Q: For the reaction $$\mathrm{N_2$(g) + 3H_2(g) \rightarrow 2NH_3(g)} at 25$$°C, $$\Delta G^\circ_{rxn} = +33.0 kJ. If$$ $$P_{N_2$} = 3.0$$ atm, $$P_{H_2$} = 3.0$$ atm, and $$P_{NH_3$} = 0.020$$ atm, what is the value of $$Q$$?

$$Q = \dfrac{(0.020)^2$}{(3.0) \times (3.0)^3$} = 4.94 \times 10$$^{-6}$$

Question 1

Which equation correctly relates the Gibbs free energy change under nonstandard conditions to the standard free energy change and the reaction quotient?

Accepted Answer

$$\Delta G = \Delta G^\circ + RT \ln Q$$

Question 2

For the reaction $$\mathrm{N_2$(g) + 3H_2(g) \rightarrow 2NH_3(g)} at 25$$°C, $$\Delta G^\circ_{rxn} = +33.0 kJ. If$$ $$P_{N_2$} = 3.0$$ atm, $$P_{H_2$} = 3.0$$ atm, and $$P_{NH_3$} = 0.020$$ atm, what is the value of $$Q$$?

Accepted Answer

$$Q = \dfrac{(0.020)^2$}{(3.0) 	imes (3.0)^3$} = 4.94 	imes 10$$^{-6}$$

Question 3

Given $$\Delta G^\circ_{rxn} = +33.0 kJ $$and $$Q = 4.94 	imes 10$$^{-6}$$ at 25°C, calculate $$\Delta G$$ for the reaction $$\mathrm{$$N_2(g) + 3H_2(g$$) ightarrow 2NH_3(g)}$$. (R = 8.314$ J/mol·K, T = 298.15$ K)

Accepted Answer

$$\Delta G = +2.7 kJ$$

Question 4

Which of the following statements is TRUE regarding the spontaneity of a reaction when $$Q \u003c K$$?

Accepted Answer

The forward reaction is spontaneous ($$\Delta G \u003c 0).$$

Question 5

At equilibrium, which of the following is always true for a chemical reaction?

Accepted Answer

$$\Delta G = 0$$

Question 6

For the reaction $$\mathrm{N_2O_4(g) \rightleftharpoons 2NO_2(g)}$$, given $$\Delta G^\circ_{rxn} = +2.8 kJ at 298 K$$, what is the equilibrium constant $$K$$?

Accepted Answer

$$K = 0.32$$

Question 7

If $$\Delta G^\circ_{rxn} is a $$large negative value, what can be said about the equilibrium constant $$K$$ and the position of equilibrium?

Accepted Answer

$$K \gg 1$$; products are strongly favored at equilibrium.

Question 8

Which equation links the standard free energy change to the equilibrium constant?

Accepted Answer

$$\Delta G^\circ = -RT \ln K$$

Question 9

A reaction has $$\Delta H^\circ_{rxn} = -802.3 kJ $$and $$\Delta S^\circ_{rxn} = -5.1 J/K at 25$$°C. Calculate $$\Delta G^\circ_{rxn} at 25$$°C.

Accepted Answer

$$\Delta G^\circ_{rxn} = -800.8 kJ$$

Question 10

For the reaction $$\mathrm{CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(g)}$$ with $$\Delta G^\circ_{rxn} = -800.8 kJ at 25$$°C, what is the approximate value of $$K at $$this temperature?

Accepted Answer

$$K \approx 10$$^{140}$$

Question 11

Which of the following best describes the effect of coupling a nonspontaneous reaction ($$\Delta G \u003e 0) $$with a spontaneous one ($$\Delta G \u003c 0$$)?

Accepted Answer

The overall process can become spontaneous if the sum of $$\Delta G$$ values is negative.

Question 12

Which equation expresses the entropy change for a reversible process at constant temperature?

Accepted Answer

$$\Delta S = \dfrac{q_{rev}}{T}$$

Question 13

Which of the following is a correct interpretation of the Boltzmann equation $$S = k \ln W$$?

Accepted Answer

Entropy is related to the number of microstates available to a system.

Question 14

If $$\Delta G^\circ_{rxn} = +42.6 kJ/$$mol for $$\mathrm{N_2$(g) + O_2$(g) \rightarrow 2NO(g)} at 25$$°C, what does this imply about the equilibrium constant $$K$$ and the spontaneity under standard conditions?

Accepted Answer

$$K \ll 1$$; the reaction is not spontaneous under standard conditions.

Question 15

Which of the following best explains why iron can be extracted from iron ore using carbon (coke) in a blast furnace?

Accepted Answer

The nonspontaneous reduction of iron oxide is coupled with the highly spontaneous oxidation of carbon, making the overall process spontaneous.

Question 16

Which of the following is NOT a correct statement about the relationship between $$\Delta G^\circ_{rxn}$$ and $$K$$?

Accepted Answer

If $$\Delta G^\circ_{rxn} \u003e 0$$, then $$K \u003e 1$$ and products are favored at equilibrium.

Question 17

Which of the following is the correct formula for the standard entropy change of a reaction?

Accepted Answer

$$\Delta S^\circ_{rxn} = \sum n_p S^\circ (	ext{products}) - \sum n_r S^\circ (	ext{reactants})$$

Question 18

Which of the following best describes the effect of temperature on the spontaneity of a reaction with $$\Delta H \u003c 0$$ and $$\Delta S \u003c 0$$?

Accepted Answer

The reaction is spontaneous only at low temperatures.

Free Energy, Nonstandard States, and Equilibrium in Chemical Reactions

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