Question

Why the dissolution of Ammonium chloride in water an endothermic rxn?

Answer 1

At room temperature (T = 300K), dissolution of ammonium chloride is an endothermic process, because the solution feels colder as the solid NH4Cl dissolves in water and absorbs energy from water to do so. Therefore the enthalpy of dissolution is POSITIVE.
For the overall free energy of the reaction to be negative, the positive change in enthalpy must have to be exceeded by the negative (T*delta S) term. This means that the delta S term must make a large POSITIVE contribution to the overall free energy of the reaction. Intuitively, you can see that the entropy of the system is increased upon dissolution, because a well ordered crystal structure of solid ammonium chloride is disrupted, as well the molecules of NH4Cl dissociates into NH4+ and Cl- ions, and these ions in turn are hydrated by water molecules.
delta G for dissolution of NH4Cl in water at room temperature is NEGATIVE. That is why it readily dissolves.
The dissolution is endothermic, but it is more than made up by the increase in entropy that results from the breakdown of crystal lattice of solid NH4Cl and its subsequent dissociation into NH4+ and Cl- ions

Answer 2

Answer:

Because the energy coming out from NH4+ and Cl- interaction isn't enough to break H+ and OH- completely, it takes some heat energy from surroundings making the reaction endothermic. ... It's quite endothermic.