The incorrect order is

Options

A. `HF lt HCl lt HBr lt Hl` : Acidic strength
B. `HF gt HCl gt HBr gt Hl` : Thermal stability
C. `HF gt HCl gt HBr gt Hl` : Boiling point
D. `HF gt HCl gt HBr gt Hl` : Bond dissociation

Answer

To determine the incorrect order from the given options regarding acidity, thermal stability, boiling points, and bond dissociation enthalpy, we will analyze each order step by step.

### Step 1: Analyze Acidic Strength
- **Acidic Strength Order**: HF < HCl < HBr < HI
- **Reasoning**: The acidity of hydrogen halides increases down the group due to the increasing size of the anions (F-, Cl-, Br-, I-). Larger anions are more stable, leading to stronger acids. Therefore, HI is the strongest acid, and HF is the weakest.

### Step 2: Analyze Thermal Stability
- **Thermal Stability Order**: HF > HCl > HBr > HI
- **Reasoning**: As we move down the group, bond length increases, which decreases bond strength and thermal stability. HF has the strongest bond due to its shorter bond length, making it the most thermally stable.

### Step 3: Analyze Boiling Point
- **Boiling Point Order**: HF > HCl < HBr < HI
- **Reasoning**: Boiling points generally increase with molecular size due to stronger Van der Waals forces. However, HF has strong hydrogen bonding, which gives it a higher boiling point than expected compared to HCl, HBr, and HI. Thus, this order is correct.

### Step 4: Analyze Bond Dissociation Enthalpy
- **Bond Dissociation Enthalpy Order**: HF > HCl > HBr > HI
- **Reasoning**: The bond dissociation enthalpy is directly related to bond strength. HF has the strongest bond (small bond length), while the bond strength decreases down the group due to increasing bond length. Thus, this order is also correct.

### Conclusion
After analyzing all the given orders, we find that the **incorrect order** is the one related to boiling points. The correct boiling point order should reflect that HF has a higher boiling point due to hydrogen bonding, while the other halides follow the trend of increasing boiling points with increasing molecular size.

### Final Answer
The incorrect order is option number 3 (Boiling Point).

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