A summary practice problem set on chemical thermodynamics to cover examples of determining whether the process is spontaneous or not, calculating the entropy of the reaction, ΔS, the entropy change associated with phase transitions, Gibbs free energy, ΔG of reaction, spontaneity based on the free energy, and etc.
The links to the corresponding topics can be found here:
- Standard Entropy Change (𝚫Sorxn) of a Reaction
- The Gibbs Free Energy
- The Effect of 𝚫H, 𝚫S, and T on 𝚫G – Spontaneity
- Entropy and State Change
- Entropy Changes in the Surroundings
- 𝚫Gorxn from the Free Energies of Formation
- Gibbs Free Energy and Hess’s Law
- Gibbs Free Energy Under Nonstandard Conditions
- Gibbs Free Energy and Equilibrium Constant
Entropy, Spontaneous and Nonspontaneous Processes
Which of the following are spontaneous processes?
a) A grain of salt dissolves in water.
b) A rusty knife turns shiny.
c) Gas burning in a stove.
d) A cup of sparkling water loses the CO2.
e) Water changing color after adding a food dye.
f) A wall of bricks is built.
Predict the sign of ΔSo, if possible, for each of the following reactions.
a) 2K(s) + Cl2(g) → 2KCl(s)
b) 2NO(g) + O2(g) → 2NO2(g)
c) CO(g) + H2O(g) → CH3OH(g)
d) N2(g) + 2O2(g) → 2NO2(g)
e) 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)
f) 2C2H2(g) + 3O2(g) → 4CO(g) + 2H2O(g)
g) C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l)
h) 2Al(s) + 3H2O(l) → Al2O3(s)+ 3H2(g)
ΔS for Phase Change
Determine the change in entropy, ΔS that occurs in the system when 1.00 mole of benzene (C6H6) melts at its melting point (5.5 °C). The heat of fusion for benzene is 9.92 kJ/mol.
Isopropanol (C3H8O) is used as rubbing alcohol. The heat of vaporization of isopropanol at its boiling point (82.5 oC) is 39.9 kJ/mol. What is the entropy change, ΔS when 1.30 mol C3H8O vaporizes at its boiling point?
Liquid nitrogen has many applications in chemical laboratories. Calculate the change in entropy, ΔS that occurs in the system when 8.60 mole of nitrogen condenses from a gas to a liquid at its boiling point (-196 °C). The heat of vaporization of nitrogen is 2.7928 kJ/mol.
What is the entropy change, ΔS when 45.0 g of diethyl ether (C4H10O) freezes at its melting point (-116.3 °C). The heat of fusion of the diethyl is 7.27 kJ/mol.
Chloroform, CHCl3, is a common organic solvent once used as an anesthetic. Given that the heat of vaporization of chloroform is 29.24 kJ/mol, calculate the entropy change, ΔS of the system when 1.00 mol of CHCl3 evaporates at its boiling point (61.2 °C). What is the entropy of the gaseous chloroform at this temperature if the standard entropy of chloroform is 295.6 J/(mol K).
Using the data in the attached Appendix, calculate the standard entropy changes, ΔS for the following reactions at 25°C:
a) 2KO2(aq)+ 2H2O(l) → 2KOH(aq) + O2(g)+ H2O2(l)
b) C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
c) C2H4(g) + H2(g) → C2H6(g)
d) C(s)graphite + H2O(g) → CO(g) + H2(g)
e) 3NO2(g) + H2O(l) → 2HNO3(aq) + NO(g)
f) 2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)
g) N2O(g) + 3H2(g) → N2H4(l) + H2O(l)
h) 2C3H7OH(l) + 9O2(g) → 6CO2(g) + 8H2O(l)
Given the ΔH°rxn, calculate the ΔSsurr for each reaction at the same temperature.
a) ΔH°rxn = -2560 kJ, T = 298 K
b) ΔH °rxn = -454 kJ, T = 298 K
c) ΔHo rxn= 59 kJ, T = 250 K
d) ΔHo rxn= 148 kJ, T = 360 K
Determine whether each reaction is spontaneous or not by calculating the ΔSuniv based on the values of ΔH °rxn, ΔS °rxn, and T. Assume that all the components in the reaction are in their standard states.
