Electrolysis is carried out in three cells
(A) 1.0 M `CuSO_4` Pt electrode
(B) 1.0 M `CuSO_4` copper electrodes
(C) 1.0 M KCl Pt electrodes
If volume of electrolytic solution is maintained constant in each of the cell, which is correct set of pH changes in (A), (B) and (C) cell respectively?

Options

A. decrease in all the three
B. increase in all the three
C. decrease, constant, increase
D. increase, constant, increase

Answer

To solve the problem regarding the pH changes during the electrolysis in the three cells, we will analyze each cell step by step.

### Step 1: Analyze Cell A (1.0 M CuSO4 with Pt Electrode)
- **At the Cathode:** The reduction reaction occurs where Cu²⁺ ions gain electrons to form copper metal (Cu).
- Reaction: Cu²⁺ + 2e⁻ → Cu
- **At the Anode:** Water is oxidized, producing hydrogen ions (H⁺) and oxygen gas (O₂).
- Reaction: 2H₂O → O₂ + 4H⁺ + 4e⁻

**Effect on pH:**
- The generation of H⁺ ions increases the acidity of the solution, leading to a decrease in pH.

### Step 2: Analyze Cell B (1.0 M CuSO4 with Copper Electrodes)
- **At the Cathode:** The reduction of Cu²⁺ ions occurs, similar to Cell A.
- Reaction: Cu²⁺ + 2e⁻ → Cu
- **At the Anode:** Copper from the anode dissolves into the solution as Cu²⁺ ions.
- Reaction: Cu → Cu²⁺ + 2e⁻

**Effect on pH:**
- There is no net change in H⁺ or OH⁻ ions in the solution because the amount of Cu²⁺ produced at the anode is equal to the amount consumed at the cathode. Therefore, the pH remains constant.

### Step 3: Analyze Cell C (1.0 M KCl with Pt Electrode)
- **At the Cathode:** Water is reduced, producing hydrogen gas and hydroxide ions (OH⁻).
- Reaction: 2H₂O + 2e⁻ → H₂ + 2OH⁻
- **At the Anode:** Chloride ions (Cl⁻) are oxidized to chlorine gas (Cl₂).
- Reaction: 2Cl⁻ → Cl₂ + 2e⁻

**Effect on pH:**
- The generation of OH⁻ ions increases the basicity of the solution, leading to an increase in pH.

### Summary of pH Changes
- **Cell A (Pt Electrode in CuSO4):** pH decreases (due to increase in H⁺).
- **Cell B (Cu Electrodes in CuSO4):** pH remains constant (no net change in H⁺ or OH⁻).
- **Cell C (Pt Electrode in KCl):** pH increases (due to increase in OH⁻).

### Final Answer
The correct set of pH changes in cells A, B, and C respectively is: **Decreases, Constant, Increases.**