Why are most exothermic reactions spontaneous?

Spontaneity in chemical reactions is often determined by the change in Gibbs free energy (ΔG). For a reaction to be spontaneous at constant temperature and pressure, ΔG must be negative. The relationship between the change in enthalpy (ΔH), temperature (T), and the change in entropy (ΔS) is given by the equation:

ΔG = ΔH - TΔS

In exothermic reactions, ΔH is negative because the system releases energy. When the energy release is substantial (indicated by a large negative ΔH), it can more than compensate for any potential increase in entropy (ΔS), especially at lower temperatures. This relationship indicates that a significant release of energy tends to drive ΔG to be negative, thus favoring spontaneity.

In summary, the large negative ΔH associated with most exothermic reactions contributes to making the free energy change negative, enhancing the likelihood of the reaction occurring spontaneously.