Problem 137 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound is formed when excess of \(\mathrm{KCN}\) is added to an aqueous solution of copper sulphate? (a) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (b) \(\mathrm{K}_{2}\left[\mathrm{Cu}(\mathrm{CN})_{6}\right]\) (c) \(\mathrm{K}\left[\mathrm{Cu}(\mathrm{CN})_{2}\right]\) (d) \(\mathrm{K}_{3}\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]\)

Step-by-Step Solution

Answer

The compound formed is \(\mathrm{K}_2\left[ \mathrm{Cu(CN)}_6 \right]\).

Step 1: Understand the Reaction Process

When you add potassium cyanide (\(\mathrm{KCN}\)) in excess to an aqueous solution of copper sulphate (\(\mathrm{CuSO}_4\)), the copper ion \(\mathrm{Cu^{2+}}\) initially reacts with \(\mathrm{CN^-}\) ions.

Step 2: Formation of Precipitate

Initially, \(\mathrm{Cu^{2+}}\) reacts with \(\mathrm{CN^-}\) to form \(\mathrm{Cu(CN)_2}\), which is a white precipitate. The reaction is as follows: \[ \mathrm{CuSO_4 + 2KCN \rightarrow Cu(CN)_2 + K_2SO_4} \]

Step 3: Complex Formation with Excess Reagent

Since \(\mathrm{KCN}\) is in excess, the \(\mathrm{Cu(CN)_2}\) formed dissolves further by reacting with additional \(\mathrm{CN^-}\) ions to form a complex ion. This process forms a soluble complex: \[ \mathrm{Cu(CN)_2 + 4CN^- \rightarrow [Cu(CN)_6]^{4-}} \]

Step 4: Final Compound Formation

The final step in the reaction process is the equilibrium between the copper-cyanide complex and potassium ions, which leads to the formation of \(\mathrm{K_2[Cu(CN)_6]}\) in solution. This is because two potassium ions balance the charge of the complex ion, resulting in the formation of \(\mathrm{K}_2\left[ \mathrm{Cu(CN)}_6 \right]\).

Key Concepts

Complex Ion Formation

Complex ion formation is a fascinating process in chemistry where central metal ions bond with several ligands to form a complex structure. In solution, metal ions such as copper can interact with ligands, which are molecules or ions that donate electron pairs, to create a more stable complex.

When you add an excess of a ligand like cyanide ions (\(\mathrm{CN^-}\)) to a metal ion like copper (\(\mathrm{Cu^{2+}}\)), the resulting interaction forms a complex ion. This is because the metal ion wants to achieve a stable electronic configuration by reaching optimal coordination with the ligands.

The central metal ion works like a core, while the ligands surround it, bridging electron-sharing bonds. This means that instead of just one or two cyanide ions attaching to the copper ion, multiple cyanide ions coordinate with it, creating a very stable complex structure. This is all driven by the copper ion's desire to lower its energy by bonding with multiple ligands.

Copper Cyanide Complex

The copper cyanide complex specifically results from the interaction of copper ions with cyanide ions in solution. When potassium cyanide (\(\mathrm{KCN}\)) is added to copper sulfate (\(\mathrm{CuSO_4}\)), initially a precipitate of copper cyanide (\(\mathrm{Cu(CN)_2}\)) is formed. This is simply because the copper ions react readily with cyanide ions present in the solution.

However, with excess \(\mathrm{KCN}\), the \(\mathrm{Cu(CN)_2}\) precipitate dissolves further to form the complex \(\mathrm{[Cu(CN)_6]^{4-}}\). This newly formed complex is much more stable and, thanks to its negative charge, it remains soluble in the solution. Two potassium ions then balance out the charge, leading to the production of \(\mathrm{K_2[Cu(CN)_6]}\).

This copper cyanide complex is an excellent example of how complex ion formation can effectively change the properties of compounds, such as their solubility in water.

Equilibrium in Chemical Reactions

Equilibrium in chemical reactions is a state where the rates of the forward and reverse reactions are equal, resulting in the concentration of reactants and products remaining constant over time. When a solution containing copper sulfate is treated with excess potassium cyanide, an intriguing transformation involving equilibrium dynamics takes place.

Initially, copper ions and cyanide ions form copper cyanide (\(\mathrm{Cu(CN)_2}\)), which precipitates out of solution. This process, however, reaches equilibrium when the formed \(\mathrm{Cu(CN)_2}\) also reacts further with excess cyanide ions to produce the soluble complex \(\mathrm{[Cu(CN)_6]^{4-}}\).

The establishment of this new equilibrium is crucial as it dictates the relative concentrations of initial reactants, the precipitated copper cyanide, and the final complex copper cyanide ions in solution. This example of equilibrium is a clear illustration of Le Chatelier's principle, where the addition of more cyanide ions shifts the equilibrium towards forming more complex ions, stabilizing the system.