Question

Which compound is formed when excess of KCN is added to an aqueous solution of copper sulphate?
A. $Cu{(CN)_2}$
B. ${K_2}[Cu{(CN)_2}]$
C. $K[Cu{(CN)_2}]$
D. ${K_3}[Cu{(CN)_4}]$

Answer 1

To solve this question, we must proceed step by step. Since excess KCN is used, there is a possibility of formation of one product in the beginning, but sue to the presence of excess reactants, the final product may be different.

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts.
Copper sulphate is the sulphate salt of copper, where the oxidation state of copper is (+2). The molecular formula of copper sulphate is $CuS{O_4}$. This salt is generally found as blue coloured crystals. On the other hand, KCN is the cyanide salt of potassium. The question proposed to us is about the product which is formed when excess of KCN is added to an aqueous solution of copper sulphate. To obtain the final product, we must go through a few steps. Let us discuss them one by one.

When excess KCN is added to the aqueous solution of copper sulphate, the first product that is formed is copper cyanide. The molecular formula of this product is $Cu{(CN)_2}$ . As the reaction progresses, this compound undergoes decomposition. Copper cyanide decomposes into cyanide and cupric cyanide. The molecular formulae of both these compounds are ${(CN)_2}$ and $C{u_2}{(CN)_2}$ respectively. This $C{u_2}{(CN)_2}$ now combines with the remaining Potassium cyanide to form the complex ${K_3}[Cu{(CN)_4}]$ . The final chemical reaction can be given as:

$2CuS{O_4} + 10KCN \to 3{K_3}[Cu{(CN)_4}] + {(CN)_2} + {K_2}S{O_4}$

Hence, Option D is the correct option.

Note: Potassium Cyanide is corrosive in nature and is a health as well as an environmental hazard. Hence, Potassium cyanide must be handled with care while performing any experiment. In the physical form, KCN is found as a white amorphous lump or a crystalline mass which has a faint odour of bitter almonds.