Question 1

Which of the following correctly represents the formula for the total entropy change of the universe for a chemical reaction?

Accepted Answer

$$\Delta S_{univ} = \Delta S_{system} + \Delta S_{surroundings}$$

Question 2

A reaction has $$\Delta S_{system} = 2.7\ \mathrm{J/K}$$ and $$\Delta S_{surroundings} = -60.0\ \mathrm{J/K}. $$What is the total entropy change of the universe for this reaction?

Accepted Answer

$$\Delta S_{univ} = -57.3\ \mathrm{J/K}$$

Question 3

Which of the following statements is TRUE regarding spontaneity and entropy change?

Accepted Answer

A reaction is spontaneous if $$\Delta S_{univ} \u003e 0$$

Question 4

Given the reaction $$2\ NO(g) + O_2$(g) \rightarrow 2\ NO_2$(g) at$$ $$25^\circ C$$, and the following standard molar entropies: $$S^\circ(NO) = 210.8\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ(O_2$) = 205.0\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ(NO_2$) = 240.1\ \mathrm{J/(mol\cdot K)}$$, calculate $$\Delta S_{rxn}^\circ.$$

Accepted Answer

$$\Delta S_{rxn}^\circ = -146.6\ \mathrm{J/K}$$

Question 5

Which formula is used to calculate the entropy change of the surroundings for a reaction at constant pressure?

Accepted Answer

$$\Delta S_{surroundings} = -\frac{\Delta H_{system}}{T}$$

Question 6

For the reaction $$Fe_2O_3(s) + 3H_2(g) \rightarrow 2Fe(s) + 3H_2O(g) at$$ $$25^\circ C$$, the standard molar entropies are: $$S^\circ(Fe_2O_3) = 87.4\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ(H_2$) = 130.6\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ(Fe) = 27.3\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ(H_2O) = 188.7\ \mathrm{J/(mol\cdot K)}. $$What is $$\Delta S_{rxn}^\circ$$?

Accepted Answer

$$\Delta S_{rxn}^\circ = 174.7\ \mathrm{J/K}$$

Question 7

Which of the following is NOT a correct statement about standard molar entropy ($$S^\circ$$)?

Accepted Answer

For substances in their natural state, $$S^\circ$$ does not equal zero.

Question 8

A reaction at $$25^\circ C$$ has $$\Delta H_{system} = -100.0\ \mathrm{kJ}$$ and $$\Delta S_{system} = 200.0\ \mathrm{J/K}. $$What is $$\Delta S_{surroundings}$$? (Use $$T = 298\ \mathrm{K})$$

Accepted Answer

$$\Delta S_{surroundings} = -335.6\ \mathrm{J/K}$$

Question 9

Which of the following best explains why the entropy of the universe increases for a spontaneous process?

Accepted Answer

Because the total disorder (randomness) of the system and surroundings increases.

Question 10

If a reaction has $$\Delta S_{system} = -50\ \mathrm{J/K}$$ and $$\Delta S_{surroundings} = 60\ \mathrm{J/K}$$, what can you conclude about the spontaneity of the reaction?

Accepted Answer

The reaction is spontaneous because $$\Delta S_{univ} = 10\ \mathrm{J/K} \u003e 0$$

Question 11

Which of the following is the correct formula for calculating the standard entropy change of a reaction ($$\Delta S_{rxn}^\circ$$)?

Accepted Answer

$$\Delta S_{rxn}^\circ = \sum n S^\circ_{products} - \sum n S^\circ_{reactants}$$

Question 12

A chemical reaction at $$25^\circ C$$ has $$\Delta H_{system} = 50.0\ \mathrm{kJ}$$ and $$\Delta S_{system} = -120.0\ \mathrm{J/K}. $$Calculate $$\Delta S_{univ}$$ and determine if the reaction is spontaneous. (Use $$T = 298\ \mathrm{K})$$

Accepted Answer

$$\Delta S_{univ} = -287.6\ \mathrm{J/K}$$, not spontaneous

Question 13

Which of the following best describes the entropy change for a reaction where the number of gas molecules increases?

Accepted Answer

$$\Delta S_{system} is $$likely positive

Question 14

Why is the standard molar entropy of a pure element in its standard state NOT zero?

Accepted Answer

Because entropy is a measure of disorder, and even pure elements have molecular motion at $$25^\circ C$$

Question 15

A reaction has $$\Delta H_{system} = -200\ \mathrm{kJ}$$ and occurs at $$T = 400\ \mathrm{K}. $$What is $$\Delta S_{surroundings}$$?

Accepted Answer

$$\Delta S_{surroundings} = 500\ \mathrm{J/K}$$

Entropy Calculations in Chemical Thermodynamics

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