Which statement is true? a. The enthalpy for the formation for two moles of a substance is half of the enthalpy for the formation of one mole of the substance. b. The enthalpy of a reaction is the heat of reaction at constant volume. c. The enthalpy of a reaction is the negative of the enthalpy of the reverse reaction. d. The enthalpy of a reaction and the enthalpy of the reverse reaction are equal.