Name: Reaction Between Ferrous Ion and Potassium Dichromate
Uploaded: 2019-07-19
Duration: 4 min 21 s
Description: This video explains the excellent oxidising agent, potassium dichromate. One mole of potassium dichromate can oxidise six moles of ferrous sulphate molecules...

Question

Explain why a solution of zinc sulphate gives a white precipitate when added to an aqueous solution of ammonia, but not when added to an aqueous solution of ammonia containing ammonium chloride.

Accepted Answer

Zinc metal has a very good tendency to form the tetrahedral and octahedral complex. The tetrahedral complex formed by zinc are ${[Zn (L)_{4}]}^{2 +}$ where $L$ represents the ligand. The octahedral complexes formed by zinc are ${[Zn (L)_{6}]}^{2 +}$ . But the octahedral complex formed by zinc are not stable due to its small size. Zinc sulphate reacts with the solution of ammonia then it forms the ${[Zn ({NH}_{3})_{4}]}^{2 +}$ i.e. a tetrahedral complex. But in case of the ammonium chloride., zinc sulphate forms ${ZnCl}_{2}$ .

Ammonia can also show the common ion effect. The combination of ${NH}_{4} Cl$ to ammonia will show the common ion effect at equilibrium.

${NH}_{3} + H_{2} O ⇌ {NH}_{4}^{+} + {OH}^{-}$

Addition of ${NH}_{4} Cl$ means there is increase in the concentration of ${NH}_{4}^{+}$ ions and reaction will go in backward direction.

$In case of ammonia solution : {ZnSO}_{4} + 4 {NH}_{3} \to {[Zn ({NH}_{3})_{4}]}^{2 +} In case of ammonium chloride solution : {ZnSO}_{4} + {NH}_{4} Cl \to {ZnCl}_{2} + ({NH}_{4})_{2} {SO}_{4}$

So, zinc metal gives the white precipitate of zinc tetrahedral complex ${[Zn ({NH}_{3})_{4}]}^{2 +}$ with ammonia. But in case of ammonium chloride, it forms the ${ZnCl}_{2}$ solution.

Explain why a solution of zinc sulphate gives a white precipitate when added to an aqueous solution of ammonia, but not when added to an aqueous solution of ammonia containing ammonium chloride.

Important Questions on Group 12 - the Zinc Group

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