When magnesium chloride dissolves in water, its $\Delta H^⦵_\mathrm{sol}$ is negative.

Since $\Delta H^⦵_\mathrm{sol} = \Delta H^⦵_\mathrm{hyd} - \Delta H^⦵_\mathrm{latt},$ this means $-\Delta H^⦵_\mathrm{hyd} > - \Delta H^⦵_\mathrm{latt}.$

Also, the value of $\Delta H^⦵_\mathrm{hyd}$ depends on strength of ionic-dipole bond, the stronger the bond, the more negative the value is.

Therefore, is the total ionic-dipole bond (hydrogen bonds) acting on an ion stronger than the lattice energy holding the ions together, in the case of magnesium chloride (I am aware for some salts, it is the reverse)?

  • 1
    $\begingroup$ It means that the energy of the interactions gained (ion with some number of solvent molecules) is larger than the energy of interactions lost (solvent-solvent in solution and ion-ion in the lattice). It does not mean that a single ion-ion pair interaction in the solid is weaker than a single solute-solvent pair interaction. $\endgroup$
    – Karsten
    Apr 9, 2020 at 2:41


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