How to Determine Spontaneous Reactions and Coupled Reactions

Question 9

For the reaction T → V, ΔG = 125. Which of the following reactions could be coupled with this reaction?

Answer:

Option a) and option d) can be coupled with reaction T → V.

For the reaction T → V, ΔG = 125. The ΔG°’ of a coupled reaction is equal to the sum of the ΔG°’ values of the individual reactions. The reaction with the negative ΔG°’ will be coupled to the reaction with the positive ΔG°’ to give an overall negative ΔG°’ for the coupled reactions.

ΔG = ΔG° + RT ln Q. RT ln Q = ΔG – ΔG° = –RT ln Keq

Here, we are given: ΔG°1 = 125. We must find which reaction is spontaneous to be coupled with the reaction T → V, which is an endergonic reaction (∆G > 0). If we check the ΔG values of the given reactions, only the reaction with ΔG value less than 125 can be coupled with T → V to make the coupled reaction spontaneous.

a) C → D, ΔG = –150. This is a spontaneous reaction as ΔG < 0, so it can be coupled with T → V. b) Y → Z, ΔG = 200. This is a non-spontaneous reaction as ΔG > 0, so it cannot be coupled with T → V. c) S → T, ΔG = 150. This is a non-spontaneous reaction as ΔG > 0, so it cannot be coupled with T → V. d) A → B, ΔG = –100. This is a spontaneous reaction as ΔG < 0, so it can be coupled with T → V. Therefore, option a) and option d) can be coupled with reaction T → V.

Question 10

For P → Q, ΔG = 75. Which of the following is true?

Answer:

The reaction is endergonic, it requires energy.

For P → Q, ΔG = 75. If the ΔG°’ value of a reaction is positive, the reaction is non-spontaneous as written under standard conditions. ΔG°’ is a measure of the maximum amount of work that can be obtained from a reaction. If ΔG°’ is negative, the reaction is spontaneous. This means that the reaction can occur under standard conditions as written without needing any energy input from the outside.

If ΔG°’ is positive, then the reaction is non-spontaneous. 75 is greater than zero, so the reaction is non-spontaneous.