If a reaction is exothermic, then ________.

A) ΔS° < 0
B) ΔS° > 0
C) ΔH° < 0
D) ΔH° > 0
E) both B and D

Exothermic reactions release heat energy, which means that the enthalpy change (ΔH°) is negative. Therefore, choice C is the correct answer.

Choices A and B refer to the entropy change, which is related to the degree of disorder in a system. Although entropy may increase or decrease during a reaction, it is not directly related to the exothermic or endothermic nature of the reaction.

Choice D is the opposite of the correct answer. Endothermic reactions absorb heat energy, so ΔH° is positive, not negative.

Choice E is partially correct, as B is a valid answer for the entropy change, but D is not correct for ΔH°.