23.2.2—An energy cycle involving enthalpy change

A solution cycle connects the direct dissolution route with an indirect route: lattice dissociation of the solid followed by hydration of the gaseous ions. Hess’s law makes the two routes equivalent.

Choose lattice formation or dissociation consistently. The indirect route is ΔHsol = ΔHlatt(dissociation) + ΣΔHhyd, with signs determined by direction.

For MgCl₂, the cycle contains one Mg²⁺ hydration and two Cl⁻ hydration steps. Omitting the coefficient on chloride gives a wrong energy balance.

Do not use one hydration value for the whole salt or mix lattice formation and dissociation arrows.