Question

Assertion $\left(A\right):$ Increasing order of acidity of hydrogen halides is

$HF<HCl<HBr<HI$

Reason $\left(R\right):$ While comparing acids formed by the elements belonging to the same group of periodic table

$H-A$ bond strength is a more important factor in determining acidity of an acid than the polar nature of the bond.

• A. $A$ is true but $R$ is false.
• B. $A$ is false but $R$ is true.
• C. Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$
• D. Both $A$ and $R$ are true but $R$ is not the correct explanation of $A.$

Answer 1

The correct option is C

Both $A$ and $R$ are true and $R$ is the correct explanation of $A.$

Analyzing the assertion:

Increasing order of acidity of hydrogen halides should be

$HF<HCl<HBr<HI$

Which is correct as per assertion.

Analyzing the reason:

Bond strength $\propto \frac{1}{\text{Bond length}}$

The bond length of $HX\left(orHA\right)$ increases down the group due to decrease in effective overlapping between orbitals of $H$ and $X.$

Also acidity is $\propto K_a\propto$ dissociation of acids to liberate $H^{+}$ ion.

Since down the group, bond length increases, bond strength decreases and thus $H^{+}$ can be easily produced by dissociation of acid.

In $HI,$ the bond length is largest in given group and thus it would be the most acidic.

Similarly, $HF$ will be having smallest bond length (because of effective overlapping between orbitals of $H$ and $F$) and thus highest bond strength which makes release of proton difficult and makes it least acidic.

$\therefore HI$ is more acidic while $HF$ is least acidic.

And there this bond strength observed to be more important factor in determining acidity of an acid than the polar nature of the bond

Hence, both Assertion and Reason is correct and Reason is correct explanation of Assertion.

$\therefore$ Correct answer is option $\left(A\right).$