If a mole of hydrogen molecule is heated to a high temperature then which of the following reactions take place ?

Options

A. `H_(2(g)) + 436 "kj mol"^(-1) rarr H_((g)) + H_((g))`
B. `2H_(2(g)) + 820 "kj mol"^(-1) rarr 2H_(2(g))`
C. `H_(2(g)) + H_(2(g)) + 436 "kj mol"^(-1) rarr H^(+)_(aq) + H_(aq)^(-)`
D. `H_(2(g)) + 200 "kj mol"^(-1) rarr H_((g)) + H_((g))`

Explanation

To determine which reactions take place when a mole of hydrogen molecules (H₂) is heated to a high temperature, we need to consider the concept of bond dissociation energy and the possible reactions that can occur.

### Step-by-Step Solution:

1. **Understanding Bond Dissociation Energy**:
- When H₂ is heated, the energy supplied can break the bonds between the hydrogen atoms in the molecule. The bond dissociation energy for H₂ is approximately 436 kJ/mol. This means that to break one mole of H₂ into individual hydrogen atoms (2 H), 436 kJ of energy is required.

2. **Analyzing the Given Reactions**:
- We need to evaluate the reactions provided in the options to see which one corresponds to the dissociation of H₂ or any other reaction that can occur at high temperatures.

3. **Option Analysis**:
- **Option A**: H₂ → 2H (This reaction represents the dissociation of hydrogen molecules into individual hydrogen atoms. It requires 436 kJ of energy, which matches our understanding of bond dissociation energy. Therefore, this reaction is correct.)
- **Option B**: H₂ + O₂ → H₂O + heat (This reaction represents combustion, which releases energy. However, it does not represent the dissociation of H₂.)
- **Option C**: H₂ + H₂ → H₂ (This reaction does not represent any meaningful chemical change and is incorrect.)
- **Option D**: H₂ + energy → 2H (This reaction also represents the dissociation of hydrogen molecules into individual hydrogen atoms, but it specifies that energy is added, which is consistent with the bond dissociation process.)

4. **Conclusion**:
- The correct reactions that take place when a mole of hydrogen molecules is heated to a high temperature are:
- **Option A**: H₂ → 2H (Dissociation of hydrogen molecules)
- **Option D**: H₂ + energy → 2H (Dissociation with energy input)

### Final Answer:
The reactions that take place are:
- H₂ → 2H (Option A)
- H₂ + energy → 2H (Option D)