2011
SKU 3023: Chemistry II
Edited by: Faridah Yusoff Ramli Hitam Norlinda Daud
SKU 3023: Chemistry II
SAFETY AND RULES IN THE CHEMISTRY LABORATORY 1.
Do not enter the laboratory without the presence of the laboratory instructor.
2. No experiment to be performed without the permission of the laboratory instructor. 3. Safety glasses must be worn at all the times in the laboratory. 4.
Laboratory coat must be worn at all the times in the laboratory.
5.
Long hair and scarf must be properly manageable.
6.
Do not wear slippers, sandals, heels or sides open shoes.
7.
Do not wear contact lenses.
8.
No smoking, drinking and eating at all the times in the laboratory.
9.
Do not throw waste organic liquids into the sink.
10. Know the location of all safety equipment. 11. Read the label on the container of a chemical twice. 12. Use a fume hood for poisonous or irritating fumes. 13. Throw all unused or contaminated chemical properly. 14. Do not return use reagent to the stock bottle. 15. Do not send your product with your report. 16. Evacuate the laboratory when a fire alarm sounds. 17. Turn off the flame and switch off the hot plate before leaving the laboratory. 18. Always practice goods housekeeping. 19. Always add acids to water. 20. Do not aim the opening of any glassware at yourself or anyone else. 21. Do not use cracked or chipped glassware.
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SKU 3023: Chemistry II 22. Never use mouth suction when using a pipette. 23. Report any accident in the laboratory. 24. Do not make fun and joke in the laboratory. 25. Do not leave any heating, vigorous or rapid reaction unattended.
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SKU 3023: Chemistry II EXPERIMENT 1: 1.1:
ENTHALPY
OBJECTIVES
1. To determine the enthalpy of neutralization of strong acid and weak acid. 2. To determine the quantity and direction of the heat transfer in the dilution of a salt.
1.2:
INTRODUCTION
Thermochemistry deals with the heat involved in the chemical and physical change and especially with the concepts of enthalpy. Enthalpy is a thermodynamic variable for reactions at constant pressure. The enthalpy of a system is defined as the internal energy plus the product of the pressure and volume.
The change in the enthalpy ( only on the difference between of reaction,
) equals the heat gained or lost at constant pressure. It depends
and.
, always refers to
The enthalpy change of reaction also called the heat . An exothermic process releases heat and results
in a decrease in the enthalpy of the system. An endothermic process absorbs heat and results in an increase in the enthalpy of the system.
J
Heat capacity is the quantity of heat required to change its temperature by 1 with the unit . Specific heat capacity, s, is the quantity of heat required to change the temperature of 1g of a
substance by 1 . Specific heat =
A calorimeter is a device used to measure the heat released or absorbed by the physical or chemical process. The coffee-cup calorimeter has been used in this experiment to measure the heat at constant pressure. The enthalpy of neutralization involves acid base reaction and for strong acid and base, the reaction is exothermic. Hn is determined by Hn
= – specific heat H2O x total mass acid-base x
T
Salt dispersion enthalpy, HS is determined experimentally with the combination of heat loss from the salt and water whenever both of them are mixed. Hs
Faridah Yusoff
= ( –heat loss H2O ) + ( –heat loss salt) 4
SKU 3023: Chemistry II 1.3
METHODS
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SKU 3023: Chemistry II 1.4:
RESULTS AND DATA
A:
Table of Data to Plot Graph
Enthalpy (heat) of solution for
Enthalpy (heat) of neutralization for acid-base reaction HCl – NaOH Trial 1
the salt
HNO3 – NaOH
Trial 2
Trial 1
Trial 1
Trial 2
Trial 2
t
T
t
T
t
T
t
T
t
T
t
T
(sec)
(°C)
(sec)
(°C)
(sec)
(°C)
(sec)
(°C)
(sec)
(°C)
(sec)
(°C)
0
31
0
29
0
30
0
30
0
23
0
23
20
31
20
29
20
30
20
30
20
23
20
23
40
31
40
29
40
29
40
30
40
23
40
23
60
31
60
29
60
29
60
30
60
23
60
23
80
31
80
29
80
29
80
30
80
23
80
23
100
31
100
29
100
29
100
30
100
23
100
23
120
31
120
29
120
29
120
30
120
23
120
23
140
31
140
