Which has/have a positive value `(s)` of `DeltaH`?

Options

A. `H_(2)(g)to2H(g)`
B. `H(g)toH^(+)(g)+e`
C. `H_(2)(l)toH_(2)O(g)`
D. `H^(+)(aq)+OH^(-)(ag)toH_(2)O`

Answer

To determine which of the given options has a positive value of ΔH (enthalpy change), we need to analyze each option based on whether the reaction is endothermic or exothermic.

### Step-by-Step Solution:

1. **Understanding ΔH**:
- If ΔH is negative, the reaction is exothermic (heat is released).
- If ΔH is positive, the reaction is endothermic (heat is absorbed).

2. **Analyzing Option A**:
- The reaction is: \( H_2 \rightarrow 2H \) (breaking the H-H bond).
- Breaking bonds requires energy input (heat). Therefore, heat is supplied to break the bond.
- Conclusion: This reaction is endothermic, so ΔH > 0. **(Positive ΔH)**

3. **Analyzing Option B**:
- The reaction is: \( H \rightarrow H^+ + e^- \) (ionization of hydrogen).
- Removing an electron from an atom requires energy (heat). Thus, heat must be supplied for ionization.
- Conclusion: This reaction is endothermic, so ΔH > 0. **(Positive ΔH)**

4. **Analyzing Option C**:
- The reaction is: \( \text{Liquid} \rightarrow \text{Vapor} \) (vaporization).
- Converting a liquid to a gas requires heat (energy input). Therefore, heat is supplied during vaporization.
- Conclusion: This reaction is endothermic, so ΔH > 0. **(Positive ΔH)**

5. **Analyzing Option D**:
- The reaction is: \( H^+ + OH^- \rightarrow H_2O \) (neutralization).
- Neutralization reactions typically release heat. Therefore, heat is evolved in this reaction.
- Conclusion: This reaction is exothermic, so ΔH < 0. **(Negative ΔH)**

### Final Conclusion:
Options A, B, and C have a positive value of ΔH, indicating they are endothermic reactions. Option D has a negative value of ΔH, indicating it is exothermic.