AP Chemistry: Chapter 17 - Spontaneity, Entropy, & Free Energy
Section 1: Multiple Choice
(1) Which of the following reactions has the largest positive value of entropy per mole of Cl2 ?
(A) H2(g) + Cl2(g) ---> 2 HCl(g)
(B) Cl2(g) + 1/2 O2(g) ---> Cl2O(g)
(C) Mg(s) + Cl2(g) ---> MgCl2(s)
(D) 2 NH4Cl (s) ---> N2(g) + 4 H2(g) + Cl2(g)
(E) Cl2(g) ---> 2 Cl (g)
(2) For which of the following processes would ΔS have a negative value?
I. 2 Fe2O3(s) ---> 4 Fe(s) + 3 O2(g)
II. Mg2+ + 2 OH¯ ---> Mg(OH)2(s)
III. H2(g) + C2H4(g) ---> 3 C2H6(g)
(A) I only
(B) I and II only
(C) I and III only
(D) II and III only
(E) I, II, and III
(3)
N2 (g) + 3 H2 (g) ---> 2 NH3 (g)
The reaction indicated above is thermodynamically spontaneous at 298 K, but becomes nonspontaneous
at higher temperatures. Which of the following is true at 298 K?
(A) ΔG, ΔH, and ΔS are all positive.
(B) ΔG, ΔH, and ΔS are all negative.
(C) ΔG and ΔH are negative, but ΔS is positive.
(D) ΔG and ΔS are negative, but ΔH is positive.
(E) ΔG and ΔH are positive, but ΔS is negative
(4) H2O (s) ---> H2O (l) When ice melts at its normal melting point, 298.16 K and 1 atmosphere, which
of the following is true for the process shown above?
(A) ΔH < 0, ΔS > 0, ΔG > 0
(B) ΔH < 0, ΔS < 0, ΔG > 0
(C) ΔH > 0, ΔS < 0, ΔG < 0
(D) ΔH > 0, ΔS > 0, ΔG > 0
(E) ΔH > 0, ΔS > 0, ΔG < 0
(5) Of the following reaction, which involves the largest decrease in Entropy?
(A) CaCO3 (s) ---> CaO (s) + CO2 (g)
(B) 2 CO (g) + O2 (g) ---> 2 CO2 (g)
(C) Pb(NO3)3 (aq) + 2 KI (aq) ---> PbI2 (s) + 2 KNO3 (aq)
(D) C3H8 (g) + O2 (g) ---> 3 CO2 (g) + 4 H2O (g)
(E) 4 La (s) + 3 O2 (g) ---> 2 La2O3 (s)
Section 2: FRQ
FRQ #1
Answer the following questions about nitrogen, hydrogen, and ammonia.
 (a)
In the boxes below, draw the complete Lewis electron-dot diagrams for N2 and
NH3.
 (b)
Calculate the standard free-energy change, ΔG°, that occurs when 12.0 g of H2(g)
reacts with excess N2(g) at 298 K according to the reaction represented below:
N2(g) + 3 H2(g)  2 NH3(g)
∆G˚298 = -34 kJ mol-1
 (c)
Given that ΔH˚298 for the reaction is −92.2 kJ mol-1, which is larger, the total
bond dissociation energy of the reactants or the total bond dissociation energy of the
products? Explain.
 (d)
The value of the standard entropy change, ΔS˚298 , for the reaction is −199 J mol1K-1. Explain why the value of ΔS˚298 is negative.
 (e)
Assume that ΔH° and ΔS° for the reaction are independent of temperature.
◦
(i)
Explain why there is a temperature above 298 K at which the algebraic
sign of the value of ΔG° changes.
◦
(ii)
Theoretically, the best yields of ammonia should be achieved at low
temperatures and high pressures. Explain.
FRQ #2
Use principles of thermodynamics to answer the following questions.
 (a)
The gas N2O4 decomposes to form the gas NO2.
◦
(i) Predict the sign of ∆H˚ for the reaction. Justify your answer.
◦
(ii) Predict the sign of ∆S˚ for the reaction. Justify your answer.
 (b) One of the diagrams below best represents the relationship between ∆G˚ and
temperature for the reaction given in part (a). Assume that ∆H˚ and ∆S˚ are independent
of temperature.
 Draw a circle around the correct graph. Explain why you chose that graph in terms of the
relationship:
∆G˚ = ∆H˚ - T∆S˚.
FRQ #3
2 Fe(s) + O2(g)  Fe2O3(s)
∆Hf˚ = -824 kJ mol–1
Iron reacts with oxygen to produce iron(III) oxide as represented above. A 75.0 g sample of Fe(s)
is mixed with 11.5 L of O2(g) at 2.66 atm and 298 K.
(a) Calculate the number of moles of each of the following before the reaction occurs.
(i)
Fe(s)
(ii) O2(g)
(b) Identify the limiting reactant when the mixture is heated to produce Fe2O3. Support your
answer with calculations.
(c) Calculate the number of moles of Fe2O3 produced when the reaction proceeds to
completion.
(d) The standard free energy of formation, ∆Gf˚ of Fe2O3 is –740. kJ mol–1 at 298 K.
(i) Calculate the standard entropy of formation ∆Sf˚ of Fe2O3 at 298 K. Include
units with your answer.
(ii) Which is more responsible for the spontaneity of the formation reaction at
298K, the standard enthalpy or the standard entropy?
The reaction represented below also produces iron(III) oxide. The value of ∆H˚ for the reaction
is –280 kJ per mol.
2 FeO(s) + O2(g)  Fe2O3(s)
(e) Calculate the standard enthalpy of formation, ∆Hf˚ of FeO(s).
FRQ #4
For the gaseous equilibrium represented below, it is observed that greater amounts of PCl3 and
Cl2 are produced as the temperature is increased:
PCl5(g)  PCl3(g) + Cl2(g)
 (a)
What is the sign of ∆S° for the reaction? Explain.
 (b)
What change, if any, will occur in DG° for the reaction as the temperature is
increased? Explain your reasoning in terms of thermodynamic principles.
 (c)
If He gas is added to the original reaction mixture at constant volume and
temperature, what will happen to the partial pressure of Cl2? Explain.
 (d)
If the volume of the reaction mixture is decreased at constant temperature to half
the original volume, what will happen to the number of moles of Cl2 in the reaction
vessel? Explain.
FRQ #5
2 C4H10(g) + 13 O2(g)  8 CO2(g) + 10 H2O(l)
The reaction represented above is spontaneous at 25°C. Assume that all reactants and
products are in their standard state.
 (a)
Predict the sign of ∆S° for the reaction and justify your prediction.
 (b)
What is the sign of ∆G° for the reaction? How would the sign and magnitude of
∆G° be affected by an increase in temperature to 50°C? Explain your answer.
 (c)
What must be the sign of ∆H° for the reaction at 25°C? How does the total bond
energy of the reactants compare to that of the products?
 (d)
When the reactants are place together in a container, no change is observed even
though the reaction is known to be spontaneous. Explain this observation.