CMY 127
Self-Assessment Exercise 7
Subject:
Reference:
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Tutorials:
Exercises:
Theme 7: Thermodynamics
Textbook Chapter 19 Exercises p. 886 - 892
The answers to the odd-numbered questions are available in Appendix R of the textbook.
You should complete these exercises before 1 September.
The focus during the tutorial sessions which run from 2 to 12 September will be on this theme.
5
10
15
18
21
24
26
29
30
37
42
43
48
53
61
63
67
Answers Even-numbered questions:
10.
18.
24.
26.
30.
42.
48.
190.7 J/Kmol Not spontaneous
(a) 898.5 kJ (b) 105.3 kJ (c) 104.08 kJ
(a) 100.89 J/Kmol (favoured – Increase T  more product-favoured)
(b) 187.95 kJ/Kmol (disfavoured – Increase T  more reactant favoured)
(c) 315.5 J/Kmol (favoured)
(d) 15.7 J/Kmol (favoured)
(a) 121.5 kJ/mol (non-spontaneous at 298 K) (b) Yes
83.28 kJ & 4.0  1014 (product favoured)
69C
128 mm Hg
Additional Exercises
1. Aluminium metal reacts with iron(III) oxide to form iron metal and alumina:
Fe2o3(S) + 2Aℓ(s)  2Fe(s) + Aℓ2O3(s)
Show that this reaction is exothermic and spontaneous at 1000 K.
[∆
812.68 J/K]
2. Boron chloride is reduced as follows with hydrogen gas:
Given: ∆ °
5.834 / ∙
2BCl3(g) + 3H2(g) → 2B(s) + 6HCl(g)
a. Determine whether this reaction takes place spontaneously at 25°C.
[∆ °
205.32
b. Determine the temperature (in ) at which the direction of spontaneity of this reaction will change.
Hint: assume that the entropy and enthalpy changes remain constant with temperature change.
[1600 K]
3. Ammonium nitrate is dangerous because it can decomposes as follows:
NH4NO3(s)  N2O(g) + 2H2O(g)
a. Calculate the quantity of energy as heat involved when 150.g of ammonium nitrate decomposes at constant pressure.
[67.54 kJ]
b. Show with calculations that this decomposition takes place spontaneously at 50.0 .
[∆
180. /
4. a. Determine the value of the change in the entropy for the following reaction at 298 K:
2SO2(g) + O2(g)  2SO3(g)
H = –198 kJ
[187 J/K]
b. Is the sign of your answer in (a) what you expected? Briefly motivate your answer.
5. Is the reaction Si(s) + 2Cℓ2(g)  SiCℓ4(g) spontaneous under standard conditions at 298 K? Answer this question by calculating
∆ ° (system), ∆ °(surroundings), and ∆ °(universe). (Define reactants and products as the system.)
[∆
2089.9 J/K]
6. Using the given data and Hess’ law, determine the standard Gibbs energy change for the following reaction:
C(s) + 2H2 (g) → CH4 (g)
∆ ° (kJ/mol)
-237.129
H2 (g) + ½O2 (g) → H2O(ℓ)
-394.359
C(s) + O2 (g) → CO2 (g)
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O(ℓ)
-237.129
[-631.488 kJ/mol]
7.
8.
In a cave in Mexico, hydrogen sulphide (H2S) reacts with oxygen to give sulphuric acid, which drips from the ceiling with a pH of
less than 1. Assume the reaction occurring at 25oC is: H2S(g) + 2O2(g) → H2SO4(ℓ)
o
o
o
o
Calculate: ∆rH , ∆rS and ∆rG . Is the reaction product favoured? Is it enthalpy or entropy driven? (Note: ∆fG values may not be
used in your calculations)
[657 kJ/mol; enthalpy driven]
NO(g) is formed from its elements: ½ N2(g) + ½ O2(g) → NO(g)
o
o
Calculate (a) ∆rG for the reaction at 25 C and (b) Use your answer in (a) to calculate Kp. (c) Comment on the connection of the
o
[86.58 kJ/mol; 6.671016]
sign of ∆rG and the magnitude of Kp.
©University of Pretoria
CMY 127
2014