Question

The correct order of relative acidity is: (a) \(\mathrm{HClO}>\mathrm{HClO}_{2}>\mathrm{HClO}_{3}>\mathrm{HClO}_{4}\) (b) \(\mathrm{HClO}_{4}>\mathrm{HClO}_{3}>\mathrm{HClO}_{2}>\mathrm{HClO}\) (c) \(\mathrm{HClO}>\mathrm{HClO}_{4}>\mathrm{HClO}_{2}>\mathrm{HClO}_{3}\) (d) \(\mathrm{HClO}_{3}>\mathrm{HClO}_{2}>\mathrm{HClO}_{4}>\mathrm{HClO}\)

Short answer

The correct order of relative acidity is (b) \(\mathrm{HClO}_{4} > \mathrm{HClO}_{3} > \mathrm{HClO}_{2} > \mathrm{HClO}\).

Step-by-step solution

Understanding the Compounds

The compounds in the given options are oxyacids of chlorine: \(\mathrm{HClO}\), \(\mathrm{HClO}_{2}\), \(\mathrm{HClO}_{3}\), and \(\mathrm{HClO}_{4}\). Their acidity depends on the number of oxygen atoms bonded to chlorine. Generally, more oxygens mean a stronger acid.

Determine the Effect of Oxygen on Acidity

For oxyacids, the acidity increases with the number of oxygen atoms. This is because more oxygen atoms increase the electron-withdrawing ability, thereby stabilizing the conjugate base after the acid donates a proton (\(\mathrm{H}^+\)). The order is \(\mathrm{HClO}_{4}\) > \(\mathrm{HClO}_{3}\) > \(\mathrm{HClO}_{2}\) > \(\mathrm{HClO}\).

Match the Correct Order to the Options

Checking the options, we see that only option (b) presents the order \(\mathrm{HClO}_{4}\) > \(\mathrm{HClO}_{3}\) > \(\mathrm{HClO}_{2}\) > \(\mathrm{HClO}\), which is consistent with the trend determined from step 2.

Key concepts

Oxyacids of Chlorine

Oxyacids of chlorine are a group of acids where chlorine is bonded to one or more oxygen atoms. These acids include hypochlorous acid (HClO), chlorous acid (HClO₂), chloric acid (HClO₃), and perchloric acid (HClO₄). The structure of these acids greatly influences their acidity levels. A primary concept to understand is that the more oxygen atoms are attached to the chlorine, the stronger the acid. This essentially creates a better electron-withdrawing environment, making it easier for the acid to donate its proton ( H}^+ ). More oxygen atoms mean that the electron density can spread out better once the acid loses a proton, increasing the stability of the resulting anion, or conjugate base. So, the key takeaway here is: the number of oxygen atoms plays a crucial role in determining the strength of these oxyacids.

Electron-Withdrawing Effect

Electron-withdrawing effect is a term used to describe how atoms or groups of atoms can pull electron density away from other parts of a molecule. This effect can significantly influence the acidity of a compound, particularly in the context of oxyacids of chlorine. In these oxyacids, oxygen atoms exhibit a strong electron-withdrawing effect. This means they decrease the electron density around the hydrogen atom in the O-H bond. With less electron density, the H atom can more easily dissociate as a proton ( H}^+ ), thereby increasing the acidity. Effects on Acidity:

• More oxygen atoms increase the electron-withdrawing effect.
• The stronger the electron-withdrawing effect, the more acidic the oxyacid.
• This effect leads to a greater stabilization of the conjugate base.

Understanding this effect helps explain why perchloric acid ( HClO₄ ) is stronger than hypochlorous acid ( HClO ). It highlights the pivotal role of electron-withdrawing groups in influencing acid strength.

Conjugate Base Stability

The stability of a conjugate base is a critical factor in determining the acidity of an acid. When an acid donates a proton ( H}^+ ), it forms its conjugate base. The more stable this conjugate base, the stronger the acid. In oxyacids of chlorine, conjugate base stability is increased when there is more effective dispersal of the negative charge that remains after proton donation. Oxygen atoms are key players here because they can distribute this charge over a larger area of the molecule. Stability Factors:

• The more oxygen atoms, the better the charge distribution.
• Better charge distribution stabilizes the conjugate base.
• Increased stability of the conjugate base correlates with increased acid strength.

This phenomenon explains why, as the number of oxygen atoms in an oxyacid increases, its acidity typically increases as well. Perchloric acid ( HClO₄ ) offers the optimal distribution for charge stability, making it the strongest among these acids.