[ddd4eva]

well the problem is that i am totally stuck with enthalpy change. i understand what it is but i dont understand the equations or cycles. can anyone help me  
thanks
xx

[dolphinsiu]

Some reactions cannot be conducted directly.So we need another feasible method to make the product. For example,
Mg(s) + 2H2O(l) --> Mg (OH)2(aq) + H2(g)

> Mg (s) --> Mg (g)
> Mg (g) ---> Mg+ (aq) + e-
> Mg+ (aq) ---> Mg2+ (aq) + e-

>>2H2O(l) + 2e- -- > H2(g) + 2OH-(aq)

Mg2+(aq) + 2e- + 2H2O(l) ---> Mg2+(aq)+ H2(g) + 2OH-(aq)
---> Mg (OH)2(aq) + H2(g)

Equlibrium cycle can be relinked by you!!!