Question

Which of the following order is true about the boiling points of HF, HCl, HBr and HI?

• A. $\mathrm{HCl}$ > $\mathrm{HF}$ > $\mathrm{HBr}$ > $\mathrm{HI}$
• B. $\mathrm{HF}$ >$\mathrm{HCl}$ >$\mathrm{HBr}$ > $\mathrm{HI}$
• C. $\mathrm{HI}$ >$\mathrm{HBr}$ > $\mathrm{HCl}$ > $\mathrm{HF}$
• D. $\mathrm{HF}$ >$\mathrm{HI}$ > $\mathrm{HBr}$ > $\mathrm{HCl}$

Answer 1

The correct option is D

$\mathrm{HF}$ >$\mathrm{HI}$ > $\mathrm{HBr}$ > $\mathrm{HCl}$

Explanation of the correct option:

(D)$\mathrm{HF}$ >$\mathrm{HI}$ > $\mathrm{HBr}$ > $\mathrm{HCl}$

• $\mathrm{HF}$ has a strong hydrogen bond, so it has the highest boiling point.
• $\mathrm{HI}$ is large in size and their van der Waal force is also high, so they have a high boiling point.
• Similarly, $\mathrm{HBr}$ boiling point is more than $\mathrm{HCl}$, because the size of $\mathrm{HBr}$ molecule is more than $\mathrm{HCl}$ so its boiling point is also high.

Explanation of incorrect options:

(A)$\mathrm{HCl}$ > $\mathrm{HF}$ > $\mathrm{HBr}$ > $\mathrm{HI}$

• The order is not correct, the boiling point $\mathrm{HCl}$ cannot be more than $\mathrm{HF}$, $\mathrm{HBr}$ and $\mathrm{HI}$, because the van der Waal force is not so strong in $\mathrm{HCl}$ than $\mathrm{HF}$.

(B)$\mathrm{HF}$ >$\mathrm{HCl}$ >$\mathrm{HBr}$ > $\mathrm{HI}$

• The boiling point $\mathrm{HCl}$ is less than $\mathrm{HI}$and $\mathrm{HBr}$ , because the van der Waal force is stronger in $\mathrm{HI}$ than $\mathrm{HCl}$.

(C)$\mathrm{HI}$ >$\mathrm{HBr}$ > $\mathrm{HCl}$ > $\mathrm{HF}$

• $\mathrm{HF}$has a strong hydrogen bond and so its boiling point cannot be less than $\mathrm{HCl}$.

Hence, option (D) is correct., the order of boiling point is $\mathrm{HF}$ >$\mathrm{HI}$ > $\mathrm{HBr}$ > $\mathrm{HCl}$.