a) ΔH °rxn = +126 kJ; ΔS °rxn = -231 J/K; T = 298 K
b) ΔH °rxn = -5 kJ; ΔS °rxn = +139 J/K; T = 298 K
Gibbs Free Energy, ΔG
Using the data for standard free energies of formations in the attached appendix, calculate the standard free energy of each reaction at 25 oC.
a) 2C2H6(g) + 7O2(g) → 4CO2(g) + 6H2O(l)
b) 6Cl2(g) + 2Fe2O3(s) → 4FeCl3(s) + 3O2(g)
c) 2CH3OH(g) + H2(g) → C2H6(g) + 2H2O(g)
d) Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)
Given the values of ΔH and ΔS, which of the following changes will be spontaneous at constant T and P?
a) ΔH = +35 kJ, ΔS = +10 J/K, T = 300. K
b) ΔH = +45 kJ, ΔS = +153 J/K, T = 312 K
c) ΔH = – 20. kJ, ΔS = +8.0 J/K, T = 298 K
d) ΔH = – 15 kJ, ΔS = -60. J/K, T = 250 K
At what temperatures will the following processes be spontaneous?
a) ΔH = +25.0 kJ, ΔS = +40.0 J/K
b) ΔH = -5.00 kJ, ΔS = -30.0 J/K
c) ΔH = -20.0 kJ, ΔS = +60.0 J/K
c) ΔH = +45.0 kJ, ΔS = -70.0 J/K
Given the data for the three combustion reactions below, calculate the free energy of the reaction producing methanol (CH3OH) from carbon monoxide and hydrogen gas.
CO(g) + 2H2(g) → CH3OH(g), ΔGo = ?
1) 2CO(g) + O2(g) → 2CO2(g), ΔGo= -514 kJ
2) 2H2(g) + O2(g) → 2H2O(g), ΔGo = -458 kJ
3) 2CH3OH(g) + 3O2(g) → 2CO2(g) + 4H2O(g), ΔGo = −1378 kJ
ΔG at Nonstandard Conditions, ΔG and Equilibrium Constant
Consider the evaporation of ethanol at 25.0 °C:
C2H5OH(l) → C2H5OH(g)
a) Determine the ΔG° at standard conditions when T = 25.0 °
b) Determine the ΔG at 25.0 °C when the pressure of ethanol is 0.845 atm.
Using data from the attached appendix (or the one from your textbook), calculate ΔG for the following reaction at 298 K and the given partial pressures:
2NOCl(g) ⇆ 2NO(g) + Cl2(g)
PNOCl = 1.00 x 10-3 atm, PNO = 2.00 x 10-2 atm, PCl2 = 1.00 x 10-2 atm
Using the data from the attached appendix (or the one from your textbook), calculate ΔG for the reaction at 298 K and the following partial pressures:
4HCl(g) + O2(g) ⇆ 2Cl2(g) + 2H2O(g)
PHCl = 50.0 mmHg, PO2 = 35.0 mmHg, P H2O = 125 mmHg, PCl2 = 182 mmHg
Using the data from the attached appendix (or the one from your textbook), calculate the equilibrium constants at 25 °C for each reaction.
a) 2SO3(g) ⇆ 2SO2(g) + O2(g)
b) NO(g) + O3(g) ⇆ NO2(g) + O2(g)
Calculate the equilibrium constants for the reactions in the previous problem at 495 K.
a) 2SO3(g) ⇆ 2SO2(g) + O2(g)
b) NO(g) + O3(g) ⇆ NO2(g) + O2(g)
Using data from the attached appendix (or the one from your textbook), calculate the equilibrium constants at 295 °C for each reaction.
a) PCl3(g) + Cl2(g) ⇆ PCl5(g)
b) N2O4(g) ⇆ 2NO2(g)
Calculate KP for the following reaction at 25°C:
H2(g) + Br2(g) ⇆ 2HBr(g) ΔG°= -106 kJ
Consider the dissociation reaction of Al(OH)3 at 25 °C:
Al(OH)3(s) ⇆ Al3+(aq) + 3OH–(aq)
Calculate ΔG° for the reaction if Ksp = 2.0 x 10-32