How is enthalpy used to predict whether a reaction is endothermic or exothermic? a. when the enthalpy of the reactants is higher than the enthalpy of the products, the reaction is endothermic. b. when the enthalpy of the products is higher than the enthalpy of the reactants, the reaction is exothermic. c. when the enthalpy change of the reaction is positive, the reaction is exothermic. d. when the enthalpy change of the reaction is positive, the reaction is endothermic.