Which among the following will boil at highest temperature?

Options

A. HCI
B. HF
C. HI
D. HBr

Answer

To determine which compound among hydrogen chloride (HCl), hydrogen fluoride (HF), hydrogen iodide (HI), and hydrogen bromide (HBr) boils at the highest temperature, we will analyze the intermolecular forces present in each compound.

### Step-by-Step Solution:

1. **Understanding Boiling Point**:
The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure. It is influenced by the strength of intermolecular forces present in the substance.

**Hint**: Recall that stronger intermolecular forces lead to higher boiling points.

2. **Identifying Intermolecular Forces**:
The primary types of intermolecular forces include:
- Hydrogen bonding
- Dipole-dipole interactions
- Van der Waals (London dispersion) forces

**Hint**: Recognize that hydrogen bonding is a particularly strong type of dipole-dipole interaction.

3. **Analyzing Each Compound**:
- **Hydrogen Fluoride (HF)**: Exhibits strong hydrogen bonding due to the highly electronegative fluorine atom.
- **Hydrogen Chloride (HCl)**: Has dipole-dipole interactions but no hydrogen bonding.
- **Hydrogen Bromide (HBr)**: Similar to HCl, it has dipole-dipole interactions but is larger in size than HCl.
- **Hydrogen Iodide (HI)**: Also has dipole-dipole interactions, but it is the largest and has the highest molecular weight among the halides.

**Hint**: Consider the electronegativity of the atoms involved and the size of the molecules.

4. **Comparing Boiling Points**:
- HF has strong hydrogen bonding, leading to a higher boiling point.
- HCl, HBr, and HI have weaker dipole-dipole interactions, with boiling points increasing with molecular weight but not exceeding that of HF.

**Hint**: Remember that while larger molecules generally have higher boiling points due to increased Van der Waals forces, hydrogen bonding in HF is a significant factor.

5. **Conclusion**:
Based on the analysis, hydrogen fluoride (HF) has the highest boiling point due to its strong hydrogen bonding, followed by hydrogen iodide (HI), hydrogen bromide (HBr), and hydrogen chloride (HCl).

**Final Answer**: Hydrogen fluoride (HF) will boil at the highest temperature.