Answer the question on basis of passage given below:
All the members of Group 15 elements form covalent hydrides with hydrogen as \(NH_3, PH_3, AsH_3, SbH_3, BiH_3\). Group 16 elements form covalent hydrides with hydrogen as \(H_2O, H_2S, H_2Se, H_2Te, H_2Po\). Group 17 elements form covalent hydrides with hydrogen as \(HF, HCl, HBr, HI\). Based on these facts answer the following question.
In aqueous solution, the acidic strength of hydrides of Group 17 elements i.e. \(HF, HCl, HBr, HI\) increases as:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(HF < HCl < HBr < HI\)
The strength of a hydrohalic acid in water depends on how easily the H–X bond breaks to release \( \ce{H^{+}} \). Down the group the halogen atom becomes larger, the H–X bond becomes longer and the bond dissociation enthalpy decreases:
H–F > H–Cl > H–Br > H–I
The weaker the bond, the more easily the proton is released, so the acidic strength increases in the reverse order: HF < HCl < HBr < HI. HF is a weak acid because of its very strong H–F bond, while HI is the strongest.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Reactivity towards Hydrogen – “… The acidic strength of these acids varies in the order: HF < HCl < HBr < HI. The stability of these halides decreases down the group due to decrease in bond (H–X) dissociation enthalpy in the order: H–F > H–Cl > H–Br > H–I.”