What is the sign of delta G for non-spontaneous reactions?

For non-spontaneous reactions, the sign of delta G (the change in Gibbs free energy) is positive. This indicates that the reaction requires energy input to proceed, meaning it will not occur on its own under standard conditions. In thermodynamic terms, a positive delta G signifies that the products of the reaction have a higher free energy than the reactants, which makes the forward reaction unfavorable without an external energy source.

In contrast, spontaneous reactions, which can occur without additional energy input, typically have a negative delta G, signifying that the products are at a lower free energy state compared to the reactants. A delta G of zero indicates that the system is at equilibrium, meaning there is no net change, and the rates of the forward and reverse reactions are equal. An undefined value does not apply in this context, as delta G will always have a defined numerical value based on the conditions of the reaction.