Q: Given the reaction: $$\mathrm{N_2$(g) + 3H_2(g) \rightarrow 2NH_3(g)} at 298 K$$, and the standard molar entropies $$S^\circ (J/$$mol·K) are: $$\mathrm{N_2$(g)} = 191.5$$, $$\mathrm{H_2$(g)} = 130.6$$, $$\mathrm{NH_3$(g)} = 192.5. $$What is $$\Delta S^\circ$$ for the reaction?

$$\Delta S^\circ = 2 \times 192.5 - [191.5 + 3 \times 130.6] = 385.0 - 583.3 = -198.3\ \mathrm{J/mol\cdot K}$$

Q: A reaction has $$\Delta H = 180.7\ \mathrm{kJ}$$ and $$\Delta S = 24.7\ \mathrm{J/K} at 298 K. $$What is $$\Delta G$$ for the reaction, and is it spontaneous?

$$\Delta G = 180.7 - (298 \times 0.0247) = 173.3\ \mathrm{kJ}$$, nonspontaneous

Question 1

Which of the following statements best describes a spontaneous process according to the Second Law of Thermodynamics?

Accepted Answer

A process that occurs on its own without outside assistance and increases the entropy of the universe.

Question 2

Given the reaction: $$\mathrm{N_2$(g) + 3H_2(g) \rightarrow 2NH_3(g)} at 298 K$$, and the standard molar entropies $$S^\circ (J/$$mol·K) are: $$\mathrm{N_2$(g)} = 191.5$$, $$\mathrm{H_2$(g)} = 130.6$$, $$\mathrm{NH_3$(g)} = 192.5. $$What is $$\Delta S^\circ$$ for the reaction?

Accepted Answer

$$\Delta S^\circ = 2 	imes 192.5 - [191.5 + 3 	imes 130.6] = 385.0 - 583.3 = -198.3\ \mathrm{J/mol\cdot K}$$

Question 3

Which equation correctly relates Gibbs free energy to enthalpy and entropy for a reaction at constant temperature?

Accepted Answer

$$\Delta G = \Delta H - T\Delta S$$

Question 4

The melting of ice at 1 atm pressure and 0°C is described by $$\Delta H = +6.01\ \mathrm{kJ}$$ and $$\Delta S = +22.0\ \mathrm{J/K}. Is $$this process spontaneous at 0°C?

Accepted Answer

No, because $$\Delta G = 0 at 0$$°C.

Question 5

Which of the following best explains why a rusted car does not spontaneously revert to a new car?

Accepted Answer

The process would decrease the entropy of the universe.

Question 6

For the freezing of elemental mercury at its normal freezing point ($$-38.9^\circC$$), with $$\Delta H_{fus} = 2.29\ \mathrm{kJ/mol}$$, what is the entropy change ($$\Delta S) $$when 50.0 g of Hg(l) freezes? (Molar mass Hg = 200.6 g/mol)

Accepted Answer

$$\Delta S = \frac{-2.29\ \mathrm{kJ/mol}}{234.2\ \mathrm{K}} 	imes 0.249 = -2.44\ \mathrm{J/K}$$

Question 7

Which of the following is true for a reaction where $$\Delta H  0$$?

Accepted Answer

The reaction is spontaneous at all temperatures.

Question 8

A reaction has $$\Delta H = 180.7\ \mathrm{kJ}$$ and $$\Delta S = 24.7\ \mathrm{J/K} at 298 K. $$What is $$\Delta G$$ for the reaction, and is it spontaneous?

Accepted Answer

$$\Delta G = 180.7 - (298 	imes 0.0247) = 173.3\ \mathrm{kJ}$$, nonspontaneous

Question 9

Which of the following best describes the entropy change when water vapor condenses to liquid water at 298 K?

Accepted Answer

Entropy decreases because the molecules become more ordered.

Question 10

Which of the following is the correct expression for the standard entropy change for the reaction $$\mathrm{2Na(s) + Cl_2$(g) \rightarrow 2NaCl(s)}$$ using the values from Table 19.1?

Accepted Answer

$$\Delta S^\circ = 2S^\circ(\mathrm{NaCl(s)}) - [2S^\circ(\mathrm{Na(s)}) + S^\circ(\mathrm{Cl_2$(g)})]$$

Question 11

Which of the following processes is NOT spontaneous under standard conditions?

Accepted Answer

A rusted car spontaneously becoming a new car.

Question 12

If $$\Delta G = 0$$ for a chemical reaction, what does this indicate about the reaction?

Accepted Answer

The reaction is at equilibrium.

Question 13

Which of the following statements about entropy ($$S) is $$correct?

Accepted Answer

Entropy is a state function that measures the disorder of a system.

Question 14

Which of the following reactions would you expect to have the largest increase in entropy?

Accepted Answer

$$\mathrm{C_6H_{12}O_6$(s) \rightarrow 6CO_2(g) + 6H_2O(g)}$$

Question 15

Which of the following is true for a process where $$\Delta S_{univ} \u003e 0$$?

Accepted Answer

The process is spontaneous.

Thermodynamics: Entropy, Spontaneity, and Gibbs Free Energy

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