Question

Which trihalide is not hydrolysed by water?

• A. $NF_{3}$
• B. $NCl_{3}$
• C. $PCl_{3}$
• D. $AsCl_{3}$

Answer 1

Answer: (A)

In the first stage of hydrolysis, an extra bond is formed by water molecule. While chlorine and the group 15 elements (except nitrogen) can expand their octet by using vacant d-orbitals of the valence shell, F and N cannot. As a result, $NF_{3}$ is extremely stable.

Trihalides of P, As, Sb and Bi hydrolyse readily.

$NF_{3}+H_{2}O\left(\right.l\left.\right) \rightarrow $ No hydrolysis

$NCl_{3}+3H_{2}O\left(\right.l\left.\right) \rightarrow NH_{3}+3HOCl$

$PCl_{3}+3H_{2}O\left(\right.l\left.\right) \rightarrow H_{3}PO_{3}+3HCl$

$2AsCl_{3}+3H_{2}O \rightarrow 6HCl+As_{2}O_{3}$

Notes: A. $SbCl_{3}$ and $BiCl_{3}$ also hydrolyze to form basic oxochlorides.

$BiCl_{2}+H_{2}O\rightleftharpoons BiOCl+2HCl$

B. Hydrolysis of $NCl_{3}$ is thought to proceed by stepwise replacement of chlorine, the lone pair on N-atom bonding to hydrogen atom of a water molecule



$NHCl_{2}+H_{2}O \rightarrow NH_{2}Cl+HClO$

$NH_{2}Cl+H_{2}O \rightarrow NH_{3}+HClO$