The first reaction in the Ostwald process for the production of nitric acid involves the combustion of ammonia 4NH3(g)+5O2(g)=4NO(g)+6H2O(g)

a) Estimate ΔH^o (in kJ) for this reaction using average bond energies.

b) Calculate ΔH^o (in kJ) for this reaction using standard heats of formation.

c) Briefly explain why the value for ΔH^o, calculated using average bond energies, is only considered to be an estimate of the standard enthalpy change for the reaction

Explanation:

Given that;

The chemical equation for the reaction is:

↔

a).

Using average bond energies;

For Bonds broken, we have:

4 × 3 N - H = 12 mol × 391 kJ/mol = 4692 kJ

5 × O = O = 5 mol × 498   kJ/mol =  2490 kJ    

            = 7182  kJ

For Bonds formed, we have:

4 × N = O = 4 mol × - 607 kJ/mol = -2428 kJ

6 × 2 O - H = 12 mol × - 467 kJ/mol = - 5604 kJ

               = - 8032 kJ

Therefore;

b) Using Standard heats of formation;

c) The value of ΔH^o, calculated using average bond energies, is only considered to be an estimate of the standard enthalpy change for the reaction because there is a premise in the calculation that all the bonds broken in the reactant and all bonds are formed in products. Meanwhile, only little bonds are broken, and only little bonds are formed during the reaction.