bohr model of the atom and assess the importance of the hydrogen emission

7

spectrum to this model. Describe a first-hand investigation into flame colour to illustrate your answer.

End of Question 33


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Marks

Question 34 — Forensic Chemistry (25 marks) (a)

Describe a sequence of tests that could be used to safely distinguish between the following organic chemicals.

3

OH

H3C

CH2

CH2

CH2

CH2

CH3

H3C

CH2

CH2

A

H3C

CH2

CH2

CH2

(c)

CH2

CH2

OH

H3C

CH2

CH2

CH2

CH

CH2

D

(i)

Compare and contrast the composition, structure and role of cellulose, starch and glycogen.

3

(ii)

Outline how sucrose and glucose could be disti nguished from each other in a school laboratory.

2

DNA analysis is one of the forensic chemist’s most important analytical tools.

(ii)

Outline the processes involved in DNA analysis and explain why it is so useful to a forensic chemist.

3

Outline an et hical issue a ssociated with t he use of D NA analysis in forensic chemistry.

2

Proteins are structural and metabolic components of all li ving things, and as a re sult their analysis is very useful in forensic chemistry. (i)

(ii)

(e)

O

Although all carbohydrates have the general formula C x(H2O)y they display a wide range of chemical and physical properties, and produce a number of very different biopolymers.

(i)

(d)

C CH2

B

C (b)

CH2

Draw the general structure of the monomer from which proteins are made, and label the functional groups it contains.

2

Describe how electrophoresis can be use d to dete rmine the origin of a protein sample.

3

Explain why discrete lines are obtained in emission spectra and assess the importance of line-emission spectroscopy in forensic chemistry. Illustrate your answer by describing a first-hand investigation into flame tests and the results observed.

7

End of paper

26

TENCHEM_QA_14.FM



D AT A SH EE T Avogadro constant, NA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.022

×

10

23

mol

–1

Volume of 1 mole of ideal gas at 100 kPa and at 0 °C (273.15 K) . . . . . . . . . . . . . . . . . . . . 22.71 L at 25 °C (298.15 K. . . . . . . . . . . . . . . . . . . . 24.79 L –14

Ionisation constant for water at 25 ° C (298.15 K), Kw . . . . . . . 1.0 × 10 3 –1 –1 Specific heat capacity of water . . . . . . . . . . . . . . . . . . . . . . . . . 4.18 × 10 J kg K

Som e useful form ulae +

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pH = –log10[H ]

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

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

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

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

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

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

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 

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

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

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

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

2OH

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Cu(s)



I



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Fe



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0.80 V



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1.10 V



H2O

1.23 V



Cl

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Cr

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Mn

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–

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Som e standard enti pot als K(s)

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0.40 V 0.52 V

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0.54 V

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0.77 V

–

–

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1.36 V

3+

+ 7--2- H2O

1.36 V 1.40 V

2+

+ 4H2O

1.51 V 2.89 V

TENCHEM_QA_14.FM

27


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HSC Trial Examination 2014

Chemistry

Solutions and marking guidelines

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TENCHEM_SS_14.FM


Section I

Part A Answer and explanation Question1

Syllabus content and course outcomes

A

9.2.3

H9, H13

The OH functional group makes this an alkanol. The longest carbon chain that contains the OH is made up of four carbons, and the OH group is attached to the second carbon from the (left-hand) end of the chain. Therefore the compound is a 2-butanol. There is an additional CH3 (methyl) group attached to the second carbon, hence the correct name is 2-methyl-2-butanol. Question2

D

9.2.1

H8, H9, H11

9.2.2

H8, H9, H13

9.2.5

H6, H7

Alkenes rapidly react with bromine and the red-brown colour of bromine is rapidly removed to give a colourless mixture if alkenes are present. Alkanes do not react rapidly with bromine and therefore the red-brown colour persists with cyclohexane longer than it does for 1-hexane. Question3

B

The reaction between alkanols and alkanoic acids is called a condensation reaction because a small molecule of water is produced (condenses) during the reaction. Question4

D

Cell D has the two electrodes with the greatest difference in standard potentials (greatest difference in reactivity) and will hence give the greatest voltage. Question5

C

9.2.5

H6, H7, H8

C is correct, as a nucleus with too many neutrons for the number of protons present in the nucleus will emit beta particles A is incorrect, as there are no electrons in the nucleus. B is incorrect, as a nucleus with too many protons for the number of neutrons will emit positrons. D is incorrect, as the number of electrons in the valence shell does not affect the stability of the nucleus. Question6

D

9.3.3

Initial pH = 2.00, hence initial [HCl] = 0.0100 mol L

H10

–1

Final pH = 4.00, hence initial [HCl] = 0.000100 mol L

–1

C1V1 = C2V2, hence 100 × 0.0100 = 0.000100 × V2 V2 = 10 000 mL

2

TENCHEM_SS_14.FM


HSC Chemistry Trial Examination Solutions and marking guidelines

Part A (Continued) Answer and explanation Question7


B

9.3.1

H11, H14

The indicator colours show the following results: methyl orange = orange, ∴ pH between 3.5 and 4.3 bromocresol green = green, ∴ pH between 3.8 and 5.5 bromothymol blue = yellow, ∴ pH < 6.1 neutral red = red, ∴ pH < 6.2 phenolphthalein = colourless, ∴ pH < 8.5 –1

The pH of the 0.0100 mol L extract is between 3.8 and 4.3. The extract is acidic. If the extract is a strong acid then the pH of a 0.0100 molar solution would be 2.00. The pH is higher than 2.00, indicating that this is a weak acid. Question8

C

9.3.2

H8, H14

9.2.4

H10, H14

9.3.5

H11, H13

The graph shows that nitrogen dioxide was added. Question9

A +

Arrhenius stated that acids ionise and produce H in solution. Question10

C

Reflux is the technique used to safely heat reaction mixtures. Question11

A

9.4.1

H3, H4, H8

A is correct; incomplete combustion of petrol will result in soot (carbon) and carbon monoxide being formed. B and C are incorrect, as carbon dioxide is produced through the complete combustion of petrol. D is incorrect, as nitrogen oxides are produced if there is too much oxygen in the cylinder, and carbon monoxide is produced if there is too little oxygen in the cylinder (both cannot be produced at the same time). Question12

C

9.4.2

H8, H10

Catalysts are added to increase the rates of reactions at lower temperatures. Question13

B

9.4.4

H8, H9, H11

9.4.3

H8, H10, H13

A coordinate covalent bond is formed between the boron of BCl 3 and the chloride ion. Question14

C

The evolution of bubbles when acid is added suggests carbonate ion is present. The apple-green colour suggests barium ion is present. Option C is the only choice that includes carbonate ion and barium ion. Question15

A

9.4.5

H11, H14

The high BOD of sample 2 indicates that it has a high organic content and therefore must have been taken early within the primary treatment plant. The low BOD of sample 1 indicates it has a very low content of organic material and hence must have been taken from a late stage (at least after aerobic respiration was complete) within the secondary treatment plant.


TENCHEM_SS_14.FM

3


Part A (Continued) Answer and explanation Question16


C

9.3.2

H9, H10

37 moles O2(g) = ------------- = 1.51 mol 24.47 2 2 moles CO2 = --- moles O 2 = --- × 1.51 = 1.0 mol 3 3 mass CO2 = moles CO 2 × 44.0 = 1.0 × 44.0 = 44 grams Question17

D

9.3.3

H7, H9, H10

heat = mC p ∆T 22 700 = m × 4.18 × 28.0 m = 194 g Question18

C

9.4.3

H10,H12,H13

9.3.5

H7, H10

9.3.3

H9, H10, H13

The graph shows that an absorbance of 0.200 corresponds to a concentration of 0.130 mg/100 mL. The litre solution will therefore contain 1.3 mg of lead. 10 g of sample will contain 1.3 mg of lead. 4 1.3 Hence 1 gram of soil will contain ------- = 0.13 mg = 1.30 × 10 g. 10 6 mass Pb  ppm Pb =  ----------------------------- mass sample  × 10

= ( 1.30 × 10

–4

g ) × 10

6

= 130 ppm Question19

B

B is the only reaction that correctly represents the formation of a transuranic element. Question20

A +

pH = – log 10 [ H ] = – log 10 [ 2 × 5.00 × 10

–3

]

= 2.00 Sulfuric acid is dibasic.

