Q: The entropy change for the surroundings ($$\Delta S_{\text{surr}}) $$during a chemical reaction at constant pressure can be calculated using which of the following equations?

$$\Delta S_{\text{surr}} = -\frac{\Delta H_{\text{sys}}}{T}$$

Question 1

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

Accepted Answer

The total energy of the universe is a constant; energy can be converted from one form to another.

Question 2

A chemical reaction is found to be spontaneous at room temperature but nonspontaneous at higher temperatures. Which combination of enthalpy ($$\Delta H) $$and entropy ($$\Delta S) $$changes is most consistent with this observation?

Accepted Answer

$$\Delta H \u003c 0$$, $$\Delta S \u003c 0$$

Question 3

Which of the following processes results in an increase in the entropy of a system?

Accepted Answer

Sublimation of dry ice (solid CO$$_2) to CO$_2$$ gas.

Question 4

The entropy change for the surroundings ($$\Delta S_{	ext{surr}}) $$during a chemical reaction at constant pressure can be calculated using which of the following equations?

Accepted Answer

$$\Delta S_{	ext{surr}} = -\frac{\Delta H_{	ext{sys}}}{T}$$

Question 5

Which of the following best explains why the entropy of a pure crystalline substance at absolute zero is zero?

Accepted Answer

There is only one possible microstate for the system.

Question 6

Given the reaction: $$2	ext{NO}(g) + 	ext{O}_2(g) ightarrow 2	ext{NO}_2(g)$$, what is the sign of the entropy change ($$\Delta S) $$for the system?

Accepted Answer

$$\Delta S \u003c 0$$

Question 7

Which of the following equations correctly relates the standard free energy change ($$\Delta G^\circ) to $$the equilibrium constant ($$K) at $$temperature $$T$$?

Accepted Answer

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

Question 8

A reaction has $$\Delta H = +40\ 	ext{kJ}$$ and $$\Delta S = +100\ 	ext{J/K}. At $$what temperature (in Kelvin) does the reaction become spontaneous?

Accepted Answer

$$T \u003e 400\ 	ext{K}$$

Question 9

Which of the following is NOT a correct statement about spontaneous processes?

Accepted Answer

A spontaneous process is always reversible.

Question 10

The Boltzmann equation relates entropy ($$S) to $$the number of microstates ($$W) as $$follows:

Accepted Answer

$$S = k \ln W$$

Question 11

Which of the following changes will increase the entropy of a sample of gas?

Accepted Answer

Allowing the gas to expand into a larger volume.

Question 12

For the reaction $$	ext{H}_2	ext{O}(l) ightarrow 	ext{H}_2	ext{O}(g) at 100$^\circC$$, which of the following is true?

Accepted Answer

$$\Delta S \u003e 0$$ and the process is spontaneous.

Question 13

Which of the following best describes a reversible process in thermodynamics?

Accepted Answer

A process that can be reversed by an infinitesimal change in a variable, restoring both system and surroundings to their original states.

Question 14

Which of the following factors does NOT increase the number of microstates (W) in a system?

Accepted Answer

Decreasing the volume.

Question 15

Which of the following is true for the standard molar entropy ($$S^\circ) of $$elements in their standard state?

Accepted Answer

$$S^\circ is $$always positive and depends on the phase and structure.

Question 16

A reaction at 25$$^\circC $$has $$\Delta G^\circ = +10\ 	ext{kJ}. $$Which of the following statements is true?

Accepted Answer

The reaction is nonspontaneous as written, but spontaneous in the reverse direction.

Question 17

Which of the following best explains why the melting of ice is spontaneous at temperatures above 0$$^\circC$$?

Accepted Answer

The increase in entropy ($$\Delta S \u003e 0) $$outweighs the positive enthalpy change ($$\Delta H \u003e 0) at $$higher temperatures.

Question 18

Which of the following is the correct expression for the standard entropy change ($$\Delta S^\circ) of a $$reaction?

Accepted Answer

$$\Delta S^\circ = \sum n S^\circ_{	ext{products}} - \sum m S^\circ_{	ext{reactants}}$$

Question 19

Which of the following statements about Gibbs Free Energy ($$\Delta G) is $$correct?

Accepted Answer

$$\Delta G is $$zero at equilibrium.

Chemical Thermodynamics: Spontaneity, Entropy, and Free Energy

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