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Thermodynamics

Practice Test

1. Potassium acetate is more soluble in water at higher temperature. What can be concluded from this information?

(A) The dissolving process is exothermic

(B) Water becomes cooler as potassium acetate dissolves in it.

(C) More concentrated potassium acetate solutions will have lower boiling points.

(D) A saturated solution of potassium acetate can be made unsaturated by cooling it.

2. For a certain reaction, ΔH° = -780.0kJ mol^-1 and ΔS°=250.0 J mol^-1 K^-1. What is ΔG° for this reaction at 25°C?

(A) -75,280 kJ mol^-1

(B) -7031 kJ mol^-1

(C) -854.5 kJ mol^-1

(D) -786.3 kJ mol^-1

3. How many mL of 8.90 M HBr solution are needed to prepare 100.mL of 2.00M HBr

(A) 4.45 (B) 8.90

(C) 17.8 (D) 22.5

4. Which changes are accompanied by an increase in entropy?

(I) sublimation of iodine

(II) freezing of ethyl alcohol

(III) refining of silver from its ore

(IV) burning of hydrazine in a rocket booster

(A) I and II (B) I and IV

(C) III and IV (D)II and III

5. As solid NH₄NO₃ dissolves in water, the solution becomes colder. Which are the correct signs of ΔG°, ΔH°, and ΔS° for this process?

	ΔG°	ΔH°	ΔS°
(A)	-	+	-
(B)	+	-	+
(C)	+	-	-
(D)	-	+	+

6. For the reaction

C₂H₄(g)+3O₂(g)à2CO₂(g)+2H₂O(l) ΔH° = -1411 kJ.

Find ΔH° for the reaction

CO₂(g) + H₂O(l) à ½ C₂H₄(g) + 3/2 O₂(g)

(A) 2822 kJ

(B) 1411 kJ

(C) 705.5 kJ

(D) -2822 kJ

7. Calculate ΔG° for this reaction:

4NH₃(g) + 5O₂(g) à 4NO(g) + 6H₂O(l)

Substance	ΔG°f (kJ mol^-1)
NH3(g)	-16.5
O2(g)	0.0
NO(g)	+86.6
H2O(l)	-237.2

(A) -134 kJ (B) -167 kJ

(C) -1011 kJ (D) -1702 kJ

8. Consider this potential energy diagram:

Which best describes the change in enthalpy and entropy for this system?

	Δ H	Δ S
(A)	+	+
(B)	+	-
(C)	-	+
(D)	-	-

9. A working flashlight battery is described by which set of characteristics?

	Ecell	Δ G	Keq
(A)	> 0	< 0	> 1
(B)	> 0	> 0	>1
(C)	< 0	< 0	< 1
(D)	< 0	> 0	< 1

10. Answer the following questions that relate to the chemistry of nitrogen.

(a) The reaction between nitrogen and hydrogen to form ammonia is represented below.

N₂(g) + 3H₂(g) à 2NH₃(g) ΔH° = -92.2 kJ

Predict the sign of the standard entropy change, ΔS°, for the reaction. Justify your answer.

(b) The value of ΔG° for the reaction represented in part (a) is negative at low temperatures but positive at high temperatures. Explain.

(c) When N₂(g) and H₂(g) are placed in a sealed container at a low temperature, no measurable amount of NH₃(g) is produced. Explain.

11. Answer parts (a) through (e) below, which relate to reactions involving silver ion, Ag⁺.

The reaction between silver ion and solid zinc is represented by the following equation:

2 Ag⁺ (aq) + Zn (s) à Zn²⁺ (aq) + 2 Ag(s)

(a) A 1.50 g sample of Zn is combined with 250. mL of 0.110 M AgNO₃ at 25°C.

(i) Identify the limiting reactant. Show calculations to support your answer.

(ii) On the basis of the limiting reactant that you indentified in part (i), determine the value of [Zn²⁺] after the reaction is complete. Assume that volume change in negligible.

(b) Determine the value of the standard potential, E^°, for a galvanic cell based on the reaction between AgNO₃ (aq) and solid Zn at 25° C.

Another galvanic cell is based on the reaction between Ag⁺ (aq) and Cu (s), represented by the equation below. At 25° C, the standard potential, E°, for the cell is 0.46 V.

2 Ag⁺ (aq) + Cu (s) à Cu²⁺ (aq) + 2 Ag (s)

(c) Determine the value of the standard free-energy change, ΔG°, for the reaction between Ag⁺ (aq) and Cu (s) at 25° C.

(d) The cell is constructed so that [Cu²⁺] is 0.045 M and [Ag⁺] is 0.010 M. Calculate the value of the potential, E, for the cell.

(e) Under the conditions specified in part (d), is the reaction in the cell spontaneous? Justify your answer.

2 NO (g) + O₂ à 2 NO₂ (g) ΔH° = - 114.1 kJ, ΔS° = - 146.5 J K^-1

12. The reaction represented above is one that contributes significantly to the formation of photochemical smog.

(a) Calculate the quantity of heat released when 73.1 g of NO (g) is converted to NO₂ (g).

(b) For the reaction at 25° C, the value of standard free energy change, ΔG°, is – 70.4 kJ.

(i) Calculate the value of the equilibrium constant, K_eq, for the reaction at 25° C.

(ii) Indicate whether the value of ΔG° would become more negative, less negative, or remain unchanged as the temperature is increased. Justify your answer.

(c) Use the data in the table below to calculate the value of the standard molar entropy, S°, for O₂ (g) at 25° C.

	Standard Molar Entropy, S° (J K^-1 mol^-1)
NO (g)	210.8
NO2 (g)	240.1