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)
To understand why the mixing of sulfuric acid (H2SO4) and water is exothermic, we can examine the molecular level interactions involved.
When sulfuric acid is dissolved in water, it dissociates into ions, releasing energy in the process. The H2SO4 molecule donates a proton (H+) to a water molecule, forming a hydronium ion (H3O+), and the remaining sulfate ion (SO4(-2)). This dissociation process releases energy due to the formation of new chemical bonds between H3O+ and SO4(-2). Since breaking the bonds in H2SO4 requires less energy than the energy released from forming the new bonds, the overall process is exothermic.
On the other hand, when solid ammonium chloride (NH4Cl) is mixed with water, it dissolves by breaking up into its constituent ions, ammonium ions (NH4+) and chloride ions (Cl-). However, in this case, the dissociation process requires an input of energy. This is because the bond between NH4+ and Cl- is relatively strong and breaking this bond requires energy. As a result, the dissolution of ammonium chloride in water is endothermic, meaning it absorbs heat from the surroundings.
In summary, the mixing of sulfuric acid with water is exothermic because the dissociation of the sulfuric acid molecule into H3O+ and SO4(-2) releases more energy than it takes to break the initial bonds. On the other hand, the mixing of solid ammonium chloride with water is endothermic because the dissolution process requires an input of energy to break the strong bond between NH4+ and Cl-.