請求高手幫忙解答...enthalpy change,,,

In most chemical and physical changes there is an associated energy change, and it is generally possible to measure the value of change even though it may not be possible to determine the absolute energy levels before and after the change. The energy change may show up as heat evolved or absorbted, as electrical energy, as light energy, or work done in expansion or contraction against or by an external pressure.
In general,
change in energy = heat absorbed or evolved - work done in expansion or contraction + other work done

Hess's law states that the enthalpy change in a chemical reaction is the same whether the reaction is brought about in one stage or through intermediate stages. Thus the enthalpy change in a reaction A to C is the same whether the reaction takes place in one stage as A －＞ C or in two stages as
A －＞ B －＞ C. This is a particular application of the law of conservation of energy.

CaCO3(s)　－＞　CaO(s) + CO2(g)　　ΔＨ＝ＥＣ　－－－（＊）
CaCO3(s) + 2H+　－＞　Ca2+ + CO2(g) + H2O 　　ΔＨ＝ＥＣ１　－－－（１）
CaO(s) + 2H+　－＞　Ca2+ + H2O　　ΔＨ＝ＥＣ２

Ca2+ + H2O　－＞　CaO(s) + 2H+　　ΔＨ＝－ＥＣ２　－－－（２）

By adding the expression of (1) and (2), we will get :
CaCO3(s) + 2H+ + Ca2+ + H2O　－＞　Ca2+ + CO2(g) + H2O + CaO(s) + 2H+　　ΔＨ＝ＥＣ１－ＥＣ２　
as 2H+ , Ca2+ , H2O these three components are appear on both sides of the reaction, we can subtract them on both sides, which results :
CaCO3(s)　－＞　CaO(s) + CO2(g)　　ΔＨ＝ＥＣ１－ＥＣ２　－－－（３）

By comparing the expression of (*) and (3), we know that EC = EC1 - EC2

=================================================

Mg(s) + H2O(l)　－＞　MgO(s) + H2(g)　　ΔＨ＝ＥＣ　－－－（４）
Mg(s) + 2H+　－＞　Mg2+ + H2(g) 　　ΔＨ＝ＥＣ３　－－－（５）
MgO(s) + 2H+　－＞　Mg2+ + H2O　　ΔＨ＝ＥＣ４

Mg2+ + H2O　－＞　MgO(s) + 2H+　　ΔＨ＝－ＥＣ４　－－－（６）

By adding the expression of (5) and (6), we will get :
Mg(s) + 2H+ + Mg2+ + H2O(l)　－＞　Mg2+ + H2(g) + MgO(s) + 2H+　　ΔＨ＝ＥＣ３－ＥＣ４　
as 2H+ , Mg2+ these three components are appear on both sides of the reaction, we can subtract them on both sides, which results :
Mg(s) + H2O(l)　－＞　MgO(s) + H2(g)　　ΔＨ＝ＥＣ３－ＥＣ４　－－－（７）

By comparing the expression of (4) and (7), we know that EC = EC3 - EC4