Standard enthalpy of reaction is defined by the difference between the products whole enthalpy and the reagents whole enthalpy. It is known that an element's (by itself or as a diatomic gas) enthalpy of formation is 0, turning out into an equality between the enthalpy of formation and the products whole enthapy. Thus, the reactions that accomplish the given questions are 1, 2, 3 and 4, since the reagents are just the elements sodium, chlorine, carbon and oxygen.
