Q: Which of the following is the correct expression for the standard enthalpy change of a reaction ($$\Delta H^\circ_{rxn}) in $$terms of standard enthalpies of formation ($$\Delta H^\circ_f$$)?

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

Question 1

A sample of gas absorbs 250 J of heat and does 150 J of work on the surroundings at constant pressure. According to the first law of thermodynamics, what is the change in internal energy ($$\Delta U) of $$the system?

$$\Delta U = q + w$$

Accepted Answer

$$\Delta U = 100\ \mathrm{J}$$

Question 2

Which of the following statements best describes a spontaneous process?

Accepted Answer

A process that occurs without outside intervention once it has started.

Question 3

Which of the following phase changes is associated with a decrease in entropy ($$\Delta S \u003c 0$$)?

Accepted Answer

Condensation of steam

Question 4

A chemical reaction has $$\Delta H = -120\ \mathrm{kJ}$$ and $$\Delta S = -200\ \mathrm{J\,K$$^{-1}$$}. At $$what temperature (in K) will the reaction change from spontaneous to nonspontaneous? Use $$\Delta G = \Delta H - T\Delta S.$$

Accepted Answer

$$T = 600\ \mathrm{K}$$

Question 5

Which of the following best describes the relationship between the standard free energy change ($$\Delta G^\circ) $$and the equilibrium constant ($$K) $$for a reaction at temperature $$T$$?

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

Accepted Answer

If $$K \u003e 1$$, $$\Delta G^\circ is $$negative and the reaction is product-favored.

Question 6

Which of the following reactions will have the largest increase in entropy ($$\Delta S$$)?

Accepted Answer

$$\mathrm{N_2O_4(g) \rightarrow 2NO_2(g)}$$

Question 7

A 50.0 g sample of water at $$25.0\ ^\circ\mathrm{C}$$ absorbs 418 J of heat at constant pressure. What is the final temperature of the water? ($$c = 4.18\ \mathrm{J\,g$$^{-1}$$\,K$$^{-1}$$}$$)

$$q = mc\Delta T$$

Accepted Answer

$$27.0\ ^\circ\mathrm{C}$$

Question 8

Which of the following is the correct balanced half-reaction for the reduction of permanganate ion ($$\mathrm{$$MnO_4$$^-}) to $$manganese(II) ion ($$\mathrm{Mn$$^{2+}$$}) in $$acidic solution?

Accepted Answer

$$\mathrm{$$MnO_4$$^-} + 8H^+ + 5e^- \rightarrow Mn$$^{2+}$$ + 4H_2O$$

Question 9

A voltaic cell is constructed with a zinc electrode in $$\mathrm{$$ZnSO_4$$}$$ solution and a copper electrode in $$\mathrm{$$CuSO_4$$}$$ solution. Which statement is correct about the direction of electron flow?

Accepted Answer

Electrons flow from the zinc electrode to the copper electrode.

Question 10

Given the standard reduction potentials: $$E^\circ(\mathrm{Ag^+/Ag}) = +0.80\ \mathrm{V}$$ and $$E^\circ(\mathrm{Cu$$^{2+}$$/Cu}) = +0.34\ \mathrm{V}$$, what is the standard cell potential ($$E^\circ_{cell}) $$for the reaction $$\mathrm{Cu(s) + 2Ag^+(aq) \rightarrow Cu$$^{2+}$$(aq) + 2Ag(s)}$$?

Accepted Answer

$$E^\circ_{cell} = +0.46\ \mathrm{V}$$

Question 11

Which of the following statements about enthalpy ($$\Delta H) is $$correct?

Accepted Answer

Enthalpy is the heat content of a system at constant pressure.

Question 12

Which of the following best describes the effect of increasing the concentration of reactants on the cell potential ($$E_{cell}) of a $$voltaic cell (assuming all other conditions are constant)?

Accepted Answer

Increasing reactant concentration increases $$E_{cell}.$$

Question 13

Which of the following is true regarding the entropy change of the surroundings ($$\Delta S_{surr}) $$when an exothermic reaction occurs at constant temperature?

Accepted Answer

$$\Delta S_{surr}$$ increases because heat is released to the surroundings.

Question 14

A reaction has $$\Delta H = +50\ \mathrm{kJ}$$ and $$\Delta S = +200\ \mathrm{J\,K$$^{-1}$$}. At $$what minimum temperature (in K) will the reaction become spontaneous?

Accepted Answer

$$T = 250\ \mathrm{K}$$

Question 15

Which of the following is the correct expression for the standard enthalpy change of a reaction ($$\Delta H^\circ_{rxn}) in $$terms of standard enthalpies of formation ($$\Delta H^\circ_f$$)?

Accepted Answer

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

Question 16

Which of the following best explains why ice melts spontaneously at temperatures above $$0\ ^\circ\mathrm{C}$$?

Accepted Answer

The increase in entropy ($$\Delta S) of $$the system outweighs the positive enthalpy change ($$\Delta H).$$

Question 17

A voltaic cell is constructed using the following half-reactions:

$$\mathrm{Fe$$^{2+}$$ + 2e^- \rightarrow Fe(s)}$$ $$E^\circ = -0.44\ \mathrm{V}$$
$$\mathrm{Cu$$^{2+}$$ + 2e^- \rightarrow Cu(s)}$$ $$E^\circ = +0.34\ \mathrm{V}$$

What is the overall cell reaction and the standard cell potential ($$E^\circ_{cell}$$)?

Accepted Answer

$$\mathrm{Fe(s) + Cu$$^{2+}$$(aq) \rightarrow Fe$$^{2+}$$(aq) + Cu(s)}$$, $$E^\circ_{cell} = +0.78\ \mathrm{V}$$

Question 18

Which of the following best describes the relationship between Gibbs free energy ($$\Delta G$$), enthalpy ($$\Delta H$$), and entropy ($$\Delta S) at $$constant temperature?

Accepted Answer

$$\Delta G = \Delta H - T\Delta S$$

Question 19

A reaction at $$25\ ^\circ\mathrm{C}$$ has $$\Delta G^\circ = +10.0\ \mathrm{kJ}. $$What is the value of the equilibrium constant $$K$$? ($$R = 8.314\ \mathrm{J\,$$mol^{-1}$$\,K$$^{-1}$$}$$)

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

Accepted Answer

$$K = 0.082$$

Question 20

Which of the following best describes the effect of temperature on the spontaneity of a reaction with $$\Delta H \u003e 0$$ and $$\Delta S \u003e 0$$?

Accepted Answer

The reaction becomes more spontaneous as temperature increases.

Thermochemistry, Thermodynamics, and Electrochemistry: Study Guide for CHEM 001B Exam 3

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