Final answer:
A reaction with a negative ΔG° is described as spontaneous, indicating an exergonic reaction where the products have less energy than the reactants.
Step-by-step explanation:
A reaction with a negative value of ΔG° can be described as spontaneous. This is because a negative ΔG indicates that the reaction releases free energy, typifying an exergonic reaction.
These reactions are characterized by their products having less free energy than the reactants, which means that they occur without an input of energy once the activation energy barrier has been surpassed. It is important to note that 'spontaneous' in this context does not necessarily mean the reaction occurs quickly; it only means that it is thermodynamically favorable under the given conditions.