4.16    use average bond energies to calculate the enthalpy change during a simple chemical reaction.

In a chemical reaction, bonds in the reactants first break and then bonds form to make the products

e.g.      Calculate the energy change for the following reaction: N2(g) + 3H2(g) ⇌ 2NH3(g)

            (the bond energies are N≡N 945 kJ/mol, H-H 436 kJ/mol, N-H 391 kJ/mol)

∆H                   = sum of all bonds broken - sum of all bonds formed

Bonds broken   = 1 x N≡N + 3 x H-H

= (1 x 945) + (3 x 436)

= 2253 kJ

Bonds formed   = 6 x N-H (each NH3 has 3 N-H bonds and there are two molecules in the equation)

= 6 x 391

= 2346 kJ

∆H                    = 2253 - 2346

= -93 kJ/mol