29
140
29
140
30
140
23
140
23
160
31
160
29
160
29
160
30
160
23
160
23
180
30
180
29
180
29
180
30
180
23
180
23
200
30
200
28
200
29
200
29
200
23
200
23
220
30
220
28
220
29
220
29
220
23
220
23
240
30
240
28
240
29
240
29
240
23
240
23
260
30
260
28
260
29
260
29
260
23
260
23
280
30
280
28
280
29
280
29
280
23
280
23
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SKU 3023: Chemistry II B:
Heat of Neutralization HCl + NaOH
HNO 3 + NaOH
Item Trial 1
Trial 2
Trial 1
Trial 2
1. Acid volume (mL)
50.0
50.0
50.0
50.0
2. Acid temperature (°C)
23.0
23.0
24.0
24.0
3. NaOH volume (mL)
50.0
50.0
50.0
50.0
4. NaOH temperature (°C)
23.0
23.0
24.0
24.0
5. NaOH concentration (molL-1) 6. Maximum temperature from graph (°C)
Section C:
1.00
1.00
30
29
30
30
Calculation of the Heat of Neutralization
HCl + NaOH Item
HNO 3 + NaOH
Trial 1
Trial 2
Trial 1
1. Average initial temperature of acid and base (°C)
23.0
23.0
24.0
24.0
2. Temperature change, T (°C)
6.6
5.7
5.1
5.7
3. Volume of final mixture (mL)
100
100
100
100
4. Mass of final mixture (g)
100
100
100
100
5. Specific heat of the mixture 6. Yielded heat (J)
4.18 Jg-1oC
Trial 2
4.18 Jg-1oC
-2758.8
-2382.6
-2131.8
-2382.6
7. Amount of OH- reacted (g)
0.85
0.85
0.85
0.85
8. Amount of H2O produced (g)
0.90
0.90
0.90
0.90
-55.176
-47.652
-42.636
-47.652
9. Yielded heat per mole H2O, Hn (kJmol-1 H2O) 10. Average Hn (kJmol-1 H2O)
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-51.414
-45.144
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SKU 3023: Chemistry II Section D:
Heat of Salt Solution (e.g: NaCl, Na2SO4, KOH, Na 2S2O3)
Salt: NaCl
Item
Trial 1
Trial 2
1. Mass of salt (g)
5.023
5.008
2. Mole of salt (mole)
0.0859
0.0857
3. mass of cup and water (g)
21.378
21.116
4. Mass of Styrofoam cup (g)
2.126
2.182
5. Mass of water (g)
19.252
18.934
6. Initial temperature of water (°C)
25.0
25.0
7. Initial temperature of water (°C) from graph
23.0
23.0
Trial 1
Trial 2
Section E:
Calculation for Heat of Salt Solution
Item
2.0
2.0
160.945
158.288
3. Salt heat released (J)
8.680
8.654
4. Total enthalpy change
-169.625
-166.942
5. Amount of OH- reacted (g)
1.4603
1.4569
6. Amount of H2O produced (g)
1.5462
1.5426
7. HS (kJ/mol salt)
-0.251
-0.251
1. Temperature change, T (°C) 2. Water heat released (J)
8. Average HS (kJ/mol salt)
Faridah Yusoff
-0.251
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SKU 3023: Chemistry II 1.5:
QUESTIONS
1.
Show your calculation for Trial 1 (in sec. C)
HCl + NaOH i)
Average initial temperature of acid and base for HCl + NaOH: Average initial =
= 23
Average final temperature of acid and base for HCl + NaOH: Average final = 29.6 ii)
Temperature change,
= 29.6
- 23.0
= 6.6
iii) Volume of final mixture = 100.0 mL iv) Mass of final mixture =100.0 g
v) Specific heat of mixture = 4.18 J vi) Yielded heat (J)
x total mass acid base x T
= - 4.18
x 100g x 6.6
=
vii) Amount of
reacted
+
1 mol
1 mol
x mol
0.05 mol
x = 0.05 mol
mass of
= 0.05 mol
= 0.85 g
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x 17.0 g/mol
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SKU 3023: Chemistry II
viii) Amount of
produced
+
1 mol
1 mol
0.05 mol
0.05 mol
mass of
= 0.05 mol
x 18.0 g/mol
= 0.9 g
ix) Yielded heat per mole
,
=
= -55176 J
= -55.176 kJ x) Average
Trial 1 + trial 2 = -55.176
+ -47.652
2
= -51.414
i)
+ NaOH Average initial temperatur e of acid and base for HCl + NaOH: Average initial =
= 24
Average final temperature of acid and base for HCl + NaOH: Average final = 29.1 ii)
Temperature change,
= 29.1
- 24.0
= 5.1
iii) Volume of final mixture = 100.0 mL iv) Mass of final mixture =100.0 g
v) Specific heat of mixture = 4.18 J vi) Yielded heat (J)
x total mass acid base x T
= - 4.18
x 100g x 5.1
=
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SKU 3023: Chemistry II
vii) Amount of
reacted
+
1 mol
1 mol
x mol
0.05 mol
x = 0.05 mol
mass of
= 0.05 mol
x 17.0 g/mol
= 0.85 g
viii) Amount of
produced
+
1 mol
1 mol
0.05 mol
0.05 mol
mass of
= 0.05 mol
x 18.0 g/mol
= 0.9 g
ix) Yielded heat per mole
,
=
= -42636 J
= -42.636 kJ x) Average
= -42.636
+ -47.652
2
= -45.144
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SKU 3023: Chemistry II 2.