4

TENCHEM_SS_14.FM



Part B Syllabus content, course outcomes and marking guide

Sample answer Question 21

(a)

Cracking is the process in which large hydrocarbon molecules are broken into smaller molecules.

9.2.1 H6, H9 • Correctly describes cracking as breaking larger molecules into smaller molecules . . . . . . . . . . . . . . . . . . 1

(b)

HDPE is high-density polyethylene and is a straight-chain polymer produced by catalytic polymerisation using Ziegler–Natta catalysts. It is more crystalline and stronger than LDPE (but is more expensive). LDPE is low-density polyethylene and is a branched-chain polymer produced by heating ethylene under pressure. It is less crystalline and weaker than HDPE but is cheaper to prepare.

9.2.1 H3,H4,H6,H7 • Describes the structure and properties of HDPE. AND • Describes the structure and properties of LDPE. AND • Accounts for the difference between these two polymers in terms of their structure. . . . . . . . . . . . . . . . . . . . 3

HDPE: unbranched polymer

LDPE: branched polymer

•

Only two of the above . . . . . . . . . . . . . . . 2

•

Only one of the above . . . . . . . . . . . . . . . 1

Question 22

(a)

60 CO 27

60

9.2.5 H7, H10 • Correctly writes the balanced nuclear decay equation . . . . . . . . . . . . . . . . . . . . . 1

0

→ 28Ni + – 1e

(b)

All radioactive isotopes will present hazards to human health and may present hazards to the environment. Iron-53 has a very short half-life and after a few weeks the radioactivity of iron-53 will have decreased to a safe level and will no longer remain an environmental hazard or health hazard. Cobalt-60 has a much longer half-life and it will remain an environmental hazard for much longer than iron-53. Cobalt-60 must be kept in a secure storage facility for decades before its radioactivity will have decreased to a safe level.

9.2.5 H3, H4, H7 • Discusses the effect of the different half-lives on the environmental and health impacts of radioisotopes. AND • Compares the environmental and health impacts of each isotope . . . . . . . . . . . . . . 2

(c)

Neutron-rich isotopes can be most easily produced in a nuclear reactor, where neutrons produced from the decay of uranium isotopes are absorbed by natural isotopes placed into the reactor chamber.

9.2.5 H3, H4, H7 • Describes how neutron-rich isotopes may be produced. AND • Includes appropriate equation . . . . . . . . . 2

59 27Co

1

60

+ 0 n → 27Co

•

•


TENCHEM_SS_14.FM



5


Part B (Continued) Syllabus content, course outcomes and marking guide


(a)

Ethanol has a non-polar carbon chain and a polar OH group. H

H

HCCOH

H H carbon chain can form dispersion forces with The non-polar non-polar compounds. The polar OH group can form dipole-dipole forces with polar compounds, and H bonds with molecules such as water. (b)

Mass changes can be used to monitor fermentation because the weight of the reaction vessel (and its contents) will decrease as the carbon dioxide that is produced by fermentation diffuses into the atmosphere: C6H12O6(aq) → 2CO2(g) + 2CH3CH2OH(aq) The initial mass of the flask containing the aqueous solution of sugar and yeast is determined. The mass of the flask and yeast is repeatedly measured – the decrease in mass is equal to the amount of carbon dioxide that is produced. The of moles of carbon dioxide can be determined fromnumber the change in mass.

9.2.3 H2,H6,H8,H9 • Draws the structural formula for ethanol. AND • Describes the type of intermolecular forces that ethanol can form with other molecules. AND •

Describes the types of compounds that can dissolve in ethanol . . . . . . . . . . . . . . . 3

•

Any two of the above. . . . . . . . . . . . . . . . 2

•


9.2.3 H2,H6,H8,H9 • Writes a balanced equation to represent the fermentation of ethanol. AND • Describes how the mass changes can be related to the moles of carbon dioxide. AND • Describes how the moles of carbon dioxide can be related to the mass of ethanol produced . . . . . . . . . . . . . . . . . . . 3 •


•


The number of moles of ethanol is equal to the number of moles of carbon dioxide. The mass of ethanol that has been produced can be determined from the number of moles of ethanol that has been produced. Question 24

(a)

Cu(s) + 2Ag (aq) → Cu (aq) + 2Ag(s)

9.2.4 H6,H7,H8,H10 • Writes the correct balanced equation. . . . 1

(b)

E cell = E cathode – E anode

9.2.4 H6, H7, H10 • Gives correct voltage . . . . . . . . . . . . . . . . 1

+

2+

= 0.80 – 0.34 = 0.46 volts

6

TENCHEM_SS_14.FM




Sample answer

(c)

9.2.4 H10, H7. H6. H13 • Completes all three boxes by inserting an appropriate answer. . . . . . . . . . . . . . . . . . 2

Direction which electrons move through wire: Electrons move towards silver electrode.

•

Completes at least two of the boxes with an appropriate answer . . . . . . . . . . . . . . . 1

V Cu electrode

Ag electrode Direction which anions move through salt bridge: Anions move towards anode Cu).

Solution: CuCl2(aq)

Solution: AgNO3(aq)

Question 25

(a)

mass ZnS = 1000 kg = 1 000 000 g

9.2.3 H2,H6,H8,H9 • Correctly calculates the moles of ZnS reacting. AND

1 000 000 moles ZnS = --------------------------------65.38 + 32.07 4

= 1.03 × 10 mole 2ZnS + 3O2 → 2ZnO + 2SO2

•

Determines the volume of SO 2 based on the moles of SO 2 previously determined. . . . 2

•


4

moles SO2 = moles ZnS = 1.03 × 10 mole 4

volume SO 2 = 1.03 × 10 × 24.79 5

= 2.54 × 10 L = 254 m (b)

3

Sulfur dioxide is an acidic gas that will decrease the pH of natural waterways: SO2(g) + H2O(l)



H2SO3(aq) + H2O(l)

H2SO3(aq) 

+

–

H3O (aq) + HSO3 (aq)

The increase in concentration of hydronium ions will lead to a decrease in pH.

9.2.3 H3,H4,H6,H10 • Discusses the impact of SO 2 on the pH of waterways. AND • Provides an ap propriate balanced chemical equation . . . . . . . . . . . . . . . . . . 2 •


Question 26

(a)

A pH meter or pH probe and data logger could be used. Calibrate these using a solution of known pH. Record the pH at the beginning and after each addition of acid. Use a suitable indicator, e.g. universal indicator paper. Place paper in the liquid and note the colour, then compare this to the chart and note the pH.

9.3.3,9.3.4 H2,H8,H11 • Identifies two methods of determining pH. AND • Describes both methods in detail. . . . . . . 3 • Identifies two methods of determining pH. AND • Describes only one method in detail . . . . 2 •


TENCHEM_SS_14.FM

Only identifies two methods of determining pH . . . . . . . . . . . . . . . . . . . . 1

7



Sample answer

(b)

Buffer solutions resist changes in pH when quantities of an acid or base are added. This corresponds to L1. Water changes pH rapidly when quantities of an acid or base are added. This corresponds to L2. The student could check this by adding an alkaline solution to L1 and L2. A buffer solution would maintain pH at the value initially shown in the graph, but would eventually increase in pH at a similar point in the graph due to the change in acid. An alkaline solution added to water would show a rapid increase in pH.

