Question

In which one of the following arrangements the given sequence is not strictly according to the properties indicated against it

• A. CO₂ < SiO₂ < SnO₂ < PbO₂ :Increasing oxidizing power
• B. HF < HCl< HBr < HI:Increasing acidic strength
• C. H₂O < H₂S< H₂Se < H₂Te:Increasing pKa values
• D. NH₃ < PH₃< AsH₃ < SbH₃:Increasing acidic character

Answer 1

The Correct Option is C

The question asks which among the given options does not strictly follow the sequence according to the indicated property. Let's analyze each option to determine the correctness:

1. CO₂ < SiO₂ < SnO₂ < PbO₂: Increasing oxidizing power

   This sequence is based on the oxidizing power of the oxides from Group 14 elements. As we move down the group from CO₂ to PbO₂, the oxidizing power generally increases. Therefore, this sequence is correct.

2. HF < HCl < HBr < HI: Increasing acidic strength

   In binary acids of halogens, the acidic strength increases from HF to HI due to the increase in bond length and decrease in bond dissociation energy. This makes HI the strongest acid, and the sequence is correct.

3. H₂O < H₂S < H₂Se < H₂Te: Increasing pKa values

   The statement implies that the pKa value increases, suggesting a decrease in acidic strength. However, in reality, H₂Te is a stronger acid than H₂O, meaning the pKa would decrease down the group. Hence, this sequence is incorrect for increasing pKa values.

4. NH₃ < PH₃ < AsH₃ < SbH₃: Increasing acidic character

   As we move down Group 15 hydrides, the acidic character increases due to the decrease in bond energy and increase in atomic size, which makes the bond weaker. Thus, this sequence is correct.

Conclusion: The sequence H₂O < H₂S < H₂Se < H₂Te: Increasing pKa values does not correctly follow the properties indicated since pKa values should decrease with the increase in acidic strength. Therefore, the correct answer is this option.