2013 Y6 H2 Chem QP

1

RAFFLES INSTITUTION 2013 YEAR 6 TERM 3 COMMON TEST Higher 2

CHEMISTRY

9647 4 July 2013 2 hours 30 minutes

Additional Materials: Materials:

OMR Answer Sheet Sheet Writing Papers Data Booklet

READ THESE INSTRUCTIONS FIRST DO NOT open this booklet until you are told to do so. Write your name and civics tutorial group in the spaces provided on the cover sheets on pages 11 pages 11 and 27 and 27.. This paper consists of three t hree sections A, B and C. Section A (20 marks) consists of 20 multiple-choice questions. Record your answers to this section in soft pencil on the separate OMR Answer Sheet. Section B (35 marks) consists of 4 structured questions. Answers to this section section are to be written written in the spaces spaces provided in the question paper. paper. Section C (40 marks) consists of 2 free-response questions. Answers to this section are to be written on the writing papers provided. You are to begin each question on a fresh page of paper. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets brackets [ ] at the end of each question or part question. question. Write in dark blue or black pen. You may be subject to penalty for writing answers answers in pencil.

This document consists of 27 printed 27 printed pages.

Raffles Institution 2013

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2

Section A (20 marks) For each question, there are four possible answers, A, B, C C and D. Choose the one you consider to be correct and record your choice on the OMR Answer Sheet provided. 1

The values of some thermochemical data for barium chloride are given in the table below. Lattice energy of barium chloride Standard enthalpy change of hydration of barium ions Standard enthalpy change of hydration of chloride ions

= = =

–1 –2065 –2065 kJ mol –1 –1 –2494 –2494 kJ mol –1 –1 –363 –363 kJ mol –1

What is the standard enthalpy change of solution of barium chloride?

2

A

–1 –5285 –5285 kJ mol –1

B

–1 –4922 –4922 kJ mol –1

C

–1 –1155 –1155 kJ mol –1

D

–1 –792 –792 kJ mol –1

The enthalpy change of neutralisation between aqueous sodium hydroxide and –1 hydrochloric acid is –57.2 –57.2 kJ mol –1 , but the enthalpy change of neutralisation –1 between aqueous sodium hydroxide and ethanoic acid is –55.2 –55.2 kJ mol –1 . Which statement best explains the numerical difference between these values?

3

A

The ionisation of ethanoic acid to give hydrogen ions in water is an endothermic process.

B

Aqueous ethanoic acid contains fewer hydrogen ions than hydrochloric acid of the same concentration.

C

The reaction between aqueous sodium hydroxide and aqueous ethanoic acid produces less than 1 mole of water.

D

A smaller volume of aqueous ethanoic acid than that of aqueous hydrochloric acid of the same concentration is required for the neutralisation.

Which statement about the equilibrium shown below is correct? [Fe(H2O)6]3+(aq) + H2O(l ) lilac

[Fe(H2O)5OH]2+(aq) + H3O+(aq) orange

> 0 H >

A

The equilibrium constant has no units.

B

The solution turns lilac when pH is increased.

C

The equilibrium constant increases when the mixture is diluted.

D

The rate constant of the backward reaction increases with temperature.


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3 4

A gaseous reactant, R was injected into a vessel and allowed to reach equilibrium with its gaseous product, P. The production of P is endothermic. The following graph shows the change in concentrations of R and P with time. concentration

time / min t1

t2

t3

Which of the following changes occurred at t 2?

5

A

More R was added.

B

The temperature was increased.

C

An inert gas was added at constant pressure.

D

The volume of the vessel was reduced at constant temperature.

