Which of the following are concentration cell?

Options

A. `Pt"|"underset(P_1)(H_2)(g)|HCl|underset(P_2)(H_2)(G)|Pt`
B. `Cd,underset((a_(1)))((Hg))"|"underset((C))Cd^(2+)"|"underset(a_2)((Hg)),Cd`
C. `Zn(s)"|"underset(c_1)Zn^(2+)"||"underset(c_2)Cu^(2+)"|"Cu`
D. `Ag"|"AgCl"|"underset(c_1)Cl^(-)(aq)"||"underset(c_2)Br^(-)(aq)"|"AgBr"|"Ag`

Answer

To determine which of the given options are concentration cells, we need to follow these steps:

### Step 1: Understand the Definition of a Concentration Cell
A concentration cell is a type of electrochemical cell where both electrodes are made of the same material but are immersed in solutions of different concentrations. The cell generates an electromotive force (EMF) due to the difference in concentration of the ions involved.

### Step 2: Identify the Condition for a Concentration Cell
For a cell to be classified as a concentration cell, the standard electrode potential (E°) of the cell must be equal to zero (E°cell = 0). This means that the two half-cells must have the same electrode potential when measured under standard conditions.

### Step 3: Analyze the Given Options
We will analyze the provided options to check if they meet the criteria for concentration cells.

1. **Zn/Zn²⁺ (1M) and Zn/Zn²⁺ (0.1M)**:
- Both electrodes are zinc, but they are in solutions of different concentrations (1M and 0.1M).
- Since they are the same metal and have different concentrations, this is a concentration cell.

2. **Cu/Cu²⁺ (1M) and Cu/Cu²⁺ (0.5M)**:
- Similar to the first case, both electrodes are copper in different concentrations.
- This is also a concentration cell.

3. **Ag/Ag⁺ (1M) and Ag/Ag⁺ (1M)**:
- Both electrodes are in the same concentration (1M).
- Since there is no difference in concentration, this is not a concentration cell.

4. **Pb/Pb²⁺ (0.1M) and Pb/Pb²⁺ (0.1M)**:
- Again, both electrodes are in the same concentration (0.1M).
- This is not a concentration cell.

### Conclusion
From the analysis, the concentration cells among the options provided are:
- Zn/Zn²⁺ (1M) and Zn/Zn²⁺ (0.1M)
- Cu/Cu²⁺ (1M) and Cu/Cu²⁺ (0.5M)