Q: Given the reaction: $$2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)$$, calculate $$\Delta S^\circ_{rxn}$$ using the following standard molar entropies: $$S^\circ(\text{H}_2\text{O}(l)) = 69.9\ \text{J K}^{-1}\text{mol}^{-1}$$, $$S^\circ(\text{H}_2(g)) = 130.7\ \text{J K}^{-1}\text{mol}^{-1}$$, $$S^\circ(\text{O}_2(g)) = 205.3\ \text{J K}^{-1}\text{mol}^{-1}.$$

$$\Delta S^\circ_{rxn} = -327\ \text{J/K}$$

Q: Calculate the entropy change in the surroundings ($$\Delta S_{surr}) $$when 2044 kJ of heat is released by the combustion of propane at 25$$^\circC$$. ($$T = 298 K)$$

$$\Delta S_{surr} = +6.86 \times 10^3$\ \text{J/K}$$

Q: Which of the following reactions has a positive $$\Delta S^\circ$$?

$$\text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{O}(g)$$

Question 1

Which of the following statements best describes the First Law of Thermodynamics?

Accepted Answer

Energy cannot be created or destroyed, only transferred or transformed.

Question 2

Which process is spontaneous under standard conditions, and why?

Accepted Answer

The conversion of diamond to graphite, because graphite is more stable.

Question 3

Which of the following best explains why ice melts spontaneously above $$0^\circC$$, even though melting is endothermic?

Accepted Answer

The increase in entropy (randomness) compensates for the absorption of heat.

Question 4

Which of the following changes results in a decrease in entropy for water?

Accepted Answer

The condensation of water on the sides of a cold glass.

Question 5

Given the reaction: $$2	ext{H}_2(g) + 	ext{O}_2(g) ightarrow 2	ext{H}_2	ext{O}(l)$$, calculate $$\Delta S^\circ_{rxn}$$ using the following standard molar entropies: $$S^\circ(	ext{H}_2	ext{O}(l)) = 69.9\ 	ext{J K}^{-1}	ext{mol}^{-1}$$, $$S^\circ(	ext{H}_2(g)) = 130.7\ 	ext{J K}^{-1}	ext{mol}^{-1}$$, $$S^\circ(	ext{O}_2(g)) = 205.3\ 	ext{J K}^{-1}	ext{mol}^{-1}.$$

Accepted Answer

$$\Delta S^\circ_{rxn} = -327\ 	ext{J/K}$$

Question 6

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

Accepted Answer

Xe(g)

Question 7

For the reaction $$	ext{Ag}^+(aq) + 	ext{Cl}^-(aq) ightarrow 	ext{AgCl}(s)$$, predict the sign of $$\Delta S^\circ$$ and explain why.

Accepted Answer

$$\Delta S^\circ \u003c 0$$, because a solid is formed from more disordered ions.

Question 8

Calculate the entropy change in the surroundings ($$\Delta S_{surr}) $$when 2044 kJ of heat is released by the combustion of propane at 25$$^\circC$$. ($$T = 298 K)$$

Accepted Answer

$$\Delta S_{surr} = +6.86 	imes 10^3$\ 	ext{J/K}$$

Question 9

Which of the following is true regarding the sublimation of dry ice (solid CO$$_2$$)?

Accepted Answer

$$\Delta H is $$positive, $$\Delta S is $$positive, and $$\Delta G is $$positive at low temperatures and negative at high temperatures.

Question 10

Given $$\Delta H = +95.7 kJ $$and $$\Delta S = +142.2 J/K $$for the reaction $$	ext{CCl}_4(g) ightarrow 	ext{C}(s, graphite) + 2	ext{Cl}_2(g)$$, at what temperature does the reaction become spontaneous?

Accepted Answer

$$T = 673 K$$

Question 11

Which of the following reactions has a positive $$\Delta S^\circ$$?

Accepted Answer

$$	ext{H}_2	ext{O}(l) ightarrow 	ext{H}_2	ext{O}(g)$$

Question 12

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

Accepted Answer

Standard molar entropies of elements are zero.

Question 13

For the reaction $$2	ext{NO}(g) + 	ext{O}_2(g) ightarrow 2	ext{NO}_2(g) at 298 K$$, $$\Delta G^\circ_{rxn} = -71.2 kJ. If$$ $$P_{NO} = 0.100$$ atm, $$P_{O_2$} = 0.100$$ atm, and $$P_{NO_2$} = 2.00$$ atm, what is the value of $$Q$$?

Accepted Answer

$$Q = 4.00 	imes 10^3$

Question 14

Which of the following best describes the relationship between $$\Delta G^\circ$$ and the equilibrium constant $$K$$?

Accepted Answer

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

Question 15

A reaction is spontaneous under a certain set of conditions. What can you conclude about $$\Delta G_{rxn}$$ and $$Q$$?

Accepted Answer

$$\Delta G_{rxn}  K$$

Question 16

Which of the following statements is true regarding the effect of temperature on the spontaneity of a reaction with $$\Delta H \u003e 0$$ and $$\Delta S \u003e 0$$?

Accepted Answer

The reaction is spontaneous only at high temperatures.

Question 17

Which of the following best explains why the entropy of a gas is greater than that of a liquid or solid at the same temperature?

Accepted Answer

Gases have more available energy states and greater dispersal of energy.

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

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