FGHS AP CHEMISTRY QUIZ #15 Entropy and Free Energy

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°

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

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

• Mg2+(aq) + 2OH-(aq) → Mg(OH)2(s)

• CaO(s) + CO2(g) → CaCO3(s)

• H2O(l) → H2O(s)

• PCl5(g) → PCl3(g) + Cl2(g)

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

• 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

Given the absolute entropies below, all given in units of J K-1 mol-1, calculate ΔS° for the formation of NH3(g) from its elements.

S° N2(g) = 192

S° H2(g) = 131

S° NH3(g) = 193

N2(g) + 3H2(g) → 2NH3(g)

• +516 J K-1 molrxn-1

• -1189 J K-1 molrxn-1

• -130 J K-1 molrxn-1

• -199 J K-1 molrxn-1

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

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

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°

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

A2(g) + 3B2(g) ⇌ 2AB3(g)

It is also found that when reactants A2(g) and B2(g) are mixed in the stoichiometric ratio under standard conditions, no AB3(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

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?

• Enthalpy

• Entropy

• Both enthalpy and entropy

• Gibb’s Free Energy