The enthalpy of ionization H_ionization is the energy required to remove an electron from an atom or ion in the gas phase. In this case, we are given the ionization potential of Mg 735 kJ/mol and the electron affinity of Mg -195 kJ/mol .The ionization potential represents the energy required to remove an electron from Mg, while the electron affinity represents the energy released when an electron is added to Mg. Since we are looking for the energy required to remove an electron from Mg, we can ignore the electron affinity of Mg in this case.Therefore, the enthalpy of ionization for the reaction Mg g Mg g + e is equal to the ionization potential of Mg:H_ionization = 735 kJ/mol