Chem 163 Ready for class? Thermidynamics
You might have to look up K’s when needed.
1. Define G, H , S.
2. Define ‘favorable condition’ and ‘spontaneous reaction’.
3. Consider melting of ice at 1.0 atm and 0°C. Predict whether each of the following quantities will be greater than, less than, or equal to, zero (i.e., > 0, < 0 or = 0). Explain each prediction in one sentence.
a. ΔH°
b. ΔS°
c. ΔG°
4. There are various ways to calculate ΔH of a reaction. Determine ΔH for the following reaction, 4 NH3(g) + 5 O2(g) → 4 NO(g) + 6 H2O(g)…..
(a) using ΔHf 0 (look up the values)..
(b) Using bond dissociation energy (look up the values)..
(c) Using Hess’s Law given..
N2(g) + O2(g) → 2 NO(g) ΔrH° = +180.8 kJ
N2(g) + 3 H2(g) → 2 NH3(g) ΔrH° = -91.8 kJ
2 H2(g) + O2(g) → 2 H2O(g) ΔrH° = -483.6 kJ
5. If a chemical reaction is endothermic and spontaneous, which of the following must be true?
a) ΔrG > 0, ΔrS > 0 and ΔrH > 0 b) ΔrG < 0, ΔrS > 0 and ΔrH > 0
c) ΔrG > 0, ΔrS < 0 and ΔrH > 0 d) ΔrG < 0, ΔrS < 0 and ΔrH < 0
e) ΔrG > 0, ΔrS > 0 and ΔrH < 0
6. The dissolution of ammonium nitrate occurs spontaneously in water at 25 °C. As NH4NO3 dissolves, the temperature of the water decreases. What are the signs of ΔrH, ΔrS, and ΔrG for this process?
a) ΔrH > 0, ΔrS < 0, ΔrG > 0 b) ΔrH > 0, ΔrS > 0, ΔrG > 0
c) ΔrH > 0, ΔrS > 0, ΔrG < 0 d) ΔrH < 0, ΔrS < 0, ΔrG < 0
e) ΔrH < 0, ΔrS > 0, ΔrG > 0
7. C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) Calculate the standard entropy change for the combustion of propane at 25 °C . Look up S0 of each compound.
8. For the following reaction at 298.2 K, 2 CO(g) + O2(g) → 2 CO2(g) has ΔS° = -173.1 J/K and ΔH° = -566.0 kJ. Calculate (a) ΔS°(surrounding) and (b) ΔS°(universe)
9. Calculate ΔrG° for the reaction at 25.0 °C, CS2(g) + 3 Cl2(g) → S2Cl2(g) + CCl4(g) given ΔrH° = -231.1 kJ and ΔrS° = -287.6 J/K.
10. At what temperature (or a range of T) will a reaction be spontaneous if ΔrH° = -12.7 kJ and ΔrS° = +135 J/K?
11. Given that C(s) + O2(g) → CO2(g) ΔrG° = -394.4 kJ
2CO(g) + O2(g) → 2 CO2(g) ΔrG° = -514.4 kJ
calculate ΔfG° of carbon monoxide, C(s) + 1/2 O2(g) → CO(g)
12. For a chemical system, ΔrG° and ΔrG are equal when
a) the system is in equilibrium. b) the reactants and products are in standard state conditions.
c) the equilibrium constant, K, equals 0. d) the reaction quotient, Q, is less than 1.
e) the reactants and products are in the gas phase.
13. Calculate ΔrG° for the reaction below at 398 K.
2 CH3OH(g) + 3 O2(g) → 2 CO2(g) + 4 H2O(g)
Species / ΔfH° (kJ/mol) / S° (J/K·mol)CH3OH(g) / -210.1 / 239.7
O2(g) / 0 / 205.1
CO2(g) / -393.5 / 213.7
H2O(g) / -241.8 / 188.8
14. The standard free energy change is -33.41 kJ/mol for the formation (ΔfG°) of AgNO3(s) from elements at 25 °C. What is the equilibrium constant for the reaction?
15. What is the equilibrium constant for reaction below at 25 °C? (R = 8.314 J/K·mol)
MgCO3(s) ßà MgO(s) + CO2(g)
given ΔfG° [MgCO3(s)] = -1028.2 kJ/mol, ΔfG° [MgO(s)] = -568.8 kJ/mol, and ΔfG° [CO2(g)] = -394.4 kJ/mol.
16. The equilibrium constant for a reaction at 25 °C is 3.1 ´ 108. What is ΔrG°?
17. Iron(III) oxide can be reduced by carbon monoxide. Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
Use the following thermodynamic data at 298 K to determine the equilibrium constant at this temperature.
Substance: Fe2O3(s) CO(g) Fe(s) CO2(g)
ΔH° f (kJ/mol): -824.2 -110.5 0 -393.5
ΔG° f (kJ/mol): -742.2 -137.2 0 -394.4
S°(J/K·mol): 87.4 197.7 27.78 213.7
18. Use the thermodynamic data at 298 K below to determine the Ksp for BaCO3 at this temperature.
Substance: Ba2+(aq) CO32-(aq) BaCO3(s)
ΔH° f (kJ/mol): -538.36 -676.26 -1219
ΔG° f (kJ/mol): -560.7 -528.1 -1139
S°(J/K·mol): 13 -53.1 112
19. Ag+(aq) + 2NH3(aq) Ag(NH3)2+(aq) Kf = 1.7 × 107 at 25°C.
What is ΔG° at this temperature?
20. Estimate the boiling point of carbon tetrachloride given the following thermodynamic parameters.
CCl4(l) / CCl4(g)ΔfH° (kJ/mol) / -128.4 / -96.0
S° ( J/K·mol) / 214.4 / 309.7
ΔfG° (kJ/mol) / -57.6 / -53.6