Question

Arrange the following:
HOF, HOCl, HOBr, HOI in order of increasing acid strength.

Answer 1

Hint: The given acids are oxyacids and they are acids which contains hydrogen, oxygen and at least one other element with at least one hydrogen atom bond to oxygen that can dissociate to form $H^{+}$ cation an anion. The acidic strength of oxoacids of halogens will depend on the electronegativity and size of the halogen atom.

Complete Solution :
- The given acids are hypofluorous acid (HOF), hypochlorous acid (HOCl), hypobromous acid (HOBr) and hypoiodous acid (HOI). Every oxoacid has basically X−OH bond, whereas most oxoacids have $X=O$ bonds present in them and the central atom in the oxoacids is $s{p}^3$ hybridized.

- The strength of an oxoacid of halogen depends on the electronegativity and size of the halogen atom. Also, the acidic strength will increase with an increase in the oxidation number of halogens.

- In all the given four oxyacids, the same atom oxygen is attached to H in all acids. As we know, the electronegativity of halogen (F, Cl, Br and I) attached to oxygen decreases down a group in a periodic table, the polarity of OH bond decreases and acid strength decreases.

- Higher the electronegativity of the halogen atom, higher will be the tendency to pull electrons and weaken the O−H bond and thus the proton ($H^{+}$) can be easily removed. Thus fluorine is the element with higher electronegativity and thus HOF will be the strongest acid and iodine has lowest electronegativity and therefore HI will be the weakest acid.

- Thus, the acids can be arranged in the order of increasing acid strength as follows
$$HOI\text{<}HOBr\text{<}HOCl\text{<}HOF$$

Note: It should be noted that due to the high electronegativity and very small size of fluorine, it can form only one oxoacid (HOF) whereas the other elements of the halogen family can form several oxoacids. Also, the oxoacids are stable in aqueous solution or in the form of salts.