• The standard enthalpy change of hydration (ΔH_hyd^?) can be calculated by constructing?energy cycles?and applying Hess’s law

Energy cycle involving enthalpy change of solution, lattice energy, and enthalpy change?of hydration

• The energy cycle shows that there are two routes to go from the ionic lattice to the hydrated ions in an aqueous solution:
  • Route 1: going from ionic solid → ions in aqueous solution (this is the?direct route)

ΔH_sol^?= Enthalpy of solution

• Route 2: going from ionic lattice →? gaseous ions? →? ions in aqueous solution (this is the?indirect route)

-ΔH_latt^??+?ΔH_hyd^??= reverse lattice enthalpy + hydration enthalpies

Lattice enthalpy usually means Lattice?formation?enthalpy, in other words bond forming. If we are breaking the lattice then this is reversing the enthalpy change so a negative sign is added in front of the term (alternatively it is called lattice?dissociation?enthalpy)

• According to?Hess’s law,?the enthalpy change for both routes is the same, such that:

ΔH_sol^??= -ΔH_latt^??+?ΔH_hyd^?

ΔH_hyd^??=?ΔH_sol^??+ ΔH_latt^?

• Each ion will have its own enthalpy change of hydration, ΔH_hyd^?, which will need to be taken into account during calculations
  • The total ΔH_hyd^??is found by adding the? ΔH_hyd^??values of both anions and cations together

Worked example: Constructing an energy cycle and energy level diagram of KCl

Answer

Energy cycle:

Energy level diagram:

Worked example: Constructing an energy cycle and energy level diagram of MgCl2

Answer

Energy cycle:

Energy level diagram: