Problem 166 - 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 \(\quad\) Anode (a) pure zinc \(\quad\) pure copper (b) pure copper impure sample (c) impure zinc impure sample (d) impure sample pure copper

Step-by-Step Solution

Answer

Option (b) pure copper (cathode) and impure sample (anode) is correct.

Step 1: Understanding Electrolysis

Electrolysis is a chemical process where electrical energy is used to drive a non-spontaneous chemical reaction. When purifying metals, the impure sample is generally made the anode.

Step 2: Identifying the Anode

In metal purification via electrolysis, the impure sample is used as the anode. This is because the anode dissolves into the solution, allowing only the pure metal ions to deposit on the cathode.

Step 3: Choosing the Cathode

The cathode must be made of pure metal so that only the pure, neutral metal atoms can deposit from the solution. This ensures that the metal deposited on the cathode is of high purity.

Step 4: Matching to the Options

Given the choices, the setup corresponds to using the impure sample as the anode and pure copper on the cathode. So, the correct option is that the cathode is pure copper and the anode is the impure sample.

Key Concepts

Anode and Cathode

In the process of electrolysis, the anode and cathode serve different but complementary roles. They are electrodes that guide a direct current (DC) through a substance to bring about a chemical change.
The **anode** is where oxidation occurs. During metal purification, the impure metal sample is made the anode. This means that the anode will gradually dissolve in the electrolyte solution as atoms lose electrons, turning into positively charged ions. For example, when purifying copper, zinc impurities dissolve into the solution, while copper ions continue their path.

• The anode is positively charged.
• Material from the anode moves into the solution as ions.

The **cathode** is the site of reduction, meaning it's the electrode where reduction (gain of electrons) occurs. It's made from pure metal so that, when ions from the solution are reduced (gain electrons), the deposited metal is extremely pure. In our example, the cathode must be pure copper to capture the copper ions effectively.

• The cathode is negatively charged.
• Pure metal ions from the solution deposit onto it.

Metal Purification

Purifying metals, like copper, via electrolysis is a method used to increase purity by using electrical energy to reverse the natural electron flow. This method is crucial in industries where metals must be free from impurities to perform effectively.
**Why Electrolysis for Metal Purification?** Using electrolysis allows for precise control over the deposited metal's purity. As the impure sample dissolves at the anode, only desirable ions join the solution and move toward the cathode.

• Ensures high purity metal is established at the cathode.
• The process discards unwanted impurities as sludge or dissolved ions.

**Common Steps in Electrolytic Purification** 1. Selection of electrodes: Impure metal as anode, pure metal as cathode. 2. Application of electric current: Drives ions from anode to cathode. 3. Recovery of pure metal: Metal ions gain electrons and deposit on cathode as pure metal. This process can take several hours or longer, depending on the scale and the degree of impurity.

Electrochemical Reactions

Electrochemical reactions are chemical reactions that involve the transfer of electrons between two substances. This transfer is integral for processes like electrolysis, where electrical energy facilitates chemical changes that would not occur spontaneously.
**Understanding the Basics** In electrochemical reactions, electrons can move through a circuit—beginning their journey at the anode and traveling toward the cathode. These reactions are the underlying processes for battery operation, corrosion, and, importantly, metal purification.

• Electrons travel from anode to cathode.
• The anode loses electrons—oxidation; the cathode gains electrons—reduction.

**Reactions in Electrolysis for Metal Purification** - At the **anode**: Metal atoms oxidize by losing electrons, entering the solution as metal ions. - At the **cathode**: These ions gain electrons and deposit as pure metal. The electrical current ensures continuous movement between the electrodes, effectively filtering out impurities and recovering pure metal in the process.