Question

Arrange the acids in terms of their strength.

Answer 1

Hint: The acid strength of the halogen acids depends on how stronger the bond is between the halogen acids. Weaker the bond, more easily it will donate protons, more will be the acidic strength.

Complete answer:
Recalling the periodic trends that ionic size increases as we move down the group. Because fluorine is at the top of the halogens, the fluorine ion is a smaller halide; therefore, the $ H - F $ bond is short. Shorter bonds are more stable, and thus the $ H - F $ bond is more difficult to break so the hydrogen ion can’t release easily. Chlorine is large and has more electrons, and therefore the $ H - Cl $ bond is longer and weaker. In presence of water it is easy to break the $ H - Cl $ bond, and the ions dissociate in solutions.
The same reasoning applied for both $ HBr $ and $ HI $ . These acids are even stronger than $ HCl $ because the $ B{r^ - } $ and $ {I^ - } $ ions are even larger. As such, the $ H - Br $ and $ H - I $ bonds are even weaker, and these compounds also readily dissociate in solution.
So, $ HI $ being bigger in size, has the lowest value of the bond strength and thus, will have the highest acidic character. On the other hand, $ HF $ being smaller in size, has the highest value of the bond strength and thus, will have the lowest acidic character.
Thus, the order of acidic character is as:
 $ HI > HCl > HBr > HF $ .

Note:
Remember, moving down the group decreases the electronegativity because the size of the element increases thus, the atom becomes easy to lose $ {H^ + } $ ion. This causes the increase in the strength down the group. And the size increases from top to bottom and hence bond decreases and thus, acidic character increases.