Final answer:
The difference in free energy between reactants and products determines the reaction's spontaneity, with ΔStotal being the ultimate criterion for spontaneous change.
Step-by-step explanation:
The difference in free energy between reactants and products influences the overall spontaneity of the reaction. The standard free energy change (ΔG) tells us if the reaction will occur spontaneously. A negative ΔG indicates that the reaction will happen without external energy input - it is exergonic. Conversely, a positive ΔG means the reaction is non-spontaneous and endergonic, requiring an input of energy.
The enthalpy (ΔH) and entropy (ΔS) changes are also crucial factors that determine the spontaneity of a reaction. However, it is the total entropy change (ΔStotal) of the universe, as expressed indirectly through ΔG (ΔG = ΔH - TΔS), that is the ultimate criterion for a reaction's spontaneity. Regardless of whether a reaction is enthalpically driven, due to heat content differences, or entropically driven, due to disorder changes, ΔStotal governs the thermodynamic favorability.