A pure sample of NH 3(g) is introduced into an evacuated vessel of constant volume. This vessel is maintained at constant temperature such that the equilibrium below is established. 2NH3(g) N2(g) + 3H2(g) The value of the final pressure is then found to be 40% greater than if only NH 3 were present. What is the mole fraction of H 2 in the reaction mixture? A

6

0.14

B

C

0.28

0.43

D

0.71

A 25.0 cm3 sample of 0.35 mol dm 3 HCOOH is titrated with 0.20 mol dm3 KOH. What is the pH of the solution after an equal volume of KOH has been added to the acid? Given: K a (HCOOH) = 1.77  104 mol dm –3. A

3.75.1/0.075


B

3.88pH

C

8.50pH

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D

11.74pH

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4 7

A diprotic acid H2 A has K a1 = 1  104 mol dm3 and K a2 = 1  108 mol dm3. The corresponding base A 2 is titrated with aqueous HC l , both solutions being 0.1 mol dm3. Which of the following diagrams best represents the titration curve? A

B

C

D

8

Reaction

Observation

Metal X is placed in a solution of metal ions Y2+.

X dissolves and Y2+ is displaced from the solution as Y.

Metal Y is placed in acid solution.

Gas bubbles form on the surface of Y.

Metal X is placed in a solution of metal ions Z2+.

No observable reaction. There is no observable reaction.

Which reaction would not occur spontaneously?

9

A

X(s)

+

2H+(aq)



H2(g)

+

X2+(aq)

B

Z(s)

+

X2+(aq)



X(s)

+

Z2+(aq)

C

Z(s)

+

2H+(aq)



H2(g)

+

Z +(aq)

D

Y(s)

+

Z2+(aq)



Z(s)

+

Y2+(aq)

An electric current of 4 A is passed for 200 minutes through molten aluminium oxide using inert electrodes. What will be the volume of gas liberated at s.t.p.? A

2.8 dm3

0.125 mol x 22.4

B

5.6 dm3

0.25 mol x 22.4

C

8.4 dm

(3/4)x0.5 mol x 22.4

D

11.2 dm3


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5 10

The sketch below shows a physical property of seven consecutive elements of Periods 2 and 3 in increasing proton number.

proton number Which physical property does this sketch depict?

11

A

melting point

B

atomic radius

C

electronegativity

D

electrical conductivity

T, U and V are oxides of Period 3 elements. The following table lists some properties of the oxides: melting point / oC

pH when mixed with water

T

920

13

U

1610

7

V

300

2

Which of the following represents the correct set of oxides? T

U

V

A

MgO

Al 2O3

P4O10

B

SiO2

Al 2O3

SO2

C

Al 2O3

MgO

SO2

D

Na2O

SiO2

P4O10


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6 12

13

Which compound will give a poor yield when synthesised directly from propene? A

1-bromopropane

B

propan-2-ol

C

1-bromopropan-2-ol

D

ethanoic acid

Halogenoalkane is a starting material for synthesising Grignard reagent, which is a useful intermediate in organic synthesis as shown below.

Which of the following is not a possible product of the following reaction?

A

B

C

D


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7 14

Compound X has the following structure:

Which of the following is formed when X is heated with iodine in alkaline solution?

15

A

B

C

D

LSD is a stimulant drug that is often used by the Hippies in the 1970s. Part of the structure of LSD is shown below.

Which of the following properties can be deduced about LSD from the partial structure shown above? A

It is insoluble in aqueous nitric acid.

B

It reacts with two moles of hydrogen bromide.

C

It reacts with ethanoyl chloride to give off pungent white fumes.

D

It gives off a fishy smell when heated with aqueous hydrochloric acid.


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8 For each of the following questions, one or more of the three numbered statements 1 to 3 may be correct. Decide whether each of the statements is or is not correct (you may find it helpful to put a tick against the statements that you consider to be correct). The responses A to D should be selected on the basis of A

B

C

D

1, 2 and 3 are correct

1 and 2 only are correct


1 only is correct

No other combination of statements is used as a correct response. 16

17

Which of the following statements regarding buffer solutions are correct? 1

A CH3COOH / CH3COO buffer can be produced by adding a strong acid to a solution of CH3COO ions.

2

Within its effective buffering range, increasing the concentrations of the components of a buffer solution will increase its buffering capacity.

3

If the pH of an acidic buffer solution is greater than the p K a value of the acid, it will have a greater capacity to neutralise added base than added acid.

Which statements are correct for a cell set up under standard conditions using the half-cells below? Co2+(aq) / Co(s) Sn2+(aq) / Sn(s)

18

E  = –0.28 V E  = –0.14 V

1

Electrons flow in the external circuit from Co electrode to Sn electrode.

