Hydrolysis of alkali and alkaline earth metals

Question

Which of the following cations are not hydrolysed in aqueous solution?
1) $ce{Mg^2+}$ 2) $ce{Ca^2+}$ 3) $ce{Na+}$ 4) $ce{K+}$

I thought nothing would undergo hydrolysis, so all the options would be correct.
But my answer key says only $ce{Na+}$ and $ce{K+}$ wouldn't hydrolyse.
Why is the case?

Maurice · Answer

Hydrolysis is a reaction with water, producing two new ions. Doubly charged ions, like $ce{Ca^{2+}}$ or $ce{Mg^{2+}}$ may react with water and produce complex ions like :$$ce{Ca^{2+} + H2O -> [Ca(OH)]^+ + H+}$$ $$ce{Mg^{2+} + H2O -> [Mg(OH)]^+ + H+}$$ These complex ions are well known in chemistry. Their existences explain why poorly soluble calcium salts like $ce{CaSO_4}$ are much more soluble in water than what could be calculated from their solubility products. The solubility product gives only the value $ce{[Ca^{2+}]}$ and does not give the concentration of the hydrated complex $ce{[Ca(OH)]^+}$.
This sort of reaction is impossible with singly charged ions like $ce{Na^{+}}$ or $ce{K^{+}}$, because these alcaline ions cannot fix a $ce{OH}$ group without being immediately dissociated.

Hydrolysis of alkali and alkaline earth metals

One Answer

Add your own answers!

Ask a Question