Question

Given the following data and using Hess’ Law, Calculate change in Enthalpy for the reaction Overall reaction : NO(g) + O (g) ------? NO2(g)

2O3(g) ------? 3O2(g) ?H = - 427 kJ

O2(g) ------? 2O(g) ?H = 495 kJ

NO(g) + O3 (g) ------? NO2 (g) + O2 (g) ?H = - 199 kJ

Answer 1

NO(g) + O (g) → NO2(g)   ; ∆H = ?                -----------------(1)

2O3(g) → 3O2(g)   ;∆H₁ = - 427 kJ                     -----------(2)

O2(g) → 2O(g)   ;∆H₂ = 495 kJ                         ------------(3)

NO(g) + O3 (g) → NO2 (g) + O2 (g)   ; ∆H₃ = - 199 kJ         -----(4)

Eqn(1) can be obtained from the remaining equations as follows:

Eqn(1) = [(1/2)xEqn(2) ] + [ (1/2)xEqn(3) ]+Eqn(4)

So ∆H = [(1/2)x∆H₁ ]+[(1/2)x∆H₂ ]+∆H₃

          = [(1/2)x(-427)]+[(1/2)x(495)]+(-199) kJ

          = -165 kJ