2

The concentration of Co 2+(aq) will increase.

3

Reduction occurs at the Sn electrode.

Element Z forms a chloride ZCl n. When 0.500 g of the chloride reacts with excess silver nitrate, 1.72 g of AgC l are formed. Another 0.500 g sample of the chloride is heated strongly. When the residue is treated with an excess silver nitrate solution, only 1.57 g of AgC l is precipitated. Which of the following statements about the element are correct? 1

Z burns in oxygen with a yellow flame.

2

The oxidation state of Z in the residue is +3.

3

Z is in Period 3.


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9 The responses A to D should be selected on the basis of A

B

C

D




1 only is correct

No other combination of statements is used as a correct response.

19

Which of the following naturally-occurring molecules have all of the features listed below?  At least five chiral carbons.  At least one secondary alcohol.  Either a carboxylic acid or a carboxylic acid derivative. 1

Thromoxane (TXA2) 2

Queuosine 3

Cortisol


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10 The responses A to D should be selected on the basis of A

B

C

D




1 only is correct

No other combination of statements is used as a correct response.

20

Citric acid, which causes the sharp taste of lemon juice, has the following structure.

Which of the following reacts in a stoichiometric ratio with 1 mol of citric acid? 1

1.5 mol of Mg(s)

2

3 mol of Na2CO3(s)

3

4 mol of HCl (g)


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11 RAFFLES INSTITUTION 2013 Year 6 Term 3 Common Test H2 Chemistry COVER SHEET

Name:

(

)

CT Group:

For Examiner ’s Use Only Marks

Section

A

/ 20

Q1

/ 10

Q2

/8

Q3

/6

Q4

/ 11

Q1

/ 20

Q2

/ 20

B

C

Penalty (Circle)

0

Total

/

1 / 95

Percentage

/ 100

Grade Penalty 

Incorrect number of significant figures.



Missing OR wrong units.



Answers written in pencil.



Did not start question in Section C on a fresh page.


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12 For Examiner’s

Section B (35 marks)

Use

This section consists of four questions. Answer all the questions in this section in the spaces provided. B1

Planning (P) The Matsutake mushroom is a delicacy added to many Japanese foods. One of the components isolated from these mushrooms is methyl cinnamate which gives these mushrooms a rich and spicy aroma. Methyl cinnamate can be prepared from cinnamic acid and methanol. O

O OH

cinnamic acid compound molar mass / g mol –1 boiling point / oC melting point / oC (a)

+

O

CH3OH

methanol cinnamic acid 148 300 133

+

H2O

methyl cinnamate methanol 32 65 –98

methyl cinnamate 162 261 38

10.8 g of cinnamic acid was reacted with 30 cm 3 of methanol, which was in excess, together with about 8 to 10 drops of concentrated sulfuric acid. The volume of methanol used was about 10 times more than the actual volume needed as a reactant. The addition of concentrated sulfuric acid to the mixture was highly exothermic. The reaction mixture was then heated under reflux for about five hours before the unreacted methanol was recovered by distillation. (i)

Write a plan for the preparation of methyl cinnamate. In your plan, you should:  give a step-by-step description of the procedures you would use to prepare methyl cinnamate, and  state the capacities of the apparatus used.

………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ……………………………………………………………………………………………………………………….


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13 For Examiner’s Use

………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. (ii)

Draw a labelled diagram of the assembled apparatus to show how you would recover the remaining methanol from the reaction mixture. Indicate on your diagram the reading on the thermometer.

(iii)

The excess methanol was used to drive the equilibrium position to the right. Suggest another reason why methanol was used in excess for this experiment.

………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. [6] Raffles Institution 2013

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14 For Examiner’s

(b)

The methyl cinnamate was purified according to the following procedure:

Use

1. Transfer the reaction mixture containing methyl cinnamate and unreacted cinnamic acid to a separatory funnel. 2. Add 50 cm3 of ethoxyethane, CH3CH2OCH2CH3, to the separatory funnel. 3. Add 50 cm3 of saturated sodium hydrogencarbonate solution to the separatory funnel. 4. Shake the mixture and discard the lower aqueous layer. stopper separatory funnel ethoxyethane layer sodium hydrogencarbonate layer tap

5. Repeat Steps 3 and 4 until no more effervescence in the aqueous layer is observed. Suggest why the mixture must be washed with saturated sodium hydrogencarbonate solution until no more effervescence was observed.

