Which one of these equations for the calculation of the lattice energy of sodium oxide, ${\mathrm{Na}}_2\mathrm{O}$ is correct?

Explain why a cation with a smaller ionic radius has a higher charge density.

Which one of the following ions will be the best polariser of the large nitrate ion? Explain your answer.

${\mathrm{Cs}}^{+} {\mathrm{Li}}^{+} {\mathrm{Na}}^{+} {\mathrm{K}}^{+}$

Which one of these ions will be most polarised by a ${\mathrm{Mg}}^{2+}$ ion? Explain your answer.

${\mathrm{Br}}^{-} {\mathrm{Cl}}^{-} {\mathrm{F}}^{-} {\mathrm{I}}^{-}$

Write an equation to represent the enthalpy change of solution of potassium sulfate.

Write an equation to represent the enthalpy change of solution of zinc chloride.

The enthalpies of solution of some metal halides are given below. What do these values tell you about the relative solubilities of these four compounds?
sodium chloride, $∆{\mathrm{H}}_{\mathrm{sol}}^{⊝} =+3.9 \mathrm{kJ} {\mathrm{mol}}^{-1}$
silver chloride, $∆{\mathrm{H}}_{\mathrm{sol}}^{⊝} =+65.7 \mathrm{kJ} {\mathrm{mol}}^{-1}$
sodium bromide, $∆{\mathrm{H}}_{\mathrm{sol}}^{⊝} =-0.6 \mathrm{kJ} {\mathrm{mol}}^{-1}$
silver bromide, $∆{\mathrm{H}}_{\mathrm{sol}}^{⊝} =+84.5 \mathrm{kJ} {\mathrm{mol}}^{-1}$

Why is the enthalpy change of hydration always exothermic?