Q: Which formula gives the standard entropy change for a reaction?

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

Question 1

Which law of thermodynamics states that the entropy of a pure crystalline substance at absolute zero is zero?

Accepted Answer

3rd Law of Thermodynamics

Question 2

A diamond spontaneously converts to graphite, but the process is extremely slow. What does this illustrate about the relationship between thermodynamics and kinetics?

Accepted Answer

Thermodynamics determines spontaneity, while kinetics determines the rate.

Question 3

Which equation correctly expresses the change in entropy for a system exchanging heat at constant temperature?

Accepted Answer

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

Question 4

Which process results in an increase in entropy?

Accepted Answer

Dissolving NaCl(s) in water

Question 5

Given the reaction: $$2	ext{NO}(g) + 	ext{O}_2(g) ightarrow 2	ext{NO}_2(g)$$, predict the sign of $$\Delta S^\circ.$$

Accepted Answer

Negative, because the number of gas molecules decreases.

Question 6

Which formula gives the standard entropy change for a reaction?

Accepted Answer

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

Question 7

Which of the following statements about entropy is correct?

Accepted Answer

Entropy increases as the number of microstates increases.

Question 8

For the reaction $$	ext{CH}_4(g) + 2	ext{O}_2(g) ightarrow 	ext{CO}_2(g) + 2	ext{H}_2	ext{O}(l)$$, which method can be used to calculate $$\Delta G^\circ_{rxn} at 25$$°C?

Accepted Answer

$$\Delta G^\circ_{rxn} = \Delta H^\circ_{rxn} - T\Delta S^\circ_{rxn}$$

Question 9

Which condition will always result in a spontaneous process?

Accepted Answer

$$\Delta H  0$$

Question 10

Which equation relates Gibbs free energy to the equilibrium constant?

Accepted Answer

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

Question 11

A reaction has $$\Delta H = -100 kJ $$and $$\Delta S = -200 J/K. At $$what temperature will the reaction change from spontaneous to nonspontaneous?

Accepted Answer

$$T = \frac{\Delta H}{-\Delta S}$$

Question 12

Which of the following best describes the entropy change when ice melts?

Accepted Answer

Entropy increases because the molecules gain more freedom of motion.

Question 13

Which process is spontaneous only at high temperatures?

Accepted Answer

Evaporation of water

Question 14

What is the entropy change for the universe ($$\Delta S_{univ}) in a $$spontaneous process?

Accepted Answer

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

Question 15

Which equation gives the relationship between Gibbs free energy and the reaction quotient Q under nonstandard conditions?

Accepted Answer

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

Question 16

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

Accepted Answer

Xe(g)

Question 17

Which statement about the entropy of a perfect crystal at 0 K is correct?

Accepted Answer

The entropy is zero.

Question 18

Which of the following is NOT a type of molecular motion contributing to entropy?

Accepted Answer

Nuclear motion

Question 19

Which process is always nonspontaneous at any temperature?

Accepted Answer

$$\Delta H \u003e 0$$ and $$\Delta S \u003c 0$$

Question 20

Which of the following statements about free energy is correct?

Accepted Answer

Free energy is the maximum amount of energy available to do work.

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

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