Question

In which case, the order of acidic strength is not correct?
(a)- HI > HBr > HCl
(b)- \[HI{{O}_{4}}\,>HBr{{O}_{4}}>\,HCl{{O}_{4}}\]
(c)- \[HCl{{O}_{4}}>HCl{{O}_{3}}>HCl{{O}_{2}}\]
(d)- \[HF\,>{{H}_{2}}O\,>N{{H}_{3}}\]

Answer 1

Hint: An acid can be defined as a species which readily gives hydrogen ions in the solution. If the hydrogen atom is weakly bonded then it will be released easily, making the acid stronger in nature. Similarly, if the hydrogen atom is strongly bonded then it cannot be released easily, making the acid a weak acid.

Complete step by step solution:

Let us go through our options one by one.

(a)- HCl, HBr, and HI are binary acids within the same group. The atom bonded to hydrogen, in our case Cl, Br, I respectively, becomes larger as we proceed down the group, which tends to weaken the bond to H. As a result, binary acids become more acidic as we move from top to bottom in the group. So, the correct order of acidic strength is: HI > HBr > HCl

(b)- \[HCl{{O}_{4}}\], \[HBr{{O}_{4}}\], and \[HI{{O}_{4}}\,\] are oxyacids with the same number of oxygens but with different central atoms. As the electronegativity of the central atom increases, the bond strength of O-H weakens, resulting in the increase in acidity. So, the correct order of acidic strength is: \[HCl{{O}_{4}}>HBr{{O}_{4}}>HI{{O}_{4}}\,\].

(c)- In the case of oxyacids, the H is always bonded to one of the oxygens. In oxyacid series such as \[HCl{{O}_{4}}\], \[HCl{{O}_{3}}\] , and \[HCl{{O}_{2}}\], with the increase in the number of oxygens bonded to the central atom, the oxidation number of the central atom also increases. This causes weakening of the O-H bond strength. As a result, an increase in the acidity. So, the correct order of acidic strength is: \[HCl{{O}_{4}}>HCl{{O}_{3}}>HCl{{O}_{2}}\].

(d)- In hydrides, acidity of the hydrides of the corresponding elements also increases with increase in electronegativity of the element. We can see that F is most electronegativity followed by O, N. As a result, the acidity increases. So, the correct order of acidic strength is: \[HF\,>{{H}_{2}}O\,>N{{H}_{3}}\].
So, the correct option is (b).

Note: In general, if a more electronegative atom is attached to the H atom, it will try to pull all the shared pairs of electron/s towards itself. This will result in hydrogen atoms easily being released, and the compound becomes a strong acid.