Chapter 3 · Question 14

Describe the process of electrolytic refining of copper with a labelled diagram explanation. What happens to the soluble and insoluble impurities during this process?

Q14

Describe the process of electrolytic refining of copper with a labelled diagram explanation. What happens to the soluble and insoluble impurities during this process?

Answer Revealed

Direct Answer:

Electrolytic refining is used to obtain highly pure metals from impure metal obtained after reduction. For copper refining: the anode (+) is a thick block of impure copper, the cathode (-) is a thin strip of pure copper, and the electrolyte is an acidified copper sulphate (CuSO₄) solution. On passing electric current: at the anode, pure copper oxidises and dissolves as Cu²⁺ ions into the electrolyte:

\text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^-

. At the cathode, Cu²⁺ ions from the electrolyte gain electrons and deposit as pure copper:

\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}

. Soluble impurities dissolve into the solution, while insoluble impurities settle down below the anode as anode mud.

Simple Explanation

Electrolytic refining is like filtering with electricity. You have a big slab of dirty copper as the positive terminal (anode) and a thin sheet of pure copper as the negative terminal (cathode). Both sit in copper sulphate solution. When you turn on the current, pure copper atoms leave the dirty slab and travel through the solution to coat the thin pure sheet. The dirt in the slab either dissolves in the solution or falls to the bottom as muddy sludge (anode mud). What you get on the cathode is exceptionally pure copper.

Exam-Ready Structure

Electrolytic refining (Section 3.4.6) is the final step in extractive metallurgy for several commercially important metals, converting impure reduced metal into high-purity metal suitable for practical use. 1. Definition: Electrolytic refining is the most widely used method for purifying (refining) metals obtained through reduction, commonly applied to copper, zinc, tin, nickel, silver, and gold. 2. Setup for copper refining (Fig. 3.12): (a) Anode (+): A thick block of impure copper. (b) Cathode (-): A thin strip of pure copper. (c) Electrolyte: An acidified aqueous solution of copper sulphate (CuSO₄). (d) Container: A tank holding the electrolyte with the electrodes immersed. 3. Process (on passing direct electric current): (a) At the anode (oxidation): Pure copper atoms from the impure anode lose electrons and enter the solution as Cu²⁺ ions:

\text{Cu(s)} \rightarrow \text{Cu}^{2+}\text{(aq)} + 2\text{e}^-

. The anode gradually becomes thinner as copper dissolves. (b) At the cathode (reduction): Cu²⁺ ions from the electrolyte gain electrons at the cathode and deposit as pure copper metal atoms:

\text{Cu}^{2+}\text{(aq)} + 2\text{e}^- \rightarrow \text{Cu(s)}

. The pure copper cathode gradually grows thicker. (c) Net effect: Pure copper is transferred from the anode to the cathode through the solution. 4. Fate of impurities: (a) Soluble impurities: Dissolve into the electrolyte solution along with the copper. (b) Insoluble impurities (such as silver, gold, and platinum that may be present as impurities in the impure copper): Do not dissolve and instead settle down at the bottom of the tank below the anode. This collection of insoluble impurities is called anode mud. Anode mud is a valuable source of precious metals that can be recovered separately.

Key Points

Electrolytic refining: impure metal → anode; pure metal → cathode; metal salt solution → electrolyte
For copper: anode = impure Cu; cathode = pure Cu; electrolyte = acidified CuSO₄ solution
At anode: $\text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^-$ (oxidation, Cu dissolves)
At cathode: $\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$ (reduction, pure Cu deposits)
Soluble impurities dissolve in electrolyte; insoluble impurities settle as anode mud

Describe the process of electrolytic refining of copper with a labelled diagram explanation. What happens to the soluble and insoluble impurities during this process?