Question

Mark the weakest acid.
A. $HF$
B. $HCl$
C. $HBr$
D. $HI$

Answer 1

Hint: To answer this question we have to know about the acidic property of halogen hydrides. As we move from fluorine to iodine among halogens the size of the halogen atom increases and acidic character also increases.

Complete Step by Step Answer:
- According to Bronsted acid base theory the species which can donate its hydrogen ion is called an acid and the species or compound which can donate hydroxyl ion acts as a base.
- The compound which can donate hydrogen ions acts as an acid. As the hydrogen ion donating tendency of a compound increases its acidic character also increases.
- As we move from hydrogen fluoride to hydrogen iodide in halogen hydrides, the size of the halogen atom that is fluorine to iodine also increases. Thus the bond of the halogen atom with hydrogen atom becomes weaker and the halogen hydride can easily release the hydrogen atom and behaves as a strong acid.
- Thus among halogen hydrides hydrogen fluoride is the weakest and hydrogen iodide is the strongest halogen acid.
- Thus we can write that $HF$ is the weakest acid.
Thus the correct option is A.

Note: Hydrogen bonds present in a compound make the compound weakest acid due to strong bond. Among the halogen hydrides only hydrogen fluoride contains hydrogen bonding between electronegative fluorine and electropositive hydrogen atom. It has different properties from other halogen hydrides.