68. Which one of the following about the standard free energy change ( Δ𝐺∘′) and the equilibrium
constant ( 𝐾𝑒𝑞 ) of an exergonic reaction, at pH 7.0 , is TRUE?
(A) Δ𝐺∘′ is positive and 𝐾𝑒𝑞 is less than one
(B) Δ𝐺∘′ is negative and 𝐾𝑒𝑞 is less than one
(C) Δ𝐺∘′ is negative and 𝐾𝑒𝑞 is greater than one
(D) Δ𝐺∘′ is positive and 𝐾𝑒𝑞 is greater than one
Understanding ΔG°’ and Keq in Exergonic Reactions at pH 7.0
Exergonic reactions release free energy and are spontaneous under standard conditions. For these reactions at pH 7.0, the standard free energy change (ΔG°’) is negative, and the equilibrium constant (Keq) exceeds 1, making option (C) correct.
Correct Answer
ΔG°’ is negative and Keq is greater than one (Option C).
This follows from the equation ΔG°’ = -RT ln Keq, where a negative ΔG°’ yields ln Keq > 0, so Keq > 1. At pH 7.0, ΔG°’ uses biochemical standard states (1 M reactants/products except [H+] = 10^{-7} M), but the relationship holds for exergonic processes favoring products.
Option Explanations
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(A) ΔG°’ positive, Keq < 1: Incorrect. Positive ΔG°’ defines endergonic reactions (non-spontaneous), with Keq < 1 favoring reactants.
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(B) ΔG°’ negative, Keq < 1: Incorrect. Negative ΔG°’ requires Keq > 1; Keq < 1 would imply positive ΔG°’.
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(C) ΔG°’ negative, Keq > 1: Correct. Matches exergonic traits: energy-releasing and product-favored at equilibrium.
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(D) ΔG°’ positive, Keq > 1: Incorrect. Positive ΔG°’ pairs with Keq < 1, not > 1.
Key Relationship Equation
The core formula is ΔG∘′=−RTlnKeq, where R is the gas constant and T is temperature in Kelvin. For exergonic reactions, this ensures products dominate at equilibrium.