…………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… [1] (c)

The ethoxyethane layer was collected and dried with anhydrous calcium chloride. The ethoxyethane was then evaporated to recover the crude methyl cinnamate, which was subsequently purified by recrystallisation and air-dried. Two possible solvents, A and B can be used for the recrystallisation of methyl cinnamate. The table below gives data about the solubility of methyl cinnamate in solvents A and B.

o

Temperature / C 10 70

Solubility of methyl cinnamate /g per 100 g of solvent Solvent A Solvent B 6.0 2.0 11.0 9.5

During recrystallisation, 50 g of one of these solvents was saturated with the methyl cinnamate at 70 oC and the solution was then cooled at 10 oC.


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15 For Examiner’s

(i)

Explain why the use of solvent A, rather than solvent B, would lead to a lower yield of recrystallised methyl cinnamate.

………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. (ii)

Calculate the final mass of methyl cinnamate that would be obtained if solvent B was used.

[2] (d)

With reference to relevant data on page 12, explain how you would check the purity of your product.

…………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………… [1] [Total: 10]


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16

For Examiner’s

B2

(a)

Iron ore is a mixture of iron( II) and iron( III) oxides. The ore goes through a series of redox reactions in the blast furnace to extract iron. Iron(II) oxide can be reduced by carbon or carbon monoxide as shown below. Reaction

∆H r  / kJ mol –1

∆Gr  / kJ mol –1

+161

+114

–11.01

–5.64

(1) FeO + C  CO + Fe (2) FeO + CO  CO2 + Fe (i)

Given the above data, calculate the standard entropy change of reaction, ∆Sr , of reactions (1) and (2).

(ii)

Comment and explain the difference in the values obtained in (a)(i).

………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. ………………………………………………………………………………………………………………………. Raffles Institution 2013

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17 For Examiner’s

(iii)

During the extraction of iron, the interior of the blast furnace has different temperatures as shown in the diagram below. This results in different reducing agents reacting with the iron ore in different regions. Using the information on page 16, deduce which reactions, (1) or (2), will take place at 700 oC and 1500 oC respectively. Write down the reaction number in the boxes below. iron ore and limestone

hot air out

Reaction ____ Reaction ____

o

700 C o

1500 C o

1900 C

hot air in slag

hot air in iron

[5] (b)

Iron(III) oxide, Fe2O3, is also reduced by carbon monoxide in the blast furnace. Using the following data, draw an energy cycle and calculate the standard enthalpy change of reaction for the reduction of iron( III) oxide by carbon monoxide to extract iron. standard enthalpy change of formation of iron( III) oxide = –824 kJ mol –1 standard enthalpy change of combustion of carbon monoxide = –283 kJ mol –1

[3] [Total: 8] Raffles Institution 2013

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18

For Examiner’s

B3

Selective precipitation is a technique in which one ion is selectively removed from a mixture of ions by precipitation. The numerical values of the solubility product of the sparingly soluble salts in water at 25 oC are given as follows: K sp of AgCl = 1.77 x 1010 ; K sp of Ag2CrO4 = 1.12 x 1012 A solution of AgNO3 is added slowly and continuously to a solution containing C l (aq) and CrO42(aq) of concentration 0.010 mol dm 3 each. It is assumed that the addition of AgNO 3 solution does not change the overall solution volume to any significant degree. (a)

Show that AgCl precipitates before Ag 2CrO4.

(b)

[2] Calculate the concentration of C l when the first trace of Ag 2CrO4 precipitate appears. 

[1] Raffles Institution 2013

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19 For Examiner’s

(c)

After the AgCl precipitate is removed and more aqueous AgNO 3 is added to the remaining solution, a suspension of reddish brown Ag 2CrO4 is obtained. With the aid of equations, describe and explain what happens when dilute nitric acid is added to this suspension of Ag 2CrO4. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. [3] [Total: 6]

B4

Use of the Data Booklet is relevant to this question. (a) The standard electrode potential of bromate(V) ions in acidic medium is represented by the equation: 2BrO3 –(aq) + 12H+(aq) + 10e – (i)

Br 2(aq) + 6H2O(l )

E  = +1.52 V

Draw a fully labelled diagram to show how the above standard electrode potential may be determined.


