The standard heat of formation of ${\text{NO}}_{\text{2}}\left(\text{g}\right)$ and ${\text{N}}_{\text{2}}{\text{O}}_{\text{4}}\left(\text{g}\right)$ are 8.0 and $\text{4}{\text{.0 kcal mol}}^{\text{–1}}$ respectively. The heat of dimerisation of ${\text{NO}}_{\text{2}}$ in kcal is

Heat of Dimerisation of NO₂

Given the reactions and enthalpy changes:

Reversing and multiplying by 2:

2 NO₂ → N₂ + 2 O₂; ΔH = -16 kcal

Also:

Adding these, we get the formation of dinitrogen tetroxide from nitrogen dioxide:

2 NO₂ → N₂O₄; ΔH = -12 kcal

Heat of dimerisation of NO₂ = -12 kcal