Advanced Placement (AP) Chemistry Practice Exam

What does a positive ΔG indicate about a reaction?

1. The reaction is thermodynamically favored

2. The reaction is at equilibrium

3. The reaction is thermodynamically unfavored

4. The reaction produces heat

The correct answer is: The reaction is thermodynamically unfavored

A positive ΔG, or change in Gibbs free energy, signifies that the reaction is thermodynamically unfavored under the given conditions. This is because a positive value of ΔG indicates that the free energy of the products is higher than that of the reactants, which means the reaction does not spontaneously proceed in the forward direction. Instead, for a reaction to occur spontaneously, it would have to have a negative ΔG, reflecting a release of free energy which can be harnessed for work. In the context of equilibrium, a reaction at equilibrium would have a ΔG of zero, meaning there is no net change occurring in the concentrations of reactants and products. Reactions that produce heat and are exothermic typically have a negative ΔH (enthalpy change) and can contribute to a negative ΔG, but a positive ΔG does not imply heat production. Hence, the accurate interpretation of a positive ΔG is that the reaction is not favored thermodynamically.