Q: The graphs show concentration vs. time for two reactions. Which statement best supports the claim that reaction 1 is thermodynamically favorable but reaction 2 is not?

At equilibrium, for reaction 1 $$\frac{[\text{product}]}{[\text{reactant}]} \gg 1$$ and $$\Delta G^\circ_{rxn} 0$$

Question 1

For the combustion of ethanol, $$\mathrm{C_2H_5OH(l) + 3\ O_2$(g) \rightarrow 2\ CO_2$(g) + 3\ H_2O(g)}$$, the standard entropy and enthalpy changes are $$\Delta S^\circ = +217.7\ \mathrm{J/(mol_{rxn}\cdot K)}$$ and $$\Delta H^\circ = -1235\ \mathrm{kJ/mol_{rxn}}. $$Based on the potential energy diagrams provided, which diagram best explains why little product is formed at 25°C, and why?

Accepted Answer

Diagram 2, because it represents a reaction with a high activation energy barrier for molecules to overcome and a very slow reaction rate, even if it is thermodynamically favorable with $$\Delta G^\circ \u003c 0.$$

Question 2

Which of the following reactions has the most negative value of $$\Delta S^\circ$$?

Accepted Answer

$$\mathrm{C_2H_2(g) + 2\ Cl_2$(g) \rightarrow C_2H_2Cl_4(l)}$$

Question 3

The dimerization of $$\mathrm{NO_2$(g)} is $$exothermic: $$2\ \mathrm{NO_2$(g) \rightleftharpoons N_2O_4(g)}. At $$which temperature is the forward reaction thermodynamically favored?

Accepted Answer

Low temperatures only

Question 4

Given the reaction $$2\ \mathrm{Na_2O_2(s) + S(s) + 2\ H_2O(l) \rightarrow 4\ NaOH(aq) + SO_2$(aq)}$$ with $$\Delta H^\circ_{298} = -610\ \mathrm{kJ/mol_{rxn}}$$ and $$\Delta S^\circ_{298} = -7.3\ \mathrm{J/(K\cdot mol_{rxn})}$$, which statement about the thermodynamic favorability at 298 K is correct?

Accepted Answer

It is thermodynamically favorable and is driven by enthalpy only.

Question 5

Using the provided standard entropy values, what is the approximate $$\Delta S^\circ$$ for the reaction $$2\ \mathrm{$$AgNO_3$$(aq) + $$CaCl_2$$(aq) \rightarrow 2\ AgCl(s) + Ca(NO_3$)_2(aq)}$$?

Accepted Answer

$$-68\ \mathrm{J/(mol\cdot K)}$$

Question 6

Given the following reactions and their $$\Delta G^\circ$$ values:

Reaction 1: $$4\ \mathrm{NH_3$(g) + 5\ O_2$(g) \rightarrow 4\ NO(g) + 6\ H_2O(l)}$$, $$\Delta G^\circ = -1011\ \mathrm{kJ/mol_{rxn}}$$
Reaction 2: $$2\ \mathrm{NO_2$(g) \rightarrow 2\ NO(g) + O_2$(g)}$$, $$\Delta G^\circ = 71\ \mathrm{kJ/mol_{rxn}}$$
Reaction 3: $$4\ \mathrm{NO_2$(g) + O_2$(g) + 2\ H_2O(l) \rightarrow 4\ $$HNO_3$$(aq)}$$, $$\Delta G^\circ = -166\ \mathrm{kJ/mol_{rxn}}$$

What is the $$\Delta G^\circ$$ for the reaction $$4\ \mathrm{NH_3$(g) + 8\ O_2$(g) \rightarrow 4\ $$HNO_3$$(aq) + 4\ H_2O(l)}$$?

Accepted Answer

$$-1248\ \mathrm{kJ/mol}$$

Question 7

In the reaction $$\mathrm{CO(g) + 2\ H_2$(g) ightleftharpoons CH_3OH(g)} at 298 K$$, $$K = 2.2 	imes 10^4$. The particle diagrams show the effect of a catalyst. Which statement best explains the results?

Accepted Answer

Although the reaction is thermodynamically favorable because $$\Delta G^\circ \u003c 0$$ based on the value of $$K$$, only the catalyzed reaction could proceed in one hour because it has a lower activation-energy pathway.

