Why Correct: A reaction is spontaneous when Delta G (Gibbs free energy change) is negative, as per the equation Delta G = Delta H - T Delta S, where negative Delta G indicates the reaction can proceed without external energy input.
Distractor Analysis: Negative Delta H (exothermic) favors spontaneity but alone doesn't guarantee it, as entropy (Delta S) and temperature also matter. Positive Delta S (increase in disorder) favors spontaneity but alone doesn't guarantee it, as enthalpy also matters. Negative Delta S (decrease in disorder) opposes spontaneity unless compensated by strongly negative Delta H.
Takeaway: The spontaneity condition Delta G 0 indicating non-spontaneity.