When is a chemical reaction considered non-spontaneous?

A chemical reaction is considered non-spontaneous when the change in Gibbs free energy (ΔG) is positive. This concept is rooted in thermodynamics, where ΔG represents the balance between enthalpy and entropy changes in a system.

When ΔG is positive, it indicates that the products of the reaction have higher free energy than the reactants, meaning that the reaction will not occur without the input of external energy. In practical terms, if a reaction is non-spontaneous, it will not proceed on its own under the given conditions, and energy must be supplied to drive the reaction forward.

The other considerations mentioned in the options provide context for spontaneity but do not define it as clearly. For instance, a ΔG of zero indicates a system in equilibrium, where no net reaction occurs, and thus it is not classified as either spontaneous or non-spontaneous. An increase in entropy, while often associated with spontaneous processes, does not classify a reaction as spontaneous on its own, since an increase in entropy can happen in reactions with a negative ΔG as well. Lastly, the release of energy characterizes exothermic reactions, which can be spontaneous or non-spontaneous depending on the ΔG; thus, energy release does