Question 8

Which pair of diagrams (A, B, C, or D) represents the change with the greatest positive value of $$\Delta S^\circ$$?

Accepted Answer

C

Question 9

The oxidation of Fe(s) is represented by a chemical equation. Which statement correctly explains whether the reaction is thermodynamically favorable?

Accepted Answer

There are more particles (including particles in the gas state) in the reactants than in the products, thus $$\Delta S_{rxn} \u003c 0. $$Because $$\Delta H is $$large and negative, the reaction will be thermodynamically favorable at low temperatures.

Question 10

The graphs show concentration vs. time for two reactions. Which statement best supports the claim that reaction 1 is thermodynamically favorable but reaction 2 is not?

Accepted Answer

At equilibrium, for reaction 1 $$\frac{[	ext{product}]}{[	ext{reactant}]} \gg 1$$ and $$\Delta G^\circ_{rxn}  0$$

Question 11

When water vapor condenses on a cold window ($$\mathrm{H_2O(g) \rightarrow H_2O(l)}$$), which of the following is true about $$\Delta H^\circ$$ and $$\Delta S^\circ$$ for the process?

Accepted Answer

$$\Delta H^\circ$$ and $$\Delta S^\circ$$ are negative

Question 12

What is the net ionic equation for the overall reaction that occurs as the cell below operates (Zn(s)/Zn$$^{2+}(aq)//Ag$^+(aq)/Ag(s$$))?

Accepted Answer

$$\mathrm{2Ag$$^{+}$$(aq) + Zn(s) \rightarrow 2Ag(s) + Zn$$^{2+}$$(aq)}$$

Question 13

Given the standard reduction potentials: $$\mathrm{Ag^+(aq) + e^- \rightarrow Ag(s)},\ E^\circ = +0.80\ V$$ and $$\mathrm{Zn$$^{2+}$$(aq) + 2e^- \rightarrow Zn(s)},\ E^\circ = -0.76\ V$$, what is the value of $$E^\circ$$ for the cell?

Accepted Answer

$$0.84\ \mathrm{V}$$

Question 14

A reversible reaction at standard conditions has a negative enthalpy change and a positive entropy change. Which of the following must be true for the equilibrium constant and the standard cell potential for the forward reaction?

Accepted Answer

Equilibrium constant $$\u003e 1$$; Standard cell potential $$= +$$

Question 15

A reaction is known to have a negative value of enthalpy and a positive value of entropy. Which of the following statements correctly describes the equilibrium constant for the reaction?

Accepted Answer

Equilibrium constant will be greater than 1 at all temperatures

Question 16

The elements K and Cl react directly to form KCl. The reaction goes essentially to completion. Which statement is true about the thermodynamic favorability of the reaction?

Accepted Answer

The reaction is favorable and driven by both enthalpy and entropy changes.

Question 17

For the synthesis of ammonia $$\mathrm{N_2$(g) + 3\ H_2$(g) ightleftharpoons 2\ NH_3$(g)}$$ with $$K = 5.6 	imes 10^5$ at 298 K and $$\Delta H^\circ_{rxn} = -91.8\ \mathrm{kJ/mol_{rxn}}$$, which equation best justifies that the reaction is thermodynamically favorable at 298 K and constant pressure?

Accepted Answer

$$\Delta G^\circ = -RT \ln K \u003c 0$$ because $$K \gg 1$$

Question 18

If the equilibrium constant for $$\mathrm{Cu(s) + 2Ag^+(aq) ightarrow Cu$$^{2+}$$(aq) + 2Ag(s)} is$$ $$3.7 	imes 10$$^{15}$$, which of the following correctly describes the standard voltage E$$^\circ$$ and the standard free energy change $$\Delta G^\circ$$ for this reaction?

Accepted Answer

$$E^\circ is $$positive and $$\Delta G^\circ is $$negative

Question 19

Given $$\mathrm{HA(aq) + H_2O(l) \rightleftharpoons A^-(aq) + H_3O^+(aq)}$$ and $$\Delta G^\circ = +35\ \mathrm{kJ/mol_{rxn}}$$, which statement justifies the claim that HA(aq) is a weak acid?

Accepted Answer

Because $$\Delta G^\circ \gg 0$$, and $$K_a \ll 1$$, and HA only partially dissociates

Unit 7 Study Guide – Thermodynamics, Entropy, Electrochemistry, and Equilibrium

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