Which of the following order is incorrect for thermal stability :-

Options

A. `BeO gt MgO gt CaO`
B. `NH_(3) lt H_(2)O lt HF`
C. `HF lt HCl lt HBr lt HI`
D. `Li_(2)SO_(2) lt Na_(2)SO_(4) lt K_(2)SO_(4) lt Rb_(2)SO_(4) lt Cs_(2)SO_(4)`

Answer

To determine which order of thermal stability is incorrect, we will analyze the thermal stability of various compounds mentioned in the question.

### Step-by-Step Solution:

1. **Understanding Thermal Stability**:
Thermal stability refers to the ability of a compound to remain stable upon heating. The more stable a compound is, the less likely it is to decompose when heated.

2. **Analyzing Alkaline Earth Metal Oxides**:
- The oxides of alkaline earth metals (Group 2) are considered.
- As we move down the group from BeO to BaO, the thermal stability of these oxides decreases.
- Therefore, the order of thermal stability for alkaline earth metal oxides is correct: **BeO > MgO > CaO > SrO > BaO**.

3. **Analyzing NH3, H2O, and HF**:
- Next, we compare ammonia (NH3), water (H2O), and hydrogen fluoride (HF).
- HF is highly polar due to the high electronegativity of fluorine, leading to strong hydrogen bonding.
- Water has moderate hydrogen bonding, while ammonia has weaker hydrogen bonding compared to HF.
- Thus, the order of thermal stability is: **HF > H2O > NH3**, which is correct.

4. **Analyzing Hydrogen Halides (HX)**:
- The thermal stability of hydrogen halides (HCl, HBr, HI, HF) is considered.
- The order based on thermal stability is inversely proportional to the size of the halogen: **HF > HCl > HBr > HI**.
- Here, HI is the least stable due to the large size of iodine, leading to weak bond strength with hydrogen.
- Therefore, the order given in the question is incorrect if it states otherwise.

5. **Analyzing Sulfates of Alkali Metals**:
- The stability of alkali metal sulfates (e.g., Na2SO4, K2SO4, etc.) increases as we move down the group.
- Thus, the order of thermal stability for alkali metal sulfates is correct: **Li2SO4 < Na2SO4 < K2SO4 < Rb2SO4 < Cs2SO4**.

6. **Conclusion**:
- The only incorrect order provided in the question is regarding the thermal stability of hydrogen halides, particularly if it suggests that HI is more stable than HF.
- Therefore, the answer to the question is that the incorrect order for thermal stability is related to the hydrogen halides.

### Final Answer:
The incorrect order for thermal stability is related to the hydrogen halides (option c).