In which cases, the order of acidic strength is not correct ?

Options

A. `HI gt HB r gt HCl`
B. `HIO_(4) gt HB rO_(4) gt HClO_(4)`
C. `HCIO_(4) gt HClO_(3) gt HClO_(2)`
D. `HF_(2) gt H_2Ogt NH_(3)`

Explanation

To determine in which cases the order of acidic strength is not correct, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option 1: HI > HBr > HCl**
- **Analysis**: This order is correct. The bond length increases from HCl to HBr to HI, leading to weaker bonds in HI. Weaker bonds dissociate more easily, making HI the strongest acid among the three.
- **Conclusion**: This option is correct.

2. **Option 2: HIO4 > HBrO4 > HClO4**
- **Analysis**: The acidic strength of oxoacids increases with the electronegativity of the central atom. As we move from HClO4 to HBrO4 to HIO4, the electronegativity of the central atom decreases, which should lead to a decrease in acidic strength. Therefore, the correct order should be HClO4 > HBrO4 > HIO4.
- **Conclusion**: This option is incorrect.

3. **Option 3: HClO4 > HClO3 > HClO2**
- **Analysis**: The acidic strength of oxoacids increases with the oxidation state of the central atom. HClO4 has the highest oxidation state, followed by HClO3 and then HClO2. Thus, this order is correct.
- **Conclusion**: This option is correct.

4. **Option 4: HF2 > H2O > NH3**
- **Analysis**: HF2 is an acid, H2O is neutral, and NH3 is a base. The acidic strength order is correct since an acid is stronger than a neutral molecule, and a neutral molecule is stronger than a base.
- **Conclusion**: This option is correct.

### Final Conclusion:
The order of acidic strength that is not correct is found in **Option 2**: HIO4 > HBrO4 > HClO4.