9.3.3, 9.3.4 H11,H13, H14 • Correctly identifies L1 as the buffer solution and L2 as water. AND • Discusses the nature of buffer solutions in detail. AND •

Provides an analysis of graph L1, describing why it must be the buffer. AND • Provides an analysis of graph L2, describing why it must be water . . . . . . . 4 •

Any three of the above. . . . . . . . . . . . . . . 3

•


•


Question 27

(a)

Examples of uses include: • as a refrigerant – used to transfer heat and cooling systems to preserve food • raw material for fertilisers to im prove crop yields

9.4.5 H4, H6 • Identifies and describes two uses . . . . . . 2

(b)

Nitrates were essential for agricultural and munitions. The majority of large nitrate supplies were located in South America, far away from Europe. Germany had restricted access to these supplies (as war was looming). Nitrogen was plentiful in the atmosphere but was unreactive. Haber used atmospheric nitrogen to produce ammonia (to be then turned into nitrates) using high pressure reactions. This was then taken up on an industrial scale by German industry.

9.4.2 H1, H3 • States importance of nitrates. AND • States problem of nitrate supplies. AND • States nitrogen is available but unreactive. AND • States that Haber was first to produce ammonia from atmospheric nitrogen on commercial scale . . . . . . . . . . . . . . . . . . . 3

(c)

8

Use a variety of sources; check for agreement between sources; check websites for sources, e.g. whether it was an organisation, a company, an individual, an educational institute, etc.

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Identifies and describes one use . . . . . . . 1

•


•


9.4.2 H14 • Gives two valid ways . . . . . . . . . . . . . . . . 2 •

Gives one valid way . . . . . . . . . . . . . . . . . 1





(a)

One example of a sequence used to prepare a standard solution is as follows: Determine the approximate mass of solid that needs to be dissolved in order to produce the volume of standard solution of the desired concentration at a standard temperature. Accurately weigh out a mass close to that

9.3.4 H11,H12,H13 • Describes the need to determine the mass of solid required to make the standard solution prior to weighing out the solid. AND • Describes weighing and dissolving solid.

previously calculated. Place the solid into a clean beaker. Add a little distilled water to dissolve the solid. Transfer this solution to a clean volumetric flask, using a filter funnel to assist. Rinse the beaker and filter funnel with distilled water, ensuring that all solution is transferred into the volumetric flask. Add sufficient distilled water to the volumetric flask to make ‘up to the mark’ (the bottom of the meniscus should sit on top of the volumetric mark) with distilled water. Stopper the flask and invert several times to ensure uniform mixing.

AND • Describes how the solution is quantitatively transferred into the volumetric flask and made up to volume. . . . . . . . . . . . . . . . . . 3 •


•


Calculate molarity using the mass weighed out, the molar mass of the solid and the volume of the volumetric flask. (b)

Amphiprotic substances will react with acids as well as with bases. Sodium hydrogen carbonate (NaHCO 3, ‘bicarbonate of soda’) is a non-toxic amphiprotic substance, as shown by the equations below. NaHCO reacting with acid: 3

+

9.3.4 H8,H11 • Names a suitable non-toxic amphiprotic substance. AND • Explains (with a description or by using equations) that amphiprotic substances can react with either acids or bases . . . . . . . . 2

–

H + HCO3 → H2O + CO2 NaHCO3 reacting with base, –

–

2–

OH + HCO3 → CO3

•


+ H2O

Question 29

(a)

The increase in the concentration of lead is greatest downstream from the factory, suggesting that the factory is the source of the lead pollution.

9.4.3,9.4.5 H11,H16 • Identifies and justifies the source of lead pollution . . . . . . . . . . . . . . . . . . . . . . 2 •

(b)

Atomic absorption spectroscopy (AAS) can measure concentrations of metal ions in solution. Atoms can absorb light of a particular frequency. A hollow cathode lead lamp emits light of the frequency that is absorbed by the lead dissolved in the sample being analysed. A solution of the sample is sprayed into a flame (atomised). The light passes through the flame where atoms absorb some of the radiation. A detector measures the intensity of the light beam. The amount of light absorbed indicates the concentration of metal ions. The greater the absorbance, the greater the concentration. The absorbance of a series of lead standard solutions in the 0 to 20 ppm range is determined and graphed, and comparison of the absorbance of the test solution with the absorbance of the standard solutions allows the concentration of lead in the test solution to be determined.


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Identifies the source of the lead pollution . . . . . . . . . . . . . . . . . . . . . . 1

9.4.4,9.4.5 H3,H4,H11,H14 • Nominates AAS as the technique that can analyse concentrations in the ppm range. AND • Describes how the absorbance measurement might be determined. AND • Describes how the concentration of lead can be determined from the absorbance measurements . . . . . . . . . . . . 3 •


•


9



Sample answer

(c)

Chlorine is added to kill pathogens and sterilise the water. Ozone (O3) is an alternative sterilant to chlorine. Fluoride is added to water to lessen the occurrence of tooth decay. Alum is added to help flocculate particles in impure water.

9.4.4 H4 • Identifies why chlorine is added to water. AND • Names, with reason, a second chemical that is added during water treatment . . . . 2 •

Question 30 (a)

F

F

Cl

C

C

9.4.4 H9, H13 • Draws the isomer. AND • Correctly names the isomer . . . . . . . . . . . 2

Cl

F Cl 1,1,1-trichloro-2,2,2-trifluoroethane

(b)

(c)

• Draws the isomer. OR • Correctly names the isomer . . . . . . . . . . . 1

Halons have been used in fire extinguishers and explosion suppressants. CFCs have been used as solvents, refrigerants, propellants for aerosols and as ‘foaming agents’ for plastics. These gases have escaped into the atmosphere from the equipment and materials in which they were used. The concentration of halons and CFCs in the atmosphere increased because these types of compounds are so unreactive, and their molecules are slow to breakdown in the atmosphere.

9.4.4 H1, H3 • Identifies sources of halons and CFCs. AND • Describes how these compounds are unreactive and are slow to decompose in the atmosphere . . . . . . . . . . . . . . . . . . . . . 2

The most common replacements for propellants in aerosol sprays are hydrocarbons. However, hydrocarbons are flammable, and also cannot be used in products that deliver medication (e.g. inhalers). In fire control applications there are few products that are as suitable as CFCs. Hydrogen fluorocarbons (HFCs) have been used as alternatives for CFCs with some success.

9.4.4 H1, H3, H8 • Identifies two classes of chemicals that are suitable replacements for CFCs. AND • Evaluates the effectiveness of each replacement . . . . . . . . . . . . . . . . . . . 2

HFCs do not contain chlorine and do not affect the ozone layer. They are more reactive than CFCs and break down much more quickly, and so contribute less to global warming.

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Section II Question 31

Industrial Chemistry Syllabus content, course outcomes and marking guide

Sample answer

(a)

Soaps are the sodium or potassium salts of long chain fatty

9.5.5 H4, H9 • Compares the structure and uses of cationic detergents and soap, including consist of a long, hydrophobic ‘tail’ and a hydrophilic anionic their effectiveness in hard water . . . . . . . 3 – ‘head’, for example CH3(CH2)14COO . The are typically used acids, made by the alkaline hydrolysis of fats and oils. They

for personal hygiene and washing, although they are not

•

effective in hard water due to the formation of a precipitate with 2+ 2+ the Mg /Ca ions present. Cationic detergents on the other hand, being cationic, do not interact with these cations and so are effective in hard water. They are used for nappy washes (due to their antiseptic properties) and in fabric/hair conditioners

Outlines the structure and use of cationic detergents, including the effectiveness in hard water.

