Which are true for a standard hydrogen electrode ?

Options

A. The hydrogen ion concentration is 1M
B. Temperature is `25^(@)C`
C. Pressure of hydrogen is 1 atmosphere
D. It contains a metallic conductor which does not absorb hydrogen.

Answer

To determine which statements are true for a standard hydrogen electrode (SHE), we will analyze the characteristics and conditions of the SHE step by step.

### Step-by-Step Solution

1. **Definition of Standard Hydrogen Electrode (SHE)**:
The standard hydrogen electrode is a reference electrode that is used to measure the electrode potential of other electrodes. It is based on the half-reaction of hydrogen ions and hydrogen gas.

**Hint**: Remember that SHE is the reference point for measuring electrode potentials.

2. **Components of SHE**:
The SHE consists of:
- A platinum electrode (inert conductor) coated with platinum black to increase surface area.
- A solution of 1 M hydrochloric acid (HCl) which provides hydrogen ions (H⁺).
- Hydrogen gas (H₂) is bubbled through the solution at a pressure of 1 bar (or 1 atmosphere).

**Hint**: Identify the key components that make up the SHE.

3. **Conditions for SHE**:
- The concentration of hydrogen ions (H⁺) is 1 M.
- The temperature is maintained at 25 °C (298 K).
- The pressure of hydrogen gas is 1 bar.

**Hint**: Conditions such as concentration, temperature, and pressure are crucial for the functioning of the SHE.

4. **Electrode Reactions**:
- When the SHE acts as an anode, hydrogen gas is oxidized to hydrogen ions:
\[ H_2 \rightarrow 2H^+ + 2e^- \]
- When it acts as a cathode, hydrogen ions are reduced to hydrogen gas:
\[ 2H^+ + 2e^- \rightarrow H_2 \]

**Hint**: Understand the oxidation and reduction reactions that occur at the SHE.

5. **Electrode Potential**:
The standard electrode potential of the SHE is defined as 0 volts at standard conditions (1 M concentration, 1 bar pressure, and 25 °C).

**Hint**: The potential of SHE is a reference point set to zero.

6. **Evaluation of Statements**:
Now, we can evaluate the statements provided in the question:
- **Statement 1**: Hydrogen ion concentration is 1 M. **(True)**
- **Statement 2**: Temperature is 25 °C. **(True)**
- **Statement 3**: Pressure of hydrogen is 1 bar. **(True)**
- **Statement 4**: It contains a metallic conductor. **(False)** - The platinum electrode is an inert conductor, but it does not refer to a typical metallic conductor.

**Hint**: Carefully analyze each statement based on the characteristics of the SHE.

### Conclusion
The true statements for a standard hydrogen electrode are:
- Hydrogen ion concentration is 1 M.
- Temperature is 25 °C.
- Pressure of hydrogen is 1 bar.

Thus, the correct options are statements 1, 2, and 3.