Compare both Hn values and give your opinion.
The average Hn of the reaction between HCl and NaOH was -51.414 reaction of
strong
while the
and NaOH was -45.144. HCl yielded higher heat per mole water because it was
stronger acid compared to A
acid
like
. The acidity constant ( hydrochloric
) of HCl is higher. It ionizes more
acid,
(HCl)
is
completely
.
ionized.
A strong acid will have an extremely large value for Ka, whereas a weak acid will have a small value. A concentrated acid has a high initial value for [HA] and a dilute acid has a low initial [HA]. pH depends on [H+] which depends on concentration and acid strength.
3. Write down the balance equation for the neutralization.
4. Show your calculation for Trial 1 ( in sec. E)
i) Temperature change,
= final – initial
= 23
- 25
=-2 =2
ii) Water heat released
x total mass
= -4.18 J
x T
x 19.252 g x 2
= -160.945 J
iii) Salt heat released
x total mass
= -0.864 J
x 5.023 g x 2
x T
= -8.680 J
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SKU 3023: Chemistry II iv) Total enthalpy change
Enthalpy salt + enthalpy water = -8.68 J + -160.945 J = -169.625 J
v) Amount of
reacted
1 mol NaCL
1 mol NaOH
0.0859 mol NaCL
Mass
0.0859 mol NaOH
= mol
x molar mass
= 0.0859 mol x 17 g/mol = 1.4603 g
vi) Amount of
produced
Mass
= 0.0859 mol x 18 g/mol = 1.5462 g
Salt dispersion enthalpy,
vii)
( ) =
= -150.474 J + -101.05 = -251.523 J = -0.251 kJ viii)
average =
=
=
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SKU 3023: Chemistry II 5.
Instead of using specific heat, what other formula can you use to calculate the heat of reaction? Explain in what condition that formula can be applied. can use the molar heat capacity. It is the energy required to raise one mole of any substance by one degree Celsius. molar heat capacity = _ J_ _ mole oC A useful relationship is
J X
g = J g mole mol oC It can be used when the value of specific heat capacity was not given and there was no value of temperature differences, T. oC
6.
What is the appropriate name should be given to the Styrofoam cup as an apparatus to measure heat of reaction? Coffee-cup calorimeter
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SKU 3023: Chemistry II 1.6:
DISCUSSION
Experiment A This experiment is carried out to determine the enthalpy of neutralization of strong acid, hydrochloric acid HCl and weak acid,
. Both of the acids reacted with the strong base
sodium hydroxide, NaOH. Both resulted with different enthalpy of neutralization. Through this experiment, the strong acid, HCl produced higher enthalpy. This happened due to its ionization property. It ionized completely, thus higher energy required and more heat was produced.
Both reactions were exothermic reactions which release heat and resulted in the negative values of heat enthalpies.
of the HCl-NaOH reaction was -51.414 kJ/mol which is near to the theoretical value of
-57.0kJ/mol. Whereas the
-NaOH reaction came out with lower value of -45.144kJ/mol. The values
were obtained by the calculation using the formula of specific heat capacity. All the acids and base were assumed to have the same specific heat capacities as same as water, 4.18kJ/mol. All the equations of the reactions must be balanced first to get the correct ratio to each other compound.
Most of the trials have decrements on the temperature during the reaction (plotted on graph). This happened due to the surrounding temperature which is much lower than in the system experimented. Thus, it lowers the temperatures of the solutions through the period of time. Although the styrofoam cup has been used, the heat transfer between the system and the surrounding still happened due to the errors on the apparatus. The lid might not been covered completely.
Experiment B
Sodium chloride salt has been used in this experiment to react with water to produce acid and base. Both trials shows decrement of 2
of the temperature which means both reactions undergo exothermic process.
There was no much difference in the result for both trials which ended up with salt dispersion enthalpies of -0.251kJ/mol salt. Balanced equation must be written first to avoid mistake in determining mol and mass of any compound.
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SKU 3023: Chemistry II 1.7:
CONCLUSION(S)
All the reactions experimented undergo exothermic process. Thus, all of the reactions give negative values of enthalpy. Strong acid has higher enthalpy of neutralization compared to weak acid. All the acids and base were assumed to have same specific heat capacity as water, 4.18kJ/mol. Coffee cup is the apparatus used to measure the enthalpy of reaction.
1.8:
REFERENCE(S)
Silberberg M. S. (2009). Chemistry: the molecular nature of matter and change. New York: The McGraw Hill Companies, Inc. Brown, LeMay (2009). Chemistry: the central science. New Jersey, Pearson Education, Inc. Eng S. C. (2009). Chemistry for Matriculation 1. Kuala Lumpur: Oriental Academic Publication.
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