Q: According to the Second Law of Thermodynamics, which of the following must be true for a process to be spontaneous?

$$\Delta S_{\text{univ}} \u003e 0$$

Q: A reaction has $$\Delta H_{\text{rxn}} = -107\ \text{kJ/mol}$$ and $$\Delta S_{\text{rxn}} = 285\ \text{J/(K·mol)}. At $$what temperature (in K) will the entropy change of the reaction equal the entropy change of the surroundings?

$$T = 107,000/285 = 375\ \text{K}$$

Q: Consider the sublimation of dry ice (solid CO$$_2). $$Which statement is true?

$$\Delta H_{\text{rxn}}^\circ is $$positive, $$\Delta S_{\text{rxn}}^\circ is $$positive, and $$\Delta G_{\text{rxn}}^\circ is $$positive at low temperature and negative at high temperature.

Q: Calculate the standard entropy change ($$\Delta S^\circ_{\text{rxn}}) $$for the reaction: $$4\text{NH}_3(g) + 5\text{O}_2(g) \rightarrow 4\text{NO}(g) + 6\text{H}_2\text{O}(g)$$ using the following data: $$S^\circ(\text{NH}_3) = 192.8$$, $$S^\circ(\text{O}_2) = 205.2$$, $$S^\circ(\text{NO}) = 210.8$$, $$S^\circ(\text{H}_2\text{O}) = 188.8 ($$all in J/K·mol).

$$\Delta S^\circ_{\text{rxn}} = [4 \times 210.8 + 6 \times 188.8] - [4 \times 192.8 + 5 \times 205.2] = 843.2 + 1132.8 - 771.2 - 1026 = 1976 - 1797.2 = 178.8\ \text{J/K}$$

Q: For a reaction with $$\Delta H_{\text{rxn}} = 35.5\ \text{kJ/mol}$$ and $$\Delta S_{\text{rxn}} = 83.6\ \text{J/(mol·K)}$$, at what temperature does the reaction become spontaneous?

$$T = \frac{\Delta H_{\text{rxn}}}{\Delta S_{\text{rxn}}} = \frac{35,500}{83.6} = 425\ \text{K}$$

Q: Which of the following is a correct application of Hess's Law for Gibbs energy?

The $$\Delta G^\circ_{\text{rxn}}$$ for a reaction is the sum of the $$\Delta G^\circ_{\text{rxn}}$$ values for the individual steps that add up to the overall reaction.

Question 1

Which of the following best describes the difference between thermodynamics and kinetics in chemical reactions?

Accepted Answer

Thermodynamics determines the direction and extent of a reaction, while kinetics determines the speed of the reaction.

Question 2

Which of the following processes results in a positive entropy change ($$\Delta S \u003e 0$$)?

Accepted Answer

Sugar dissolving in tea.

Question 3

According to the Second Law of Thermodynamics, which of the following must be true for a process to be spontaneous?

Accepted Answer

$$\Delta S_{	ext{univ}} \u003e 0$$

Question 4

Which of the following statements about entropy is correct?

Accepted Answer

Entropy increases as a solid sublimes to a gas.

Question 5

For the reaction $$2	ext{NH}_3(g) ightarrow 	ext{N}_2(g) + 3	ext{H}_2(g)$$, what is the sign of the entropy change ($$\Delta S$$)?

Accepted Answer

Positive, because the number of gas molecules increases.

Question 6

A reaction has $$\Delta H_{	ext{rxn}} = -107\ 	ext{kJ/mol}$$ and $$\Delta S_{	ext{rxn}} = 285\ 	ext{J/(K·mol)}. At $$what temperature (in K) will the entropy change of the reaction equal the entropy change of the surroundings?

Accepted Answer

$$T = 107,000/285 = 375\ 	ext{K}$$

Question 7

Which of the following has the highest standard molar entropy at 298 K?

