Question

How does the oxyacid strength of the halogens vary as the number of oxygens in the formula increases?

Short answer

The strength of oxyacids of halogens increases as the number of oxygen atoms in the formula increases. This occurs because additional oxygen atoms make the O-H bond more polar, facilitating the donation of an H+ ion, and also stabilize the conjugate base by delocalizing the negative charge over the multiple oxygen atoms attached to the central halogen atom.

Step-by-step solution

- Understanding Acid Strength

Acid strength is determined by the ability of the acid to donate a proton (H+) and the stability of its conjugate base. An acid is stronger when it easily donates the H+ ion and results in a stable conjugate base.

- Role of Oxygen in an Oxyacid

In an oxyacid, a halogen (X) is attached to an oxygen atom, and the oxygen atom is also attached to a Hydrogen atom (making an OH bond). When an oxyacid donates a proton (steps to behave as an acid), it's the Hydrogen atom connected to an Oxygen atom that is given.

- Increasing the Number of Oxygen Atoms

If the number of oxygen atoms in the formula increases, these additional oxygen atoms will also be linked to the halogen atom. These oxygen atoms are highly electronegative which means they draw electron density towards themselves. This makes the O-H bond more polar and makes it easier to donate the H+ ion, thus increasing the acid strength.

- The Stability of the Conjugate Base

The additional oxygen atoms do not only improve the ability to donate an H+ ion, but they also stabilise the conjugate base. After the H+ ion is given away, a negative charge is left on the oxygen atom. That negative charge can be spread (delocalized) over the multiple oxygen atoms attached to the central Halogen atom. As a result, it stabilizes the negative charge on the conjugate base, thus making the acid stronger. Hence, as we increase the number of oxygen atoms in the formula of the oxyacid of a halogen, the strength of the acid increases.