Question

Arrange in decreasing order of acid strength.

$HF,HCl,HBr,HI$

• A. $HF>HCl>HBr>HI$
• B. $HCl>HBr>HI>HF$
• C. $HI>HBr>HCl>HF$
• D. $HBr>HI>HCl>HF$

Answer 1

The correct option is C

$HI>HBr>HCl>HF$

One of the ways to define acidity is how easily will the molecule release $H^{+}$ ion.

Thus, we can conclude that $HF$ is a weak acid. What could be the reason? This is because of Hydrogen bonding and thus the $HF$ molecule would not release a proton easily.

Now that $HF$ is established as a weak acid, let us assess the other Hydracids. In each case, Hydrogen bonds with $Cl,BrandI$. In terms of size, $I>Br>Cl.$

Thus, the bond strength is in the order $HCl>HBr>HI$.

As the size of the anion increases, the internuclear distance between the Hydrogen and Halogen atoms increases. For hydracids, as we go down the Halogen group, it becomes more easier to release $H^{+}$ ions. Thus, order of acid strength is $HI>HBr>HCl>HF$