OR • Outlines the structure and use of soap, including the effectiveness in hard water . . . . . . . . . . . . . . . . . . . . . . . . 2

(because they adhere to fibres). Cationic detergents are typically quaternary ammonium salts, as shown below, with a hydrophobic ‘tail’ and a cationic ‘head’. CH3

CH3 (b)

(i)

Identifies the structure or effect in hard water of cationic detergents.

OR • Identifies the structure or effect in hard water of soap . . . . . . . . . . . . . . . . . . . . . . 1

cationic detergent

N

•

+

CH3

Sodium hydrogencarbonate formation can be modelled in the school laboratory by transferring 100 mL of

9.5.6 H12 • Describes a safe method for modelling

saturated NaCl(aq) and 100 mL of NH 3(aq) to a 1 L conical flask and swirling. This should be done in a fume cupboard to avoid inhaling the toxic ammonia fumes.

one step of the Solvay process in the school laboratory . . . . . . . . . . . . . . . . . . . 3 •

Outlines a method for modelling one step of the Solvay process in the school laboratory . . . . . . . . . . . . . . . . . . . 2

•

Identifies one step in modelling part of the Solvay process in the school laboratory . . . . . . . . . . . . . . . . . . . 1

Pieces of CO2(s) are then added with constant swirling in the fume cupboard until the solution is saturated with CO2 and is cold, at which point solid NaHCO 3 forms. The CO2(s) should be handled with tongs and while wearing insulating gloves because it causes burns on skin contact due to its very low temperature. The NaHCO3 can be isolated by filtration, washed with cold, deionised water, and dried in air. (ii)

2NaCl(aq) + CaCO3(s) → CaCl2(s) + Na2CO3(s) m ( NaCl ) = 125 000 g

125 000 n ( NaCl ) = -------------------- = 2138.95 ( 22.99 + 35.45 )

9.5.6 H8, H10, H14 • Correctly calculates the mass of sodium carbonate produced to three significant figures, showing working. . . . . . . . . . . . . 2 •

2138.95 n ( Na 2 CO 3 ) = ------------------- = 1069.47 2

Correctly calculates the number of moles of NaCl used. . . . . . . . . . . . . . . . . . . . . . . 1

m ( Na 2 CO3 ) = 1069.47 × ( 22.99 × 2 + 12.01 + 16.00 × 3 )

= 113 353 g = 113 kg


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Question 31

Industrial Chemistry (Continued) Syllabus content, course outcomes and marking guide

Sample answer

(c)

(i)

The demand for rubber increased dramatically during World War II and supplies of natural rubber (made from latex, obtained from rubber trees) were unable to meet this demand for a number of reasons. Synthetic rubber was developed at this time and is a copolymer of styrene with 1,3-butadiene. Since then, a number of types of synthetic rubbers have been developed and are used in

9.5.1 H4, H9 • Outlines how increasing demand for an identified natural product has been met by chemical industry. AND • Uses a specific example. . . . . . . . . . . . . . 2

many applications such as tyres, medical equipment and • protective clothing. (ii)

2NO2(g)

N2O4(g)

Identifies industrial replacement identified an natural product. . . . . . . . .for . . an .. 1

9.5.2 H8, H10, H14 • Calculates the value of Q. AND • Uses this value to describe the microscopic and macroscopic features of the system . . . . . . . . . . . . . . . . . . . . . . . . 3

[ N 2 O4 ] K = ------------------- = 3.2 at 25° C 2 [ NO 2 ]

 3.4   ------2  Q = --------------- = 0.32 2  4.6  ------ 2

•

The value of Q is not equal to the value of K, so the system is not at equilibrium. Q is lower than K, and as a result the system must be shifting to the right to increase

Writes the correct equilibrium constant expression. AND • Calculates Q . . . . . . . . . . . . . . . . . . . . . . . 2 •

the concentration of N2O4 and increase the value of Q

Writes the correct equilibrium constant expression . . . . . . . . . . . . . . . . . 1

until it is equal to K. This means that at a macroscopic level the brown colour of the mixture will be fading and the pressure in the system will be decreasing. At a microscopic level the rate of the decomposition of NO 2 will be faster than the rate of its formation. (d)

(i)

Electrolysis of dilute NaCl: 2H2O(l) → 2H2(g) + O2(g) Electrolysis of concentrated NaCl: 2NaCl(aq) + 2H2O(l) → Cl2(g) + H2(g) + 2NaOH(aq)

9.5.4 H3, H8 • Writes a labelled equation for the electrolysis of dilute NaCl (electrolysis of water). AND • Writes a labelled equation for the electrolysis of concentrated NaCl(aq) . . . 2 •

Writes a labelled equation for the electrolysis of dilute NaCl (electrolysis of water).

OR • Writes a labelled equation for the electrolysis of concentrated NaCl(aq) . . . 1

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Question 31


Sample answer

(ii)

The diaphragm and membrane processes for NaOH 9.5.4 H3, H4 production involve the oxidation of chloride ions at a • Compares and contrasts the diaphragm and steel anode and the reduction of water, to produce membrane processes for NaOH hydrogen gas at a steel or graphite cathode. In the production, including purity of product diaphragm process the anode and cathode and social consequences . . . . . . . . . . . . . 3 compartments are separated by an asbestos diaphragm. Sodium ions pass through the diaphragm from the anode • Outlines aspects of the diaphragm and – to cathode compartments to produce NaOH( aq), but unreacted Cl ions also pass through, contaminating the NaOH with NaCl. Asbestos exposure causes the fatal lung disease asbestosis, and so the diaphragm process has unacceptable social consequences. In the membrane • process an ion-selective membrane separates the two + compartments and only permits Na ions to pass through. As a result the NaOH is of very high purity. It also avoids the use of asbestos and as a result the membrane process does not have the social consequences of the diaphragm process.

(e)

membrane NaOH production,processes includingfor purity of product or social consequences. . . . . . . . . . . . . . . 2 Response contains one correct comparison of the membrane and diaphragm processes . . . . . . . . . . . . . . . . 1

9.5.3 H3, H7, H8 Elemental sulfur is obtained using the Frasch process, or by recovery of sulfur from natural gas and petroleum. In the Frasch • Thoroughly explains the formation of sulfuric acid from sulfur by describing the process, superheated steam is pumped into underground sulfur chemistry involved, including an assessment deposits where it melts the sulfur, and compressed air is of the equilibrium conditions required for pumped in to force the molten sulfur to the surface. Sulfur is SO3 production. insoluble in water so the sulfur obtained in this way is pure. AND In the Contact process, the sulfur is melted and sprayed along • with an abundant supply of oxygen into a furnace, where it is burnt to produce SO2(g). The increase in surface area produced • by spraying molten sulfur into the furnace ensures that a 100% yield of SO2 is obtained very rapidly:

Response includes chemical equations.. . 7

The SO2(g) is converted to SO3 by reaction with oxygen:

Explains the formation of sulfuric acid from sulfur by describing the chemistry involved, including the equilibrium conditions required for SO3 production. AND • Response includes chemical equations.. . 6

2SO2(g) + O2(g)

•

S(l) + O2(g) → SO2(g)

2SO3(g)

in an exothermic equilibrium reaction. It is in this step that the reaction conditions are crucially important, and they must be carefully controlled to maximise the rate and yield. Because the forward reaction is exothermic, high temperatures – while favouring the rate – would lower the yield of SO 3 by shifting the equilibrium position to the left. As a result, a compromise temperature of approximately 650°C is used along with a V2O5 catalyst to increase the rate. The catalyst is present in three separate catalyst beds, and the temperature is reduced as the gaseous mixture passes over each of the beds. In this way a high rate is maintained and the conversion to SO3 is increased to almost 100%, because the successively lower temperatures favour the forward reaction. Although Le Chatelier’s principle predicts that high pressure would favour the production of SO 3 in this reaction, in practice almost 100% conversion is achieved at 1–2 atmospheres, and so costly and dangerous high pressure conditions are not warranted.

