Question

Why is HOCl a stronger acid than HOBr ?

Answer 1

Hint: Both the molecules present are acid in nature and can donate ${H^ + }$ ion. The presence of electron withdrawing groups will affect their acidic strength. The chlorine is more electronegative atom than bromine because it is smaller in size and has high charge density.

Complete step by step answer:
First, let us understand what an acid is. Then, we will be able to distinguish between them.
The acid is defined as the species which can donate ${H^ + }$ ions. This concept was given by Bronsted and Lowry; so, named as Bronsted-Lowry concept.
The acid which can easily donate the ${H^ + }$ ion will be strong acid and the other one weak. The presence of electron withdrawing groups and electron donating groups have a large impact on the acidity of a molecule. The electron withdrawing groups attracts the electrons towards itself thereby resulting in easy donation. Thus, they increase the acidity of the molecule.
On the other hand the electron donating groups give electrons thereby resulting in decrease in acidity of the molecule.
Now, as we are done with our background let us see the molecules given in question.
The HOCl and HOBr both these can easily donate ${H^ + }$ ions. So, both are good acids. But we have to choose strong among them. The groups attached are Cl and Br; both are electron withdrawing in nature. So, both will increase the acidity of the molecules.
Now, the difference comes at this point. The chlorine is more electronegative than bromine. As a result, it will attract electrons more towards itself than bromine. This will result in a donation of ${H^ + }$ more easily in HOCl than in HOBr. So, the HOCl will be stronger acid than HOCl.

Note: It must be noted that both are strong acids. But when we have to choose one among these then we see the difference they have. Both have different halogen atoms attached. Further, we know that on moving top to bottom in the periodic table, the electronegativity decreases. And the electronegativity increases the acidic strength of the compound.