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20 For Examiner’s

(ii)



Use E data to predict and explain what might happen when a solution of acidified bromate( V) ions is added separately to the following reagents in  and . Write balanced equations for any reactions that occur. 

Excess aqueous potassium iodide

…………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….…..



Acidified potassium dichromate(VI)

…………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. [6] Raffles Institution 2013

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21 For Examiner’s

(b)

The lead –acid accumulator is a storage battery because it can be recharged. It consists of a series of cells, and each cell contains an electrolyte of sulfuric acid as well as two electrodes, one made of lead and the other, lead dioxide. The relevant reduction potentials are given below. PbSO4(s) + 2e –

Pb(s) + SO42(aq)

PbO2(s) + 4H+(aq) + SO42(aq) + 2e –

E = 0.13 V PbSO4(s) + 2H2O(l ) E  = +1.47 V

(i)

Write the overall equation for the reaction upon discharge, and calculate the voltage delivered by the battery if it contains a series of 6 cells.

(ii)

State the polarity (i.e. ‘+’ or ‘–’) of the respective electrodes by filling up the box below. electrode Pb PbO2

(iii)

polarity

Suggest a reason why the battery is rechargeable. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….…..

(iv)

Explain why the measurement of the electrolyte density can be used to indicate the extent of discharge of the cell. …………………………………………………………………………….….. …………………………………………………………………………….….. …………………………………………………………………………….….. [5] [Total: 11]


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22

Section C (40 marks) This section consists of two questions. Answer all questions from this section. Begin each question on a fresh page of paper . C1

This question is mainly about organic compounds that are found in some animals and bacteria. (a)

Mandelonitrile is synthesised and stored in special glands by some species of millipedes such as Apheloria corrigata and Pachydesmus crassicutis. When the millipede is attacked, the stored mandelonitrile undergoes an enzyme-catalysed reaction to yield a mixture of toxic hydrogen cyanide and benzaldehye, which is ejected to ward off predators. CN

O OH

enzyme

mandelonitrile

(i)

H

benzaldehyde

+

HCN

hydrogen cyanide

The reverse of the above reaction can be carried out in the laboratory using a trace amount of aqueous potassium cyanide. Describe the mechanism of this reverse reaction.

Under suitable conditions, mandelonitrile can be converted into compounds W and X as shown in the reaction scheme below. NH2

OH

W

Cl

O

CN OH

mandelonitrile

Cl

mandelic acid

X

(ii)

With reference to the different hybridisation states of the nitrogen atoms in the –CN and –NH2 groups, explain why mandelonitrile, unlike compound W, is effectively neutral and not basic.

(iii)

When one mole of compound X is treated with one mole of dimethylamine, (CH3)2NH, a neutral organic product Y is obtained. Suggest the structure for Y and explain the different reactivities of the two chlorine atoms in X.

(iv)

In an experiment, 0.20 mol of compound X was added to 1 dm 3 of 0.10 mol dm –3 sodium hydroxide solution. Calculate the approximate pH of the resultant solution formed, with reasoning shown. [10]


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23 (b)

Benzaldehyde, present in the toxic secretion of the Apheloria corrigata millipedes, is known to be repellent to ants. When treated with concentrated sodium hydroxide, benzaldehyde undergoes the following reaction first studied by Stanislao Cannizzaro in 1853: O





concentrated NaOH

H

OH

O



O-Na+

+

(i)

Determine the oxidation number of each of the labelled carbon atoms, ,  and , in the three compounds and hence, or otherwise, state the type of reaction which occurs.

