Question

What effect on melting point would be observed if a very high bp solvent like methylnaphthalene was used in a recrystalization and traces of it were left unevaporated in the purified material?

Answer 1

Melting point and boiling point are one of the colligative properties. When we have high boiling liquid as a contaminant with some crystals the melting point of pure crystals decreases. At the melting point there is a phase change and it is in equilibrium and hence change in Gibss free energy is zero. Change in enthalpy is compensated by a change in entropy.

$\Delta G = \Delta H - T\Delta S$

$\Delta H = T\Delta S$

$T_{melting} = \frac{\Delta H}{\Delta S}$

Thus in the case of pure solid, there is no relative change in entropy. Whereas, when there is an impurity of high boiling point is present in the solid crystals the change in entropy is very high. This means the denominator in the above equation is increasing. and to compensate for this change in enthalpy should be decreased. So the energy required to melt the impure solid is less than that of the pure solid. Thus when there is an impurity of high BP present in the crystals it will decrease the melting point.