Hess's Law

Correct, but there are few things for reminder:
1. The stoichiometric coefficient of O2 is 3/2, don't write 1.5. It is not acceptable in exam.
2. Enthalpy change of formation of CO2 and that of formation of H2O are required because CO2 and H2O are formed in the routes the reaction goes through,
i.e. delta H1 = enthalpy change of Mg with HCl + enthalpy change of formation of CO2 + enthalpy change of formation of H2O

2012-01-17 22:12:50 補充：
By the way, the equation given is not correct.
Since
total enthalpy change of clockwise direction = total enthalpy change in anti-clockwise direction,
we have delta H1 = delta Hf + delta H2
delta Hf = delta H1 - delta H2