(ii)

Using benzaldehyde as the reactant, the mechanism for Cannizzaro reaction is shown below: -

O

O

O -

-

-

:OH

H

H

OH

H -

+ H2O

O

OH

-

-

:O

+

H

O

O

O

-

O

H

+

-

O

O

O -

O

-

OH

O H2O

-

O

+

+



Deuterium, D, is the  isotope of hydrogen. With the aid of the above mechanism, suggest the deuteriumcontaining organic product(s) that would be obtained for each of the following reactions: (1)

O H

(2)

O D


concentrated NaOD, D2O

concentrated NaOH, H 2O

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24 (iii)

Suggest suitable reagents and conditions to carry out the following 3-step synthesis in which one of the steps must be the Cannizzaro reaction. Show all intermediate products f ormed. O H H O

O O

[7] (c)

Compound L is produced by bacterial species such as Bifidobacterium and Streptococcus. In the laboratory, it can be obtained from a six-carbon compound K. K does not react with sodium nor phenylhydrazine. Upon analysis, it is found to contain 5.6% H and 44.4% O by mass. When K is refluxed with aqueous sodium hydroxide and the resultant mixture distilled, the distillate does not contain any organic compound. Acidification of the residual mixture left from the distillation yields L as the only organic compound. Warming 1 mole of L with alkaline aqueous iodine produces 1 mole of ethanedioate ions and 1 mole of a yellow precipitate. (i)

Determine the molecular formula of K.

(ii)

Suggest a suitable structural formula each for K and L. [3] [Total: 20]


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25 C2

Magainins are a class of proteins isolated from the African Clawed Frog, Xenopus laevis, found to exhibit potent broad-spectrum antimicrobial activity, and are evaluated for use as drugs f or treating diseases. A major disadvantage of using peptide-based drugs is that their properties may be lost in the body of the patient due to enzymatic hydrolysis. This may be overcome by modifying the structure of the amino acids used. Typical proteins consist of -amino acids, so named because the amino group is on the -carbon atom, next to the carboxylic acid group.



An -amino acid, where R is the side group. A magainin peptide may be modified using -amino acids. A  -amino acid has the amino group on the -carbon atom, which is the second carbon atom from the acid group.



A -amino acid, where R is the side group. (a)

Draw the structure of a tripeptide that consists of all three of the following -amino acids: R = –CH2CH(CH3)2 -leucine: R = –CH2CH2CH2CH2NH2 -lysine: R = –CH2CH2COOH -glutamic acid: [1]

(b)

Explain how a modified peptide synthesised from -amino acids may be resistant to enzymatic hydrolysis in the body. [1]

(c)

A magainin protein can be modified by replacing some of its -amino acids with the corresponding -amino acids, with the same R groups. Explain why such a modified protein exhibits similar biological properties. [2]

(d)

Describe with an aid of a diagram, the structure of the alpha helix. [3]

(e)

Generally, helix structures of peptides synthesised from  -amino acids are more stable than the helix structure of a typical peptide of -amino acids. Suggest a reason for this observation. [1]


9647/Y6/T3/13

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26 (f)

Aspartic acid is one of the amino acids found in a magainin peptide. Suggest a possible synthesis scheme of aspartic acid from ethene.

[6] (g)

The pK a values of aspartic acid are listed below: pK 1 = 1.95 pK 2 = 3.71 pK 3 = 9.66 (i)

Sketch the titration curve when a solution containing the following species A+ is titrated with aqueous sodium hydroxide.

A+

(h)

(ii)

Draw the structure of the most abundant species present when the pH of an aspartic acid solution is adjusted to 2.83.

(iii)

Describe the movement of the species from part (ii) in an electric field. [4]

Suggest a simple chemical test to distinguish the following two compounds. X: CH3CH2CH2CONH2

and

Y: CH3CH2CH2COO –NH4+ [2] [Total: 20]

   END OF PAPER   


9647/Y6/T3/13

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27 RAFFLES INSTITUTION 2013 Year 6 Term 3 Common Test H2 Chemistry COVER SHEET

Name:

(

)

C

CT Group:

For Examiner’s Use Only

Marks

Section

C1a – b

/ 17

C1c

/ 3

C2

/ 20

Penalty 

Incorrect number of significant figures.



Missing OR wrong units.



Answers written in pencil.



Did not start question in Section C on a fresh page.


9647/Y6/T3/13

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2013 Y6 H2 Chem QP

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