Which has/have a positive value of `DeltaH`?

Options

A. `H_(2(g)) rarr 2H_((g))`
B. `H_((g)) rarr H_((g))^(+) + e`
C. `H_(2)O_((l)) rarr H_(2)O_((g))`
D. `H_((aq))^(+) + OH_((aq))^(-) rarr H_(2)O`

Answer

To determine which processes have a positive value of ΔH (change in enthalpy), we need to identify whether the processes are endothermic or exothermic. A positive ΔH indicates that heat is absorbed during the process, which is characteristic of endothermic reactions.

### Step-by-Step Solution:

1. **Understanding ΔH**:
- ΔH represents the change in enthalpy. A positive ΔH indicates an endothermic process where heat is absorbed.

2. **Analyzing the First Option**:
- **Process**: Hydrogen gas (H₂) dissociates into atomic hydrogen (H).
- **Explanation**: This process requires energy to break the H-H bond, which means heat is absorbed.
- **Conclusion**: ΔH is positive.

3. **Analyzing the Second Option**:
- **Process**: A gaseous hydrogen atom (H) loses one electron to become a cation (H⁺).
- **Explanation**: This requires ionization energy, which means heat is absorbed.
- **Conclusion**: ΔH is positive.

4. **Analyzing the Third Option**:
- **Process**: Liquid water (H₂O) is converted into steam (gaseous water).
- **Explanation**: This process requires latent heat of vaporization, indicating that heat is absorbed.
- **Conclusion**: ΔH is positive.

5. **Analyzing the Fourth Option**:
- **Process**: H⁺ ions and OH⁻ ions combine to form water (H₂O).
- **Explanation**: This is an exothermic process where energy is released when bonds are formed.
- **Conclusion**: ΔH is negative.

### Final Answer:
The processes that have a positive value of ΔH are:
- 1st option (dissociation of hydrogen gas into atomic hydrogen)
- 2nd option (ionization of hydrogen atom)
- 3rd option (vaporization of water)