2 Fe(s) + 6 HCl(aq) →2 FeCl3(aq) + 3 H2(g) ΔHa Fe2O3(s) + 6 HCl(aq) → 2 FeCI3(aq) + 3 H2O(l) ΔHb 2 H2(g) + O2(g) → 2 H2O(l) ΔHc Show how these equations must be summed together according to Hess's Law to determine ΔH for the combustion of iron (target equation shown below). Also show clearly how the ΔH values of each of the three reactions must be manipulated to determine the enthalpy of combustion of iron. 4 Fe(s) + 3 O2(g) → 2 Fe2O3(s) ΔH = ?

Answer : The enthalpy of combustion of iron is,

Explanation :

According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

The combustion reaction of iron is:

The intermediate balanced chemical reaction will be,

(1) ;    

(2) ;   

(3) ;   

Now we are multiplying reaction 1 by 2, reaction 3 by 3 and reverse reaction of 2 by 2 and then adding all the equations, we get:

(1) ;    

(2) ;   

(3) ;   

The expression for enthalpy of combustion of iron will be,

Therefore, the enthalpy of combustion of iron is,