[Vicstar]

When NH₄Cl(s) is combined with H₂O(l) in what I assume is a process of dissolving a solute into a solvent, what rationale is there to propose that it is endothermic rather than what I thought it to be exothermic?

I mean, when the ions form, the entropy of the solution increases and hence, would that not imply an exothermic reaction? It is however, endothermic.

[constant thinker]

When ionic solids are dissolved by water, energy is required to break up the crystal structure. Whether or not the solvation of an ionic compound is endothermic or exothermic depends on the lattice energy of the compound.

[Disclaimer] I'm suffering from summer brain drain so I may have messed up a term.

[Yggdrasil]

The change in entropy of a process is unrelated to the change in enthalpy of a process.  For example, the transition from liquid water to water vapor causes an increase in entropy but is an endothermic process.