Chem enthalpy change

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) .. ΔH = -376 kJ mol⁻¹
MgCl2(aq) + H2O(l) → MgO(s) + 2HCl(aq) .. ΔH = -(-92.3) = +92.3 kJmol⁻¹
H2(g) + (1/2)O2 → H2O(l) .. ΔH = -286 kJ mol⁻¹

Add the above three thermochemical equations together, and cancel 2HCl(aq),MgCl2(aq), H2(g) and H2O(l) on the both sides.
Mg(s) + (1/2)O2(g) → MgO(s)
ΔH = (-376) + 92.3 + (-286) = -569.7 kJ mol⁻¹

Equation of the enthalpy change of reaction between magnesium and hydrochloric acid: Mg+2HCl→MgCl2+H2
Equation of the enthalpy change of reaction between magnesium oxide and hydrochloric acid: MgO+2HCl→MgCl2+H2O
Equation of the enthalpy change of formation of water: H2+ (1/2)O2→H2O
Equation of the enthalpy change of formation of magnesium oxide: Mg+ (1/2)O2→MgO


《我畀唔到你睇個enthalpy cycle 真係唔好意思，我又唔知道你識唔識畫，如果你識畫，你就明白點解下面係咁計。》

△H=(-376)+(-286)-(-92.3)=-569.7 kJmol^1

2013-07-09 22:21:51 補充：
△H is the enthalpy change of formation of magnesium oxide