The synthesis of ammonia gives off heat:

(g) + (g)

H (298) = -22.1 kcal per 2 moles of

Ammonia molecules absorb heat when they dissociate.

The energy released in forming one NN and three
HH bonds is not enough to break apart six NH bonds in the two ammonia molecules.

Raising the temperature therefore should favor dissociation, because the extra thermal energy is put to work pulling molecules apart.

This is borne out by the measured values of for the reaction.