What rationale is there for the mixing of sulfuric acid aqueous and water being exothermic but the mixing of solid ammonium chloride with water to be endothermic?
Solid NH4CL dissociates in water, and the dissociation process requires some energy, because the NH4+ - Cl- ionic bond is a strong one.
H2SO4 dissociates also, but the bond between H+ and HSO4- is weaker than the bond between H+ and H2O. Similarly, the HSO4- ion's bond between SO4(-2) and H+ is weaker than the bond between H+ and H2O. Therefore, both the H2SO4 and the HSO4- break up exothermically, forming H3O+ and SO4(-2)
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