AP MCQ 09ABCDEF PART 1

Question 01:

Photosynthesis is a chemical reaction that occurs in nature, but one that is generally not thermodynamically favored. It is forced forward by the presence of light. Which of the following is another example of a thermodynamically unfavored process, which must be forced to occur by the use of an external source of energy?

A galvanic cell
A battery
Electrolysis
Any process with a negative ΔG°

Question 02:

Which of the following combinations of conditions is found to be thermodynamically favored at high temperatures, but unfavored at low temperatures?

An exothermic reaction where the entropy increases
An exothermic reaction where the entropy decreases
An endothermic reaction where the entropy increases
An endothermic reaction where the entropy decreases

Question 03:

Which of the following chemical reactions has a positive ΔS°?

Mg²⁺_(aq) + 2OH^-_(aq) → Mg(OH)_2(s)
CaO_(s) + CO_2(g) → CaCO_3(s)
H₂O_(l) → H₂O_(s)
PCl_5(g) → PCl_3(g) + Cl_2(g)

Question 04:

What is true of the absolute entropy values, S°, of all substances?

They are all positive
They are all negative
They can be positive or negative depending upon the temperature and the reaction
More information is required to answer this question

Question 05:

Given the absolute entropies below, all given in units of J K^-1 mol^-1, calculate ΔS° for the formation of NH_3(g) from its elements.

S° N_2(g) = 192
S° H_2(g) = 131
S° NH_3(g) = 193

N_2(g) + 3H_2(g) → 2NH_3(g)

+516 J K^-1 mol_rxn^-1
+189 J K^-1 mol_rxn^-1
-130 J K^-1 mol_rxn^-1
-199 J K^-1 mol_rxn^-1

Question 06:

Which situation described below, is the one with the greatest entropy?

A piece of copper wire at 298 K
A sample of PCl₃ gas at 298 K in a 1.00 L container
A sample of PCl₃ gas at 298 K in a 5.00 L container
A 10.0 L container of water at 298 K

Question 07:

Which combination of ΔG°, K and rate of reaction, best describes a thermodynamically favored reaction?

ΔG is negative, K > 1 and the rate of reaction is fast
ΔG is negative, K > 1 and the rate of reaction is slow
ΔG is negative, K > 1 and the rate of reaction can be fast or slow
ΔG is positive, K < 1 and the rate of reaction is slow

Question 08:

Sodium nitrate is observed to be highly soluble in water under standard conditions. It is also observed that the temperature of the solution drops quite dramatically during the solution process. Which statement best describes these observations and the change in entropy that occurs?

Entropy must increase, since all exothermic reactions have increases in disorder
Entropy must increases since in order to make ΔG° negative, a positive TΔS° must be subtracted from a positive ΔH°
Entropy must decrease, since all endothermic reactions have decreases in disorder
Entropy must decrease since in order to make ΔG° positive, a positive TΔS° must be subtracted from a negative ΔH°

Question 09:

It is found that the reaction below has ΔG° = -35 kJ mol^-1 and ΔH° = -100 kJ mol^-1.

A_2(g) + 3B_2(g) ⇌ 2AB_3(g)

It is also found that when reactants A_2(g) and B_2(g) are mixed in the stoichiometric ratio under standard conditions, no AB_3(g) is formed. What is the best explanation for this observation?

The reaction is thermodynamically unfavored
Entropy decreases in the forward direction making the reaction unfavored
The forward reaction is not under kinetic control
Despite being thermodynamically favored, the reaction is a very slow one with a high activation energy

Question 10:

The combustion of a solid hydrocarbon in oxygen, leads to the production of two gases and the evolution of energy. The reaction is found to be thermodynamically favored. What is the driving force for this reaction?