Why is the reaction of barium hydroxide octahydrate crystals with dry ammonium chloride an endothermic reaction?
A better question might be why is it spontaneous if it is an endothermic reaction.
The reaction may be summarized as follows:
Now, as you know, this reaction is spontaneous, but as it proceeds it extracts energy from the surroundings; so much so that the reaction vessel becomes visibly icy. Why should the reaction be spontaneous when bonds are being broken? Because the reaction is entropy driven.
Gaseous ammonia and aqueous barium chloride supply a thermodynamic driving force to the reaction by their increased entropy, by the greater potential for disorder. I do not have the thermodynamic parameters to hand but