For a reaction to be spontaneous, which of the following must be true about ΔG?

• A. It must be positive
• B. It must be negative
• C. It must be zero
• D. It may be positive or zero

Answer: (B)

For a reaction to be spontaneous, the change in Gibbs free energy (ΔG) must be negative. This is a crucial concept in thermodynamics that indicates the favorability of a reaction. When ΔG is negative, it means that the reaction can proceed without the input of additional energy, and it can lead to an increase in the entropy of the universe, thus favoring spontaneous processes.

A negative ΔG reflects that the products of the reaction have lower free energy compared to the reactants, contributing to the thermodynamic stability of the system. Spontaneous reactions often lead to the release of energy, which can manifest as heat or work.

In contrast, if ΔG is positive, the reaction is not spontaneous and requires energy input to proceed. If ΔG is zero, the reaction is at equilibrium, meaning there is no net change in the concentrations of reactants and products over time, and thus it is also not spontaneous in either direction. Therefore, the requirement for spontaneity is that ΔG must be negative, underscoring the relationship between free energy and the directionality of chemical processes.