Outlines the formation of sulfuric acid from sulfur, including the equilibrium conditions required for SO3 production. AND • Response includes chemical equations . . 5 •

Outlines the formation of sulfuric acid from sulfur, including the at least one aspect of the equilibrium conditions required for

SO3 production. AND • Response includes one chemical equation . . . . . . . . . . . . . . . . . . 4 •

Outlines the formation of sulfuric acid from sulfur. AND • Response includes one chemical equation . . . . . . . . . . . . . . . . . . 3

(e) continues on next page


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Question 31


Sample answer

(e) (continued)

•

The next step is the conversion of SO 3 to sulfuric acid. To do this, the SO3 is bubbled into concentrated sulfuric acid to produce oleum: H2SO4(l) + SO3(g) → H2S2O7(l)

Response contains one correct statement about the production of sulfuric acid. AND • Response contains a relevant chemical equation . . . . . . . . . . . . . . . . . . 2

The oleum is then carefully diluted with water to produce

•

sulfuric acid of 98% purity: H2S2O7(l) + H2O(l) → 2H2SO4(l)

OR about the production of sulfuric acid. • Response contains a correct, relevant chemical equation . . . . . . . . . . . . . . . . . . 1

The final step is carried out in this way to avoid safety issues

Response includes one correct statement

associated with the highly exothermic nature of the reaction between SO3 and water. There are no rate/yield issues associated with the production of oleum or its conversion to sulfuric acid.

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Question 3 2

Shipwrecks, Corrosion a nd C onservation Syllabus content, course outcomes and marking guide

Sample answer

(a)

Porous objects such as wooden artefacts become saturated with seawater after being submerged for long periods. Seawater contains a range of salts (mainly sodium chloride) and these salts become trapped within the structure of the wooden artefact. When the seawater is allowed to evaporate, the concentration of salts in solution increases (water evaporates, salts do not). As the artefact dries the dissolved salts start to crystallise.

9.6.7 H1, H5 • Identifies components in seawater that will cause damage. AND • Describes the process of evaporation as the loss of water leading to an increase in salt concentration and the eventual crystallisation of salts. AND The formation of these salt crystals throughout the wooden artefact causes considerable damage by distorting the shape of • Explains how crystallisation of salts will cause damage . . . . . . . . . . . . . . . . . . . . . . 3 the artefact, causing cracks to form. Replacing water with a suitable substitute (such as PEG) ensures that the wood does not shrink or distort.

(b)

(i)

Anode half-equation: +


•


9.6.3 H1,H6, H7, H8,H10,H13 • Correctly provides the anode half-equation. AND • Correctly provides the cathode half-equation . . . . . . . . . . . . . . . . . . . . . . 2

–

2H2O(l) → 4H (aq) + O2(g) + 4e Cathode half-equation: 2+

•

–

Cu (aq) + 2e → Cu(s)

• (ii)


Volta and Galvani were the first to systematically

9.6.1

investigate electrochemical techniques. Importantly, Volta designed the first reliable source of direct current – the Voltaic pile (using Cu and Sn).

•

H1,H2,H7,H13

Places Davy’s work in context with the other scientists mentioned in the stimulus material. Davy developed reliable electrical power supplies based AND on Voltaic piles (using Ag and Zn). He also developed a • Outlines Davy’s contributions to the development of technology. range of electrolysis techniques used to split AND compounds into their component elements (the first • Outlines how other scientists (most electrolysis experiments). obviously, Faraday) used Davy’s work The techniques developed by Davy were investigated by to a further understanding of his assistant (Faraday) who came to understand the electrolytic reactions. mathematical relationships between the amount of AND electricity consumed and the amount of chemical • Concludes by describing the continuing reaction that occurred. importance of electrolysis techniques in the Davy’s electrolysis techniques allowed him to isolate production of modern materials. . . . . . . . 4 reactive metals including sodium, potassium, magnesium and strontium. • Any three of the above. . . . . . . . . . . . . . . 3 The importance of Davy’s work is most evident in the production of modern materials (aluminium and its alloys), which are produced by electrolysis techniques that Davy pioneered.


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Question 32

Shipwrecks, Corrosion and Conservation (Continued) Syllabus content, course outcomes and marking guide

Sample answer

(c)

Steels are metal alloys in which the main metal component is iron. Samples 3, 4 and 5 are types of steel. Sample 3 is a low carbon steel that is easily and cheaply produced. It is used to produce pipes, metal sheeting and in modern building materials, but will have low corrosion resistance. Sample 4 has a chromium content greater than 10.5% and such steels are called stainless steels. Stainless steels resist corrosion due to the formation of chromium oxides at the surface and are used in the manufacture of cutlery, kitchen sinks and bathroom fittings.

(d)

9.6.2 H8,H12,H13,H14 • Identifies one steel alloy from the list provided and states a possible use for this type of alloy. AND • Discusses how the composition of this steel alloy affects its properties and uses. AND • Identifies a second steel alloy from the list provided and states a possible use for this type of alloy. AND • Discusses how the composition of the second alloy is related to its properties and uses . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Sample 5 is also a stainless steel but, being a more complex alloy, will be more expensive to produce. This a specialist alloy that is resistant to corrosion (due to chromium) and stronger (due to molybdenum, vanadium and manganese) than other stainless steels. It is marine steel used in ship building. •

Only three of the above . . . . . . . . . . . . . . 3

•

Only two of the above . . . . . . . . . . . . . . . 2

•


(i)

A series of six identical test tubes were prepared as follows: Experiment to determine effect of pH:

Test tube 1 contained a solution of 0.10 M H2SO4 that was heated to expel oxygen. Test tube 2 contained a solution of 0.10 M NaOH that was heated to expel oxygen. Test tube 3 contained distilled water without anything being added that was heated to expel oxygen.

9.6.5, 9.6.6 H3, H6, H8, H10, H13, H14 • Describes a suitable set of controls. AND • Describes an experimental design that would investigate the effect of dissolved oxygen on rates of corrosion. AND • Describes an experimental design that would investigate the effect of pH on rates of corrosion . . . . . . . . . . . . . . . . . . . . . . . 3

3

A 1 cm sample of iron metal was placed into each test • tube. Each test tube was then sealed and examined at the end of day 1 and day 2 for signs of corrosion. • Experiment to determine effect of oxygen

Only two of the above . . . . . . . . . . . . . . . 2 Only one of the above . . . . . . . . . . . . . . . 1

Test tube 4 contained a solution of 0.10 M NaCl that was heated to expel oxygen. Test tubes 5 and 6 contained a solution of 0.10 M NaCl that was not heated. 3

A 1 cm sample of iron metal was placed into each test tube. Test tubes 4 and 5 were sealed. Test tube 6 was periodically shaken during the next two days. Each test tube was observed for signs of corrosion over the next two days.

