1 - Ms Brown's Chemistry Page
AP Chemistry Name _________________________________
Period ___ Date ___/___/___
16 ( Entropy and Free Energy
PRACTICE TEST
1. Which of the following represents an increase in entropy?
a) freezing of water l(s
b) boiling of water l ( g
c) crystallization of salt from a supersaturated solution aq ( s
d) the reaction 2 NO(g) ( N2O2(g) fewer gas molecules on product side
e) the reaction 2 H2(g) + O2(g) ( 2 H2O(g) fewer gas molecules on product side
2. The enthalpy of vaporization of methanol (CH3OH) is 35.3 kJ/mol at the boiling point of 64.2 (C. Calculate the entropy change for methanol going from a liquid to vapor.
a) 600. J/K·mol d) -105 J/K·mol
b) 551 J/K·mol e) -551 J/K·mol
c) 105 J/K·mol
at the b.p. ΔG = 0, at equilibrium , so……..
35.3 kJ mol -1/337.2K = 0.105 kJ mol-1K-1 =
105 J mol-1K-1
3. Calculate the standard entropy change for the following reaction,
Cu(s) + ½ O2(g) ( CuO(s)
given that
|S([Cu(s)] = 33.15 J/K·mol |
|S([O2(g)] = 205.14 J/K·mol |
|S([CuO(s)] = 42.63 J/K·mol |
a) 195.66 J/K d) -93.09 J/K
b) 93.09 J/K e) 195.66 J/K
c) -45.28 J/K
Prod – react
[42.63]-[(33.15)+1/2(205.14)] = -93.09
4. In which of the following reactions do you expect to have a decrease in entropy?
a) Fe(s) ( Fe(l) increase
b) Fe(s) + S(s) ( FeS(s) increase
c) 2 Fe(s) + 3/2 O2(g) ( Fe2O3(s)
d) HF(l) ( HF(g) increase
e) 2 H2O2(l) ( 2 H2O(l) + O2(g) increase
5. The formation ½ A2 + 2 B2 + C ( CAB4 has an enthalpy of formation of -104 kJ and a change in entropy of -60.8 J/K at 30 (C. What is (G and spontaneity of the reaction?
a) -85.6 kJ, spontaneous
b) -18.3 kJ, not spontaneous
c) +18.3 kJ, spontaneous
d) +85.6 kJ, not spontaneous
e) -85.6 kJ, not spontaneous
Use ΔG = ΔH-TΔS and don’t forget to convert ΔS to kJ
6. If (H and (S are both negative or positive, then (G has a ___________ sign.
a) positive d) large
b) negative e) no
c) variable
If both ΔS and ΔH are the same sign then sign of ΔG will be temperature dependent
7. At what temperature would a given reaction become spontaneous if (H = +119 kJ and
(S = +263 J/K?
a) 452 K d) 2.21 K
b) 2210 K e) 363 K
c) 382 K
Set ΔG =0 and solve for T, don’t forget to convert ΔS to kJ
8. The free energy change for a given reaction is -36.2 kJ. What is the equilibrium constant at 298 K?
a) 0.985 d) 8.32 x 10-7
b) 2.22 x 106 e) 3.25 x 106
c) 1.01
Use ΔG = -RTlnK
9. Given the following information, calculate (G( for the reaction below at 25(C:
SnCl4(l) + 2 H2O(l) ( SnO2(s) + 4 HCl(g)
(H( = 133.0 kJ and (S( = 401.5 J/K
a) -252.6 kJ d) 122.9 kJ
b) -13.4 kJ e) 252.6 kJ
c) 13.4 kJ
Use ΔG = ΔH-TΔS and don’t forget to convert ΔS to kJ
10. Given the following information, calculate (G( for the reaction below at 25(C:
2 H2O2(l) ( 2 H2O(l) + O2(g)
|Compound |(H((kJ/mol) |S((J/K·mol) |
|H2O2(l) |-187.8 |109.6 |
|H2O(l) |-285.8 |69.9 |
|O2(g) |(( |205.1 |
a) -37700 kJ d) -233.5 kJ
b) -342.6 kJ e) -157.9 kJ
c) -233.5 kJ
Use Hess’s Law to calculate ΔH & ΔS
ΔH = ΣΔHprod – ΣΔHreact same for ΔS. Use ΔG = ΔH-TΔS and don’t forget to convert ΔS to kJ
11. For the process at 25(C
I2(g) ( I2(s)
what are the signs of (G, (H, and (S?
| |(G |(H |(S |
|a) |+ |− |− |
|b) |− |− |− |
|c) |− |+ |+ |
|d) |− |− |+ |
|e) |+ |+ |+ |
Because I2 is a solid at 25oC, going from (g) ( (s) will be spontaneous
12. If a process is exothermic and not spontaneous, then what must be true?
a) (S > 0 d) (S < 0
b) (H > 0 e) (H = 0
c) (G = 0
If not spontaneous, ΔS must drive rxn backward bc ΔH is exothermic which drives rxn forward, so ΔS is negitive
13. For any reaction at equilibrium, which of the following is true?
a) (H < 0 d) (H = 0
b) (S = 0 e) (G = 0
c) (S < 0
14. All of the following have (G(f = 0 EXCEPT
a) O2(g) d) Ca(s)
b) Br2(g) e) Hg(l)
c) H2(g)
bc Br2 is a liquid at 25oC, energy is needed to form Br2 gas.
Answers:
1B 2C 3D 4C 5A 6C 7A 8B 9C 10C 11B 12D 13E 14B 15B
15. Ammonium nitrate spontaneously dissolves in water at room temperature and the process causes the solution to become quite cold. Which of the following is TRUE about the dissolution of ammonium nitrate?
a) The process is exothermic. untrue
b) Its solubility will be greater in warmer water. T increases, to much heat, shift right and become more soluble
c) (S( for the reaction is negative.
d) All solutions of ammonium nitrate are supersaturated.
e) All solutions of ammonium nitrate are cold. They can warm up
Answers:
|1. | |6. | |11. | |
|2. | |7. | |12. | |
|3. | |8. | |13. | |
|4. | |9. | |14. | |
|5. | |10. | |15. | |
From the AP Exam formula sheet:
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