Question 1

Which of the following best explains the entropy change when solid iodine sublimes at room temperature?

Accepted Answer

Entropy increases because the iodine molecules spread out into the gas phase.

Question 2

Given the standard enthalpy of formation for liquid acetone, $$\Delta H_f^\circ = -247.6\ \mathrm{kJ/mol}$$, and for vapor acetone, $$\Delta H_f^\circ = -216.6\ \mathrm{kJ/mol}$$, calculate the entropy change when 1.00 mol of liquid acetone vaporizes at $$25^\circ\mathrm{C}.$$

Accepted Answer

$$\Delta S = 104\ \mathrm{J/K}$$

Question 3

At the normal boiling point of o-xylene, $$\Delta H^\circ_{vap} = 36.2\ \mathrm{kJ/mol}$$ and $$\Delta S^\circ_{vap} = 86.7\ \mathrm{J/(mol\cdot K)}. $$What is the normal boiling point of o-xylene?

Accepted Answer

$$418\ \mathrm{K}$$

Question 4

Which of the following pairs has the higher molar entropy?

Accepted Answer

$$\mathrm{C_3H_8(g)}$$ has higher entropy than $$\mathrm{CH_4$(g)}.$$

Question 5

Consider the reaction: $$\mathrm{Ba(OH)_2\cdot 8H_2O(s) + 2NH_4NO_3(s) \rightarrow 2NH_3(g) + 10H_2O(l) + Ba(NO_3$)_2(aq)}. $$Predict the sign of $$\Delta S^\circ$$ for this reaction.

Accepted Answer

$$\Delta S^\circ is $$positive because the number of moles and disorder increases.

Question 6

For the reaction $$2\mathrm{H_2$(g)} + \mathrm{O_2$(g)} \rightarrow 2\mathrm{H_2O(l)}$$, what is the sign of $$\Delta S$$?

Accepted Answer

Negative, because the number of gas molecules decreases.

Question 7

Which process results in a positive entropy change?

Accepted Answer

$$\mathrm{H_2O(l)} \rightarrow \mathrm{H_2O(g)}$$

Question 8

Calculate $$\Delta S^\circ_{rxn}$$ for the reaction: $$\mathrm{N_2H_4(l) + 2H_2O_2(l) \rightarrow N_2$(g) + 4H_2O(g)}$$ given the following standard molar entropies: $$S^\circ_m(\mathrm{N_2H_4(l)}) = 121.4\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ_m(\mathrm{H_2O_2(l)}) = 109.6\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ_m(\mathrm{N_2$(g)}) = 191.6\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ_m(\mathrm{H_2O(g)}) = 188.7\ \mathrm{J/(mol\cdot K)}.$$

Accepted Answer

$$\Delta S^\circ_{rxn} = +605.8\ \mathrm{J/(mol\cdot K)}$$

Question 9

For the reaction $$2\mathrm{H_2O(g)} \rightarrow 2\mathrm{H_2$(g)} + \mathrm{O_2$(g)}$$, given $$S^\circ_m(\mathrm{H_2O(g)}) = 188.7\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ_m(\mathrm{H_2$(g)}) = 130.6\ \mathrm{J/(mol\cdot K)}$$, $$S^\circ_m(\mathrm{O_2$(g)}) = 205.0\ \mathrm{J/(mol\cdot K)}$$, calculate $$\Delta S^\circ_{rxn}. $$

Accepted Answer

$$\Delta S^\circ_{rxn} = +88.8\ \mathrm{J/(mol\cdot K)}$$

Question 10

A reaction has $$\Delta S_{system} = -88.8\ \mathrm{J/(mol\cdot K)}$$ and releases $$483.6\ \mathrm{kJ} of $$heat at $$298\ \mathrm{K}. $$What is $$\Delta S_{universe}$$?

Accepted Answer

$$\Delta S_{universe} = 1531.2\ \mathrm{J/(mol\cdot K)}$$

Question 11

For the reaction $$\mathrm{NH_3$(g) + HCl(g) \rightarrow NH_4Cl(s)}$$ with $$\Delta H = -176\ \mathrm{kJ/mol}$$ and $$\Delta S = -285\ \mathrm{J/(mol\cdot K)}$$, under what conditions is the reaction spontaneous?

Accepted Answer

Spontaneous at low temperatures.

Question 12

For the vaporization of a liquid at a given pressure, which statement is correct regarding $$\Delta G$$?

Accepted Answer

$$\Delta G is $$positive at low temperatures but negative at high temperatures.

Question 13

If a reaction is endothermic and non-spontaneous at $$25^\circ\mathrm{C}$$, what can you conclude?

Accepted Answer

It may be spontaneous at higher temperature.

Question 14

The standard Gibbs energy of formation ($$\Delta G_f^\circ) $$for $$\mathrm{CS_2$(g)} is$$ $$+66.85\ \mathrm{kJ/mol}. $$What does this indicate about $$\mathrm{CS_2$(g)}$$?

Accepted Answer

It is less stable than its elements in their standard states.

Question 15

Calculate the standard free energy change, $$\Delta G^\circ$$, for the oxidation of ethyl alcohol to acetic acid: $$\mathrm{CH_3CH_2OH(l) + O_2$(g) \rightarrow CH_3COOH(l) + H_2O(l)}$$, given $$\Delta G_f^\circ$$ values: $$\mathrm{CH_3CH_2OH(l)} = -174.8\ \mathrm{kJ/mol}$$, $$\mathrm{O_2$(g)} = 0$$, $$\mathrm{CH_3COOH(l)} = -392.5\ \mathrm{kJ/mol}$$, $$\mathrm{H_2O(l)} = -237.2\ \mathrm{kJ/mol}.$$

Accepted Answer

$$\Delta G^\circ = -454.9\ \mathrm{kJ}$$

Question 16

Given $$\Delta H^\circ_{rxn} = -74.8\ \mathrm{kJ/mol}$$ and $$\Delta S^\circ_{rxn} = -80.8\ \mathrm{J/(mol\cdot K)}$$ for the formation of methane at $$298\ \mathrm{K}$$, calculate $$\Delta G^\circ_{rxn}.$$

Accepted Answer

$$\Delta G^\circ_{rxn} = -50.7\ \mathrm{kJ/mol}$$

Question 17

For the reaction $$\mathrm{N_2$(g) + O_2$(g) \rightarrow 2NO(g)}$$, the standard free energy change is $$\Delta G^\circ = 173.20\ \mathrm{kJ}. $$What does this imply?

Accepted Answer

The reaction is nonspontaneous and NO will decompose into its elements.

Question 18

Which of the following reactions is always spontaneous under standard conditions?

Accepted Answer

$$\mathrm{H_2$(g) + Br_2$(l) \rightarrow 2HBr(g)}$$; $$\Delta H = -73\ \mathrm{kJ/mol}$$, $$\Delta S = +114\ \mathrm{J/(mol\cdot K)}$$

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