Meaning of standard Gibbs free energy: Which description best captures what ΔG° represents for a chemical reaction?

Introduction / Context:
ΔG° is a thermodynamic state function that predicts spontaneity under standard conditions and links directly to the equilibrium constant Keq by ΔG° = −RT ln Keq. It is not a kinetic parameter and not a vague “residual energy.”

Given Data / Assumptions:

• Standard state: 1 bar pressure, 1 M activities for solutes (idealized).
• Temperature typically 298 K unless otherwise specified.
• Equilibrium composition depends on Keq, which depends on ΔG°.

Concept / Approach:
Gibbs free energy is a state function; ΔG° compares the standard molar Gibbs energies of products and reactants, weighted by stoichiometry. Negative ΔG° favors products (Keq > 1); positive ΔG° favors reactants (Keq < 1). It is distinct from activation energy (Ea), which governs rate.

Step-by-Step Solution:
Recall definition: ΔG° = Σν G°(products) − Σν G°(reactants). Relate to equilibrium: ΔG° = −RT ln Keq. Interpretation: sign/magnitude indicate position of equilibrium under standard conditions. Thus, “difference in standard Gibbs energies” best captures the meaning.

Verification / Alternative check:
Thermodynamic tables list standard Gibbs energies of formation; summation gives ΔG° and predicts Keq consistent with experiments.

Why Other Options Are Wrong:
“Residual energy” is undefined; “energy required per mole regardless of conditions” ignores dependence on standard state; activation energy is kinetic, not ΔG°.

Common Pitfalls:
Confusing ΔG° with ΔG (which depends on Q) or with the activation barrier; forgetting that ΔG° is condition-specific (standard state).

Final Answer:
Difference in standard Gibbs energies of products and reactants (related to Keq).