What does it indicate when ΔG is negative?

When ΔG (Gibbs free energy change) is negative, it indicates that the reaction is spontaneous in the forward direction. A negative ΔG means that the products have a lower free energy than the reactants, suggesting that energy is released as the reaction proceeds. This release of energy often makes the reaction favorable and able to occur without needing an external input of energy.

In the context of thermodynamics, a spontaneous reaction is one that can occur naturally under the given conditions, driven by the tendency to reach a more stable state, which corresponds to a lower Gibbs free energy. Therefore, a negative ΔG signifies not just the favorability of the reaction but also implies that the equilibrium will shift towards the products when the reaction is allowed to proceed.

The other options, while related to thermodynamic principles, do not accurately describe the significance of a negative ΔG. For instance, equilibrium is characterized by a ΔG of zero, and a non-spontaneous reaction would have a positive ΔG. Similarly, suggesting that the reaction requires more energy to proceed contradicts the meaning of ΔG being negative, as it indicates favorability rather than a need for additional energy input.