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Question 32

Shipwrecks, Corrosion and Conservation (Continued) Syllabus content, course outcomes and marking guide

Sample answer

(ii)

The pH experiments show lowering pH increases the 9.6.5, 9.6.6 H3, H6, H8, H10, H11, H13, H14 rate of corrosion (the iron wire is more pitted in the test • Predicts the results expected for tube that contains five drops of sulfuric acid). each experiment. Iron in low pH tube (Tube 1) corrodes rapidly due to the AND + presence of H (aq) ions: • Draws valid conclusions based on the + 2+ predicted results. Fe(s) + 2H (aq) → Fe (aq) + H (g) 2 AND The oxygen experiments show that the greatest amount • Provides an equation, or equations, to of corrosion occurred for test tube 6 due to the presence support their reasoning. . . . . . . . . . . . . . . 3 of the greatest amount of dissolved oxygen: • Any two of the above. . . . . . . . . . . . . . . . 2 2+ – 2Fe(s) + O2(g) + 2H2O(l) → 2Fe (aq) + 4OH (aq) •

(e)

Paint is a relatively cheap product and painting can be an effective method of corrosion protection. However, the physical barrier that paint provides is easily damaged (scratched) and needs to be monitored as repainting will become necessary. Terrestrial objects (like bridges and above-ground pipes) can be easily and relatively cheaply repainted. Painting underwater structures (boats, oil rigs, wharfs, pipes) and underground structures (pipes) is difficult and corrosion of these structures is better prevented using sacrificial anodes. Sacrificial protection requires an electrolyte to be present between the cathode and the anode – unless an electrolyte is present (as seawater or moist ground), sacrificial protection will not work. A sacrificial anode is a metal that is more reactive than iron and is attached to the surface of the steel object to be protected from corrosion. Magnesium, zinc and aluminium – all being more reactive than iron – are good choices of metals to use as sacrificial anodes. The sacrificial anode is oxidised in preference to the iron in the steel, hence it protects steel from corrosion. For example, zinc could be used as the sacrificial anode and would be oxidised in preference to steel: 2+

9.6.4 H3,H4,H6, H8,H13,H14 • Describes how painting protects steel against corrosion. AND • Provides reasons as to why painting is not always the best corrosion prevention method. AND • Identifies a suitable metal that could be used as the sacrificial anode. AND • Explains that the steel object becomes the cathode and that oxygen is reduced at this site. AND • Writes an equation (either an overall equation or an appropriate half equation) that describes the corrosion process. AND • Identifies when painting or using sacrificial anodes is the more appropriate corrosion prevention method . . . . . . . . . . . . . . . . . . 6 •

–

Zn(s) → Zn (aq) + 2e

The steel object is thus protected from corrosion by the • sacrifice of the more active metal. The steel object becomes an • inert cathode and the reduction of oxygen, in the presence of water, occurs at the inert steel cathode: –

–

O2(g) + 2H2O(l) + 4e → 4OH (aq) As long as some active metal (in this case, Zn) remains attached to the steel structure (underground pipe, or ship), it will remain protected from corrosion.


Any five of the above . . . . . . . . . . . . . . . . 5 Any four of the above . . . . . . . . . . . . . . . 4 Any three of the above. . . . . . . . . . . . . . . 3

•


•


The zinc anodes can be periodically replaced more cheaply than digging up the pipe or repainting the ship, hence sacrificial anodes provide a more efficient and effective method of corrosion protection in marine and below-ground environments (where repainting and monitoring of a paint layer are difficult). Copyright © 2 014 Neap

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Question 33

The Chemistry of Art Syllabus content, course outcomes and marking guide

Sample answer

(a)

The Pauli exclusion principle states that no two electrons in an

9.8.3 H6, H7 atom can have the same set of quantum numbers, and the result • Describes the Pauli exclusion principle. of this is that each orbital can hold a maximum of two electrons AND • Hund’s rule. . . . . . . . . . . . . . . . . . . . . . . . 3 of opposite spin. Hund’s rule states that each orbital in a degenerate set (orbitals of the same energy) will contain one electron before any of them contain two. For example, this means that each3 of the 3p orbitals will be occupied by one electron in a 3p configuration.

(b)

(i)

Transition metal complexes consist of a metal atom or ion surrounded by a set of coordinating ligands. Each donor atom on the ligand donates an electron pair to an empty orbital on the metal, meaning that the ligands act as Lewis bases and the metal acts as a Lewis acid. This type of bonding is called coordinate covalent bonding because both electrons in the bond are donated by one atom. The ligands’ electrons interact with the metal’s d-electrons, splitting them into two sets of different energy. Electrons can be excited from the lower energy set to the higher energy set by the absorption of visible light, and as a result the complex appears coloured. This is illustrated by the two cobalt complexes, pink

• Outlines the Pauli exclusion principle. OR • Hund’s rule. . . . . . . . . . . . . . . . . . . . . . . . 2 • Identifies the Pauli exclusion principle. OR • Hund’s rule. . . . . . . . . . . . . . . . . . . . . . . . 1 9.8.3, 9.8.5 H6, H7 • Describes the bonding in transition metal complexes, including an explanation of why the two cobalt complexes have different colours. . . . . . . . . . . . . . . . . . . . 4 •

Outlines the bonding in transition metal complexes, identifying that the complexes have different ligands.

OR • Describes the bonding in transition metal complexes. OR

hexaquacobalt(II) and blue tetrachlorocobalt(II). The • different number and types of ligands split the d-orbitals to different extents, changing the wavelength of light absorbed in electron excitation, and thus changing the • colour of the complexes.

Describes how colour arises in transition metal complexes which contain different ligands . . . . . . . . . . . . . . . . . . . . 3 Outlines the bonding in transition metal complexes.

OR • Outlines how colour arises in transition metal complexes with different ligands . . 2 •

Response contains one correct statement about colour in transition metal complexes.

OR • Response contains one correct statement about the bonding in transition metal complexes. . . . . . . . . . . . . . . . . . . . 1 (ii)

2+

2

2

6

2

6

9.8.4 H6 • Identifies the electron configuration of

7

Co : 1s , 2s , 2p , 3s , 3p , 3d

2+

Co

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using orbital notation . . . . . . . . . . . 1



Question 33

The Chemistry of Art

(Continued) Syllabus content, course outcomes and marking guide

Sample answer

(c)

(i)

To observe a transition metal changing oxidation states the following procedure can be used: • • • •

9.8.4 H12, H13 • Outlines the p rocedure including reagents used. Transfer 2 mL of FeCl 2(aq) to a test tube. AND Add acidified potassium permanganate solution • Includes safety precaution(s) taken. dropwise to the test tube, mixing after each addition. AND Safety glasses should be worn to prevent eye contact • Identifies one colour change . . . . . . . . . . 3 with the acidified KMnO4(aq), which is corrosive. • Rubber gloves should be worn to prevent skin contact with the corrosive acidified KMnO4(aq).

The green colour of the Fe yellow/orange as Fe

3+

2+

•

ion changes to pale –

is formed. The purple MnO4 ion 2+

will be discolourised as it is added to the Fe . (ii)

The relevant half-equations for this procedure are: Oxidation: 2+

3+

–

Fe (aq) → Fe (aq) + e Reduction: –

(d)

(i)

+

–

Outlines the procedure, including reagents used . . . . . . . . . . . . . . . . . . . . . . 2 Response contains one correct reagent for observing a transition metal change colour. . . . . . . . . . . . . . . . . . . . . . 1

9.8.4 H8 • Identifies the two half-equations associated with the procedure described in part (c) (i), correctly labelled as oxidation or reduction . . . . . . . . . . . . . . . . . . . . . . . 2

2+

MnO4 (aq) + 8H (aq) + 5e → Mn (aq) + 4H2O(l)

•

Ancient cultures used pigments as cosmetics. For

9.8.1

example, (red mercury(II) sulfide, HgS)and was used cinnabar in Ancient Egyptian culture as a rouge a lipstick.

•

Mercury is toxic, causing damage to the central nervous system, and learning and behaviour disorders in children.

Identifies one correct half-equation associated with the procedure described in part (c)(i) . . . . . . . . . . . . . . . . . . . . . . . 1 H2, H3

Outlines a specific cosmetic aoruse forofburial of thepigment dead. as a AND • Outlines a health hazard associated with its use . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 •

Outlines a use of a specific pigment as a cosmetic or for burial of the dead.

