Question

What is observed when excess of dilute $NaOH$ solution added to an aqueous solution of aluminium chloride?

• A. A white precipitate which dissolves in excess $NaOH$
• B. A permanent white precipitate is formed immediately
• C. A green precipitate forms which turns red on standing in air
• D. No change is noted at first; a white precipitate is formed on standing

Answer 1

Answer: (A)

A white precipitate which dissolves in excess $NaOH$

This reaction may proceed mostly in one direction due to solubility. eg: aluminum hydroxide is not very soluble (0.0001 g/100 mL at 20${}^{o}C$). so the overall reaction will be :

$3NaOH+AlC{l}_3⟶3NaCl+Al(OH{)}_3$

$NaOH\leftrightharpoons N{a}^{+}+O{H}^{-}$

$AlC{l}_3\leftrightharpoons A{l}^{3+}+3C{l}^{-}$

$N{a}^{+}+C{l}^{-}\leftrightharpoons NaCl$

$A{l}^{3+}+3O{H}^{-}\leftrightharpoons Al(OH{)}_3$

$2H_{2}O\leftrightharpoons H_3{O}^{+}+O{H}^{-}$

$H_3{O}^{+}C{l}^{-}\leftrightharpoons HCl+H_{2}O$