Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

When the sample of copper with zinc impurity is to be purified by electrolysis, the appropriate electrodes are [2002] Cathode Anode (a) pure zinc pure copper (b) pure copper impure sample (c) impure zinc impure sample (d) impure sample \(\quad\) pure copper

Step-by-Step Solution

Answer

(b) pure copper impure sample.

Step 1: Understanding Electrolysis

Electrolysis is a process where electricity is used to induce a chemical reaction. In purification by electrolysis, impurities are separated as the metal is transferred from an impure electrode to a pure one.

Step 2: Identifying Anode and Cathode Roles

In electrolysis, the anode is where oxidation (loss of electrons) occurs, and it typically consists of the impure metal. The cathode is where reduction (gain of electrons) occurs, and it consists of the pure metal. Electrons travel from the anode to the cathode.

Step 3: Setting Up the Electrodes

In electrolytic purification of metals, impure metal is used as an anode since it will dissolve into the electrolyte solution. Pure metal is used as the cathode, as it will attract metal ions from the solution, leaving impurities behind.

Step 4: Choosing the Correct Electrodes for Copper Purification

Here, the impure sample containing copper is used as the anode so that copper dissolves, while pure copper is used as the cathode to attract copper ions. The impurities from the impure sample will either remain in solution or settle as anode sludge.

Step 5: Conclusion and Answer Selection

Option (b) is correct: "pure copper impure sample" as it correctly aligns with the roles of anode and cathode in purifying the sample by electrolysis. Copper will transfer from impure sample (anode) to pure copper (cathode).

Key Concepts

Copper Purification

Copper purification through electrolysis is a fascinating process that allows you to obtain copper in its purest form. This method is crucial when you have copper mixed with impurities like zinc. During electrolysis, an electrical current is passed through the electrolyte solution containing dissolved copper ions.

Here's how copper goes from impure to pure:

• At the start, the impure copper sample is the source of mixed metals.
• As the current flows, copper ions migrate towards the pure copper cathode.
• These ions are then deposited as pure copper at the cathode.
• Impurities like zinc remain in the solution or accumulate as sludge at the bottom.

This technique ensures that the copper harvested is highly refined and suitable for various industrial applications.

Electrode Identification

In the electrolysis process, recognizing which electrode serves as the anode or cathode is essential to the purification outcome.

Anode and Cathode Functions:

• The anode is the electrode where oxidation occurs. In copper purification, it is the impure copper sample. This is due to the oxidation of copper ions, which dissolve from the impure anode into the solution.
• The cathode is where reduction takes place, and is typically pure copper. The copper ions from the solution gain electrons at the cathode and form solid copper.

The movement of copper ions from the anode to the cathode not only transforms impure copper into pure copper but also effectively separates contaminants.

Oxidation and Reduction

Understanding oxidation and reduction reactions—collectively called redox reactions—is key to grasping electrolysis in copper purification.

Oxidation:

• Occurs at the anode.
• Involves loss of electrons.
• Copper atoms in the impure sample become copper ions ( ext{Cu} ightarrow ext{Cu}^{2+} + 2 ext{e}^{-}).

Reduction:

• Occurs at the cathode.
• Involves gain of electrons.
• Copper ions in the solution convert back into copper atoms ( ext{Cu}^{2+} + 2 ext{e}^{-} ightarrow ext{Cu}).

Through this flow of electrons, the electrolysis process elegantly purifies the copper.

Electrolytic Refining

Electrolytic refining of copper is a specific application of electrolysis focusing on enhancing metal purity. This method is highly efficient in isolating copper from mixtures containing other metals.

The Process:

• The impure copper, serving as the anode, releases copper ions into the solution—this involves dissolving away the impurities.
• Simultaneously, at the cathode, these ions are meticulously deposited as pure copper.
• Unwanted elements either stay dissolved or settle at the base as a byproduct called anode mud or slime.

This technique not only purifies the copper but also allows for recovery of other valuable materials from the sludge, making it environmentally and economically beneficial.