OR • Identifies a health hazard associated with the use of a specific pigment . . . . . . . . . . 1


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Question 33



Sample answer

(ii)

Infrared radiation (IR) can be u sed to examine artworks and is particularly useful for studying underdrawings. Near-IR radiation from a lamp is shone onto a painting and the radiation penetrates through most pigments to the white ground underneath, from which it is reflected. Any charcoal drawing present will absorb the radiation, and as a result, a black image is formed showing any underdrawings. Pigments containing copper may also absorb IR radiation and thus IR reflectance can be used to identify them by comparison with known spectra. Some copper pigments decompose when subjected to far-IR radiation and so care must be taken to only use near-IR radiation in analysis of this type.

9.8.2 H3 • Describes how IR and UV radiation can be used to study artworks. . . . . . . . . . . . . 3 •

Outlines how IR and UV radiation can be used to study artworks . . . . . . . . . . . . . . . 2

•

Identifies one use of IR or UV radiation in the study of artworks . . . . . . . . . . . . . . 1

Ultraviolet radiation is used to identify pigments because they reflect different wavelengths in the UV-region depending on their composition. As a result, the reflectance spectrum of a section of the artwork can be compared to reference spectra for pigment identification. Furthermore, zinc oxide and green malachite (CuCO3·Cu(OH)2) fluoresce under UV light to produce a yellow and mauve colour respectively, which makes their identification very easy. (e)

The hydrogen emission spectrum was fundamentally important 9.8.2 to the development and acceptance of the Bohr model of the atom. Prior to the Bohr model, the accepted atomic model was Rutherford’s ‘planetary model’, in which electrons orbited the nucleus of an atom like planets orbiting a star. This model was based on classical mechanics but it could not explain why the electrons did not lose energy as they orbited, ultimately spiralling into the nucleus. When hydrogen atoms are excited by an electric current the emission spectrum is observed. It consists of coloured lines on a black background at discrete wavelengths. Bohr explained this emission spectrum using Plank’s concept of energy quantisation. He suggested that electrons orbit the nucleus in orbits of fixed energy and radius, without losing energy. He also postulated that there were a fixed number of orbits, each with its own characteristic energy and radius. The orbits are also known as shells, and Bohr assigned the quantum number n to them, where n = 1 is the shell closest to the nucleus. Bohr proposed that electrons can be excited to a shell of higher energy level by absorbing exactly the energy

Thoroughly describes the Bohr model of the atom, including its limitations. AND • Describes the hydrogen emission spectrum and assesses its importance to the Bohr model. AND • Describes how flame colours are related to the Bohr model using the results of a first-hand investigation. . . . . . . . . . . . . . . 7 •

Thoroughly describes the Bohr model of the atom, including its limitations. AND • Describes the hydrogen emission spectrum. AND • Describes how flame colours are related to the Bohr model using the results of a

difference between them, and that this energy is re-emitted when the electron relaxes back to its ground state. Bohr was • able to calculate energy differences between orbits that corresponded to the experimental observation of the lines in the visible region of the hydrogen emission spectrum. He also calculated the frequencies of the lines he expected in the UV • and IR regions of the spectrum, and these were later experimentally confirmed. Thus the hydrogen emission spectrum was the experimental basis on which Bohr’s model was based, and as such it was of crucial importance. (e) continues on next page

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H1, H6, H7

•

first-hand investigation. . . . . . . . . . . . . . . 6 Describes the Bohr model of the atom and relates it to the hydrogen emission spectrum and to the results of flame tests . . . . . . . . 5 Outlines the Bohr model of the atom and relates it to the hydrogen emission spectrum and to the results of flame tests . . . . . . . . . . . . . . . . . . . . . . . . 4



Question 33



Sample answer

(e) (continued) Despite its success at calculating and then predicting later experimental observations of the H emission spectrum, the Bohr model did have its limitations. It could not explain why only orbits of fixed energy and radius were allowed. It could not be used to calculate the energy of lines in the spectra of atoms with more than one electron. Nor could it explain why

•

Outlines the Bohr model of the atom and relates it to the hydrogen emission spectrum or to the results of flame tests . . . . . . . . . . . . . . . . . . . . . . . . 3

• Outlines the hydrogen emission spectrum. OR

some lines in the H emission spectrum were closely spaced • doublets which were split by a magnetic field. Nevertheless, it was a great leap forward from Rutherford’s classical • mechanical model.

Outlines the. Bohr flame tests . . . . .model . . . . .of . . the . . .atom . . . . or ....2

Response contains one correct statement about the hydrogen emission spectrum. Bohr’s ideas of quantised energy levels can be visualised using OR the results of flame tests. To carry out a flame test, a clean • Response contains one correct statement platinum loop is dipped into a solid metal salt such as copper about the Bohr model of the atom or nitrate, and then into a non-luminous Bunsen flame. Electrons flame tests . . . . . . . . . . . . . . . . . . . . . . . . 1 in the atoms of the sample absorb energy from the flame and become excited to a higher energy level. When they relax back to the ground state the energy is emitted in the form of a specific wavelength of light in the visible region, bluish-green in the case of copper. Because the number of protons and the electron configuration of each element is unique, the energy differences between shells is also unique, and as a result each element has a characteristic flame colour.


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Question 3 4

Forensic C hemistry Syllabus content, course outcomes and marking guide

Sample answer

(a)

In order to distinguish compounds A to D, the following sequence of tests could be carried out: •

•

9.9.1 H12, H14 • Describes a sequence of tests to safely distinguish compounds A to D . . . . . . . . . 3

Transfer 1 mL of each compound to a separate test tube and add 1 mL of saturated Na2CO3(aq) to each. The one in • which gas bubbles form is compound B (an alkanoic acid). Transfer 1 mL of each of the remaining samples to clean,

•

separate test tubes and add 1 mL of Br 2(aq) to each. The one in which the orange colour of the bromine water decolourises immediately is sample D (an alkene).

(b)

•

Transfer 1 mL of e ach of the remaining samples to clean, separate test tubes and add 1 mL of acidified KMnO4(aq). The sample which decolourises the permanganate is sample B (the alcohol).

•

The remaining sample is sample A, the alkane. This can be confirmed by adding 1 mL of Br2(aq) to the sample in the presence of UV light. The sample should slowly decolourise the bromine water.

•

Hydrocarbons such as A and D are toxic by inhalation so to avoid inhaling the vapours the experiment should be done in a fume cupboard. (i)

22

Identifies a test to distinguish one of the compounds . . . . . . . . . . . . . . . . . . . . .1

Cellulose, starch and glycogen have one common 9.9.2 H9, H13 feature, which is that they are all condensation polymers • Compares and contrasts cellulose, starch of glucose. As a result, their composition is the same. Cellulose is formed from β-glycoside linkages and as a result, the polymer chains have a flat, ribbon-like • structure that can pack very closely together. The extensive intermolecular hydrogen bonding between polymer chains results in a very strong material that is also difficult to digest, making cellulose perfect for its • role as a plant structural material. Starch is composed of helical amylose and branched amylopectin molecules. It is much easier to digest and hydrolyse, and is used by plants as a glucose storage molecule, for example, in tubers such as potatoes. As a result, starch is the major carbohydrate in the human diet. Glycogen is a highly branched polymer of glucose, and is the major glucose storage molecule in animals and fungi. In humans it is stored extensively in muscle tissue and the liver. It is rapidly hydrolysed during exercise to provide glucose for muscle cells.

(ii)

Outlines two correct tests to safely distinguish two compounds . . . . . . . . . . . 2

and glycogen in terms of composition, structure and function . . . . . . . . . . . . . . . . 3 Outlines similarities and/or differences between cellulose, starch and glycogen . .2 Identifies one difference or similarity between two of the polysaccharides . . . . .1

Glucose is a red ucing sugar whereas sucrose is not. As 9.9.2 H8, H9, H12 • Outlines the test for a reducing sugar a result they can be distinguished by heating their 2+ including reagent used and positive solutions with Benedict’s reagent (alkaline Cu test result. . . . . . . . . . . . . . . . . . . . . . . . . .2 solution). Glucose will cause the formation of a brick-red precipitate from the clear blue solution, • Identifies that Benedict’s reagent whereas no colour change will be observed for sucrose. can be used to distinguish glucose from sucrose . . . . . . . . . . . . . . . . . . . . . . .1

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Question 34

Forensic Chemistry


Sample answer

(c)

(i)

DNA analysis involves the following steps. •

The complementary strands in the DNA sample are separated by heating, and enzymes are added to catalyse DNA replication.

