explain how the dissolving of sodium hydroxide can be exothermic, whereas for ammonium chloride the process is endothermic.

Question

when a solid substance dissolves, two processes are of prime importance: the breaking way of particle from the solid and the joining of solute and solvent particles. in the terms of these two process, explain how the dissolving of sodium hydroxide can be exothermic, whereas for ammonium chloride the process is endothermic.

Answer 1

Whether dissolution is endothermic or exothermic will depend on the lattice energy of the salt and the hydration energy when water surrounds the ions.  Or as you put it, breaking the particle from the solid (lattice energy) and joining solute and solvent particles (hydration energy).  If exothermic, as with NaOH, this suggests that the lattice energy is greater than the hydration energy, and vice versa for NH4Cl.  That is, for NaOH, it takes more energy to break the lattice apart than what is generated by the water surrounding the ions.  The ∆E is negative, and this is exothermic.