Accepted Answer

N$$_2H$_4(g)$$

Question 8

The standard Gibbs energy of formation ($$\Delta G_f^\circ) $$for pure elements in their standard states is:

Accepted Answer

Zero

Question 9

Consider the sublimation of dry ice (solid CO$$_2). $$Which statement is true?

Accepted Answer

$$\Delta H_{	ext{rxn}}^\circ is $$positive, $$\Delta S_{	ext{rxn}}^\circ is $$positive, and $$\Delta G_{	ext{rxn}}^\circ is $$positive at low temperature and negative at high temperature.

Question 10

Which of the following best explains why water can freeze spontaneously below 0°C?

Accepted Answer

The entropy decrease of the system is offset by a larger entropy increase in the surroundings.

Question 11

Calculate the standard entropy change ($$\Delta S^\circ_{	ext{rxn}}) $$for the reaction: $$4	ext{NH}_3(g) + 5	ext{O}_2(g) ightarrow 4	ext{NO}(g) + 6	ext{H}_2	ext{O}(g)$$ using the following data: $$S^\circ(	ext{NH}_3) = 192.8$$, $$S^\circ(	ext{O}_2) = 205.2$$, $$S^\circ(	ext{NO}) = 210.8$$, $$S^\circ(	ext{H}_2	ext{O}) = 188.8 ($$all in J/K·mol).

Accepted Answer

$$\Delta S^\circ_{	ext{rxn}} = [4 	imes 210.8 + 6 	imes 188.8] - [4 	imes 192.8 + 5 	imes 205.2] = 843.2 + 1132.8 - 771.2 - 1026 = 1976 - 1797.2 = 178.8\ 	ext{J/K}$$

Question 12

For a reaction with $$\Delta H_{	ext{rxn}} = 35.5\ 	ext{kJ/mol}$$ and $$\Delta S_{	ext{rxn}} = 83.6\ 	ext{J/(mol·K)}$$, at what temperature does the reaction become spontaneous?

Accepted Answer

$$T = \frac{\Delta H_{	ext{rxn}}}{\Delta S_{	ext{rxn}}} = \frac{35,500}{83.6} = 425\ 	ext{K}$$

Question 13

Which of the following is a correct application of Hess's Law for Gibbs energy?

Accepted Answer

The $$\Delta G^\circ_{	ext{rxn}}$$ for a reaction is the sum of the $$\Delta G^\circ_{	ext{rxn}}$$ values for the individual steps that add up to the overall reaction.

Question 14

A reaction has $$\Delta H_{	ext{rxn}} = -137.5\ 	ext{kJ/mol}$$ and $$\Delta S_{	ext{rxn}} = -120.5\ 	ext{J/(mol·K)}. $$How does the spontaneity of the reaction change as temperature increases?

Accepted Answer

The reaction becomes less spontaneous as temperature increases.

Question 15

Which of the following best describes the entropy change when a crystalline solid dissolves in water?

Accepted Answer

Entropy increases because the ions become more dispersed.

Question 16

Which of the following is true about the entropy of a perfect crystal at absolute zero (0 K)?

Accepted Answer

It is zero.

Question 17

Consider the reaction $$2	ext{H}_2	ext{S}(g) + 	ext{SO}_2(g) ightarrow 3	ext{S}(s) + 2	ext{H}_2	ext{O}(g)$$ with $$\Delta G^\circ_{	ext{rxn}} = -102\ 	ext{kJ/mol} at 298 K. If $$the partial pressures are $$P_{	ext{H}_2$$	ext{S}} = 2.00$$ bar, P$$_{	ext{SO}$$_2} = 1.50$$ bar, and $$P_{	ext{H}_2$$	ext{O}} = 0.0100$$ bar, is the reaction more or less spontaneous under these conditions compared to standard conditions?

Accepted Answer

Less spontaneous, because the reaction quotient Q is greater than 1.

Gibbs Energy and Thermodynamics: Spontaneity, Entropy, and Free Energy

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