+ Enthalpy of Reaction: State and Stoichiometry Use the data below to answer the questions. Substance AH (kJ/mol) C(g) CF.(g) CH(g) 718.4 -679.9 -74.8 217.94 -268.61 H(g) HF(g) Keep in mind that the enthalpy of formation of an element in its standard state is zero. Calculate the enthalpy change, AH, for the "expansion" of methane; CH(g) C(g) + 4H(g) Calculate the enthalpy change, AH, for the reverse of the formation of methane CH(g) +C(s) + 2H(g)