Enthalpy change for a reaction (∆H) is given by the expression,
∆H = ∆U + ∆ngRT
Where,
∆U = change in internal energy
∆ng = change in number of moles
For the given reaction,
∆ng = ∑ng (products) – ∑ng (reactants)
= (2 – 2.5) moles
∆ng = –0.5 moles
And,
∆U = –742.7 kJ mol–1
T = 298 K
R = 8.314 × 10–3 kJ mol–1 K –1
Substituting the values in the expression of ∆H:
∆H = (–742.7 kJ mol–1 ) + (–0.5 mol) (298 K) (8.314 × 10–3 kJ mol–1 K –1 )
= –742.7 – 1.2
∆H = –743.9 kJ mol–1