Question

Which of the following halogen acids is least acidic
(A) $HI$
(B) $HCl$
(C) $HF$
(D) $HBr$

Answer 1

Hint: Acidity is defined as the tendency of the compound to lose a proton. The rate of loss of protons is directly proportional to the nature of acidity. Loss of proton is directly linked to the stability of anion after the loss of ${H^ + }$. Hence, more the stability of the anion, more is the acidity.

Complete Step by Step Solution:
As we move from top to bottom in a group, the size of the atom increases and its electronegativity decreases. Thus, ${I^ - }$has the largest size whereas ${F^ - }$has the smallest size.

Thus, after the removal of ${H^ + }$, iodide ion is most stabilised because the negative charge is delocalized on a greater surface. Whereas in case of fluoride, the negative charge is accommodated on such a small area that is highly unstable and very reactive.
$HF$is least acid halogen acid can be also understood from the fact that the bond between $H$and $F$is so strong that loss of proton does not take place very easily and it does not dissociate easily.

Hence the order of acidity of halogen acids is $HI > HBr > HCl > HF$
The above order can also be explained by the HSAB principle according to which hard acid reacts with hard bases to form stable compounds. Similarly, soft acid forms compounds with soft bases. Thus, a compound of hard- hard or soft-soft does not dissociate easily.
${H^ + }$is a hard acid whereas ${F^ - }$is a hard base. Hence, the bond in $HF$is very strong. But ${I^ - }$is a weak base so the bond in $HI$is weak and hence, ${H^ + }$is donated easily.
So the correct answer is C.

Note: The above-mentioned definition of acids was proposed by Bronsted. Different scientists gave different acid-bases concepts. Arrhenius' concept is based on dissociation of compounds in water to give ${H^ + }$ as acid and $O{H^ - }$ as bases. Lewis talked about acid and bases in terms of electrons. A compound that loses electrons is called a base and one that accepts electrons is known as acid.