Question 1

Which of the following is an example of a chemical change?

Accepted Answer

$$2 \mathrm{H}_2\mathrm{S}(g) + 3 \mathrm{O}_2(g) \rightarrow 2 \mathrm{SO}_2(g) + 2 \mathrm{H}_2\mathrm{O}(l)$$

Question 2

Given the following data: 
$$\mathrm{CO}(g) + \frac{1}{2} \mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g)$$ 
$$\Delta G = -94.3$$ kcal/mol 
$$\mathrm{C}(s) + \mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g)$$ 
$$\Delta G = -94.1$$ kcal/mol 
Calculate $$\Delta G$$ for $$\mathrm{C}(s) + \frac{1}{2} \mathrm{O}_2(g) \rightarrow \mathrm{CO}(g)$$ and compare with the value for $$\Delta G^\circ$$ for CO(g) in the table of thermodynamic data.

Accepted Answer

$$\Delta G = 0.2$$ kcal/mol

Question 3

Given the following reactions and their $$\Delta G^\circ$$ values: 
$$\mathrm{COCl}_2(g) + 2 \mathrm{NH}_3(g) \rightarrow \mathrm{CON}_2H_4(s) + 2 \mathrm{NH}_4Cl(s)$$ $$\Delta G^\circ = -332.0 kJ$$ 
$$\mathrm{COCl}_2(g) + \mathrm{H}_2O(l) \rightarrow \mathrm{CO}_2(g) + 2 \mathrm{HCl}(g)$$ $$\Delta G^\circ = -191.8 kJ$$ 
$$\mathrm{NH}_4Cl(s) \rightarrow \mathrm{NH}_3(g) + \mathrm{HCl}(g)$$ $$\Delta G^\circ = 94.2 kJ 
$$Calculate the value of $$\Delta G^\circ$$ for the reaction: $$\mathrm{CON}_2H_4(s) + \mathrm{H}_2O(l) \rightarrow \mathrm{CO}_2(g) + 2 \mathrm{NH}_3(g)$$

Accepted Answer

$$\Delta G^\circ = 34.4 kJ$$

Question 4

A chemist claims that the reaction $$\mathrm{S}_8(s) + 8 \mathrm{I}_2(g) \rightarrow 8 \mathrm{SI}_2(g) at 25$^\circC is $$spontaneous. Which of the following statements best explains how to determine if the reaction is thermodynamically possible?

Accepted Answer

Calculate $$\Delta G$$ for the reaction; if $$\Delta G \u003c 0$$, the reaction is spontaneous.

Question 5

For oxygen difluoride, $$\mathrm{OF}_2$$, $$\Delta G_f^\circ is 140.6 kJ/$$mol. Is the preparation of $$\mathrm{OF}_2$$ from its elements at 25$$^\circC $$spontaneous? Explain briefly.

Accepted Answer

No, because $$\Delta G_f^\circ is $$positive, so the reaction is nonspontaneous under standard conditions.

Question 6

For the reaction $$\mathrm{NH}_4^+(aq) + \mathrm{HCO}_3^-(aq) \rightarrow \mathrm{NH}_3(g) + \mathrm{H}_2O(l) + \mathrm{CO}_2(g)$$, $$\Delta H is $$given as $$-176.9 kJ. If $$the absolute entropy, $$S^\circ$$, of $$\mathrm{NH}_4^+(aq) is$$ $$94.8 J/K$$·mol, and data for the other species are given, how would you calculate $$\Delta G$$ for the reaction at 25$$^\circC$$?

Accepted Answer

Use $$\Delta G = \Delta H - T\Delta S$$, where $$\Delta S is $$calculated from the standard entropies of products minus reactants.

Question 7

The absolute entropies of $$\mathrm{Fe}(s)$$ and $$\mathrm{Fe}(g) at 298 K $$are given as 27.28 J/K·mol and 187.6 J/K·mol, respectively. If the enthalpy of vaporization of iron is 349.7 kJ/mol, estimate the normal boiling point of iron.

Accepted Answer

$$T = \frac{\Delta H_{vap}}{\Delta S_{vap}} = \frac{349700\ \mathrm{J/mol}}{187.6 - 27.28\ \mathrm{J/K\cdot mol}} \approx 2187 K$$

Question 8

The heat of vaporization of ethanol is 39.3 kJ/mol and the normal boiling point of ethanol is 78.3$$^\circC. $$Use this data to calculate $$\Delta S$$ for this phase change.

Accepted Answer

$$\Delta S = \frac{\Delta H_{vap}}{T} = \frac{39300\ \mathrm{J/mol}}{351.45\ \mathrm{K}} \approx 112 J/K$$·mol

Question 9

The heat of vaporization of carbon disulfide, $$\mathrm{CS}_2$$, at 25$$^\circC is 28 kJ/$$mol. What is the entropy change when 1 mol of $$\mathrm{CS}_2$$ vapor in equilibrium with liquid condenses to liquid at 25$$^\circC$$?

Accepted Answer

$$\Delta S = -\frac{28000\ \mathrm{J/mol}}{298\ \mathrm{K}} = -94 J/K$$·mol

Question 10

For mercury, the enthalpy of vaporization is 58.51 kJ/mol and the entropy of vaporization is 92.92 J/K·mol. Estimate the normal boiling point of mercury.

Accepted Answer

$$T = \frac{\Delta H_{vap}}{\Delta S_{vap}} = \frac{58510\ \mathrm{J/mol}}{92.92\ \mathrm{J/K\cdot mol}} \approx 630 K$$

Question 11

The excess production of ozone ($$\mathrm{O}_3) in $$the lower atmosphere causes rubber to deteriorate, green plants to turn brown, and persons with respiratory disease to have difficulty breathing. If $$\Delta H$$ for the formation of $$\mathrm{O}_3$$ from $$\mathrm{O}_2 is $$positive, is the formation favored at high, low, or intermediate temperatures?

Accepted Answer

High temperatures, because the positive $$\Delta S$$ and $$\Delta H$$ make $$\Delta G$$ more negative as $$T$$ increases.

Question 12

A natural gas contains primarily methane, $$\mathrm{CH}_4. 
a) $$Write a balanced equation for the combustion of methane gas to $$\mathrm{CO}_2(g)$$ and $$\mathrm{H}_2O(l). 
b) $$How much heat is produced in burning 1 mole of $$\mathrm{CH}_4$$ under standard conditions if reactants and products are brought to 298 K and $$\Delta H_f^\circ is $$formed?

Accepted Answer

$$\mathrm{CH}_4(g) + 2 \mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g) + 2 \mathrm{H}_2O(l)$$; $$\Delta H = -890 kJ/$$mol

Question 13

Which of the following best describes the maximum amount of useful work that can be accomplished under standard conditions by a chemical system?

Accepted Answer

The maximum useful work is equal to the change in Gibbs free energy, $$\Delta G.$$

Question 14

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

Accepted Answer

Entropy is a measure of the disorder or randomness of a system.

Question 15

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

Accepted Answer

$$T = \frac{\Delta H}{\Delta S} = \frac{-100000\ \mathrm{J}}{-200\ \mathrm{J/K}} = 500 K$$

Thermodynamics and Spontaneity in General Chemistry

Study Guide - Practice Questions

Study Guide - Flashcards