Enthalpy Change

The second ionisation energy is the energy that is required to remove the second electron from a positive charge ion. The second ionization energy is greater than the first ionization energy. For example: Li → Li+ + e- and second ionization energy is Li+ → Li2+ + e-

According to the question 5-1 should be done to get the second ionisation energy.

M (g) → M2+ +2e- - M (g) → M+ +e-

Second ionisation energy is the amount of energy required to take out an electron from the monopositive cation.
.......(5)
....(3)
On subtracting eq(iii) form e (v) we get,