•

The process of heating and replication is repeated many times to produce large quantities of DNA for analysis, and is known as the polymerase chain reaction.

•

The DNA strands are then cut at various specific locations within the base sequence by restriction enzymes, which produces short fragments of DNA for separation by electrophoresis.

•

The mixture of base sequences is deposited onto a gel, and a potential difference applied across it. This causes the base sequences (which are charged) to separate as they move through the gel at a rate that depends on their size (mass) and overall charge.

•

Once separated, the base sequences are dyed so that they become visible, and are then compared to standards, for example, those of a suspect.

9.9.4, 9.9.5 H3, H4 • Outlines the process of DNA analysis. AND • Explains why DNA analysis is useful . . . 3 • Outlines the process of DNA analysis. OR • •

Explains why DNA analysis is useful . . . 2 Response contains one correct statement about DNA analysis . . . . . . . . . . . . . . . . . 1

DNA analysis is extremely useful because DNA is present in almost all biological samples left at a crime scene and it can be used to match a suspect with a crime scene, or to exclude them. Different sample types can be compared, for example, saliva on a cup compared with skin left under fingernails. The PCR reaction means that extremely small samples can be analysed, and the fact that DNA contains ‘fingerprint’ regions unique to each individual means that a suspect can be successfully eliminated or linked to a crime scene. (ii)

(d)

An ethical issue associated with the use of DNA analysis 9.9.4 H4 is the establishment of DNA databases in some • Outlines an ethical issue associated with jurisdictions. The collection and storage of individuals’ DNA analysis. . . . . . . . . . . . . . . . . . . . . . 2 DNA for later analysis – and an increasing reliance on DNA evidence – could result in an unfair assumption of • Identifies an ethical issue associated with DNA analysis. . . . . . . . . . . . . . . . . . . . . . 1 guilt should that DNA also be found at a crime scene, since the presence of DNA alone does not indicate guilt and other substantiating evidence is also necessary. This is especially true for individuals already convicted of an earlier, unrelated crime.

(i)

9.9.3

H amino group H2N

O C R


Draws and correctly labels an amino acid . . . . . . . . . . . . . . . . . . . . . . . . 2

•

Draws a correct amino acid . . . . . . . . . . . 1

C OH

H9

•

acid group

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Question 34

Forensic Chemistry


Sample answer

(ii)

Electrophoresis relies on the movement of charged 9.9.3, 9.9.5 H3 species in an electric field. The sample containing a • Describes how electrophoresis can be used mixture of proteins is placed in the middle of the solid to analyse a protein sample. . . . . . . . . . . .3 phase, for example, a polyacrylamide gel soaked in a • Outlines how electrophoresis can be used buffer solution. A DC power source is connected to to analyse a protein sample. . . . . . . . . . . .2 opposite ends of the gel, and the proteins in the sample migrate to one of the electrodes or the other at a rate • Response contains one correct statement dependent on their size and overall charge. The overall about electrophoresis . . . . . . . . . . . . . . . . 1 charge on the proteins will depend upon the pH of the buffer, because acidic side groups will be ionised to different extents depending on the pH. After separation on the gel the proteins in the sample are visualised by staining, and can be compared to known samples which are processed on the same gel under the same conditions. Because different animal species have different protein profiles, comparison of the unknown sample with reference samples can be used to determine the srcin of the sample.

(e)

The Bohr model of the atom, proposed in an attempt to explain the hydrogen emission spectrum, states that electrons in atoms move around the nucleus in orbits of fixed or quantised radii and energy. Using this model, Bohr was able to overcome the deficiencies in the Rutherford model of the atom, explain the discrete lines observed in the visible region of the hydrogen emission spectrum, and calculate the energy of these lines.

9.9.6 H3, H6, H7 • Thoroughly explains how line-emission spectra occur and why discrete lines are observed. AND • Explains why line-emission spectroscopy is useful in forensic chemistry and provides

Furthermore, Bohr was able to calculate the expected spectral lines in the IR and UV regions of the hydrogen emission spectrum, which were later confirmed by experimental data. Bohr’s model suggests that electrons are able to move from an orbit of lower energy to one of higher energy by absorption of exactly the energy corresponding to the difference between the two energy levels. Electrons relax back to the ground state by emission of this same quantum of energy. As a result, discrete lines are observed in the emission spectra of elements when their atoms are excited (for example, by heat from a Bunsen flame), corresponding to the energy released when electrons in an excited state relax back to the ground state. Each atom has a characteristic line-emission spectrum because the number of protons in the nucleus and the electron configuration of each element is different. As a result, the energy differences between orbits, and the number and type of excitations possible, are different. These principles can be demonstrated in the school laboratory

an assessment. AND • Describes flame tests, including at least two flame colours. . . . . . . . . . . . . . . . . . .7 •

Explains how line-emission spectra occur and why discrete lines are observed. AND • Explains why line-emission spectroscopy is useful in forensic chemistry. AND • Describes flame tests, including at least two flame colours . . . . . . . . . . . . . . . . . . .6 •

Explains how line-emission spectra occur and why discrete lines are observed. AND • Outlines why line-emission spectroscopy is

using flame tests. In these tests a small sample of a metal salt useful in forensic chemistry. such as copper(II) chloride is introduced into the blue Bunsen AND • Outlines how flame colours can be flame on the end of a clean platinum wire, and the observed in the laboratory, citing one characteristic blue-green flame colour is observed. If strontium specific example . . . . . . . . . . . . . . . . . . . .5 chloride is used, a characteristic crimson-red colour is observed. Using this simple laboratory test, the presence of metal ions in a water sample, for example, can be identified. (e) continues on next page

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Question 34

Forensic Chemistry


Sample answer

(e) (continued)

•

The fact that elements exhibit unique line-emission spectrum makes line-emission spectroscopy an extremely useful tool for the forensic chemist. For example, it allows a wide variety of sample types to be analysed for the elements they contain. This could allow a paint sample found at a crime scene to be matched with paint used on particular models of car, or from particular paint factories. It could allow specific heavy metals to be identified in tissue samples where heavy metal poisoning is suspected. Furthermore, the principles upon which line-emission spectroscopy are based have been used to develop atomic absorption spectroscopy, which allows the quantitative analysis of metals in samples. Using this technique, the amount of lead, for example, in a tissue sample, or arsenic in a waterway, can be measured to the ppm or ppb level, making it extremely useful to the forensic chemist.

Outlines how line-emission spectra occur and why discrete lines are observed. AND • Outlines why line-emission spectroscopy is useful in forensic chemistry. AND • Identifies one flame colour . . . . . . . . . . . 4 •

OR • Outlines line-emission spectroscopy and relates it to flame tests . . . . . . . . . . . . . . . 3 • Outlines line-emission spectroscopy. OR • Outlines its use in forensic chemistry. OR • How flame colours can be observed in the laboratory . . . . . . . . . . . . . . . . . . . . 2 •


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Outlines line-emission spectroscopy and its use in forensic chemistry.

Response contains one correct statement about line-emission spectroscopy, the Bohr model of the atom, the use of emission spectroscopy in forensic chemistry, or flame tests . . . . . . . . . . . . . . . . . . . . . . . . 1

25

2014 Chemistry - NEAP Trial With Solutions

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