Problem 26 - A Complete Resource Book in Chemistry for JEE Main

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) \(\mathrm{HClO}_{3}>\mathrm{HClO}_{2}>\mathrm{HClO}_{4}>\mathrm{HClO}\)

Step-by-Step Solution

Answer

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

Step 1: Understanding Relative Acidity

The acidity of an oxyacid (such as the given HClO, HClO_2, HClO_3, and HClO_4) generally increases with the oxidation state of the central atom. For chlorine, higher oxidation states lead to a greater ability to donate protons (H+), resulting in stronger acids.

Step 2: Analyzing Chlorine Oxidation States

Determine the oxidation state of chlorine in each oxyacid: - HClO: Chlorine has an oxidation state of +1. - HClO_2: Chlorine has an oxidation state of +3. - HClO_3: Chlorine has an oxidation state of +5. - HClO_4: Chlorine has an oxidation state of +7. Higher oxidation states indicate stronger acids.

Step 3: Evaluating Acid Strength

Since HClO_4 has the highest oxidation state (+7), it is the strongest acid. HClO, with the lowest oxidation state (+1), is the weakest acid. Thus, the relative acidity increases in the order: HClO < HClO_2 < HClO_3 < HClO_4.

Step 4: Selecting the Correct Order from Options

Compare the conclusion from step 3 with the given options: - Option (a): HClO > HClO_2 > HClO_3 > HClO_4 (incorrect) - Option (b): HClO_4 > HClO_3 > HClO_2 > HClO (correct) - Option (c): HClO > HClO_4 > HClO_2 > HClO_3 (incorrect) - Option (d): HClO_3 > HClO_2 > HClO_4 > HClO (incorrect) The correct answer is option (b).

Key Concepts

Understanding Oxyacids

Oxyacids are a type of acid that contains oxygen. They are composed of hydrogen, oxygen, and another element (the central atom, often a non-metal). In the case of hydrochloric oxyacids, the central atom is chlorine. Oxyacids are common in chemistry and play essential roles in both industrial and biological processes.
These acids differ from binary acids like hydrochloric acid (HCl) because they also contain oxygen in their molecular structure. The strength of an oxyacid largely depends on the central non-metal atom and how it influences the molecule's ability to yield protons in solution.

• For example, in the oxyacid series HClO, HClO_2, HClO_3, and HClO_4, the variations in acidity are largely drawn from the differences in the oxidation states of chlorine.
• In general, the higher the oxidation state of the central atom, the stronger the oxyacid.

The electron-withdrawing effect of oxygen and the electronegative nature of chlorine in different oxidation states significantly influence how easily the hydrogen ion can dissociate.

The Role of Oxidation State

The concept of oxidation state is pivotal in determining the properties of oxyacids. An oxidation state is a hypothetical charge an atom would have if all bonds to atoms of different elements were completely ionic. This state helps in understanding not only the formula but also the reactivity and acidity of compounds.
In the context of oxyacids in our exercise (HClO, HClO_2, HClO_3, HClO_4),

• HClO has chlorine in the +1 oxidation state.
• HClO_2 has chlorine in the +3 oxidation state.
• HClO_3 has chlorine in the +5 oxidation state.
• HClO_4 has chlorine in the +7 oxidation state.

Higher oxidation states mean a stronger acid. Why? Because the central atom's enhanced ability to stabilize an additional negative charge allows the molecule to more readily release a proton when dissolved in water. Thus, a higher oxidation state corresponds to stronger acids capable of donating protons more effectively.

Proton Donation in Acids

Proton donation is a key process in determining the strength of an acid. When an acid donates a proton (typically written as \( \text{H}^+ \)), it usually forms a conjugate base. The willingness and ability of an acid to donate a proton is one measure of its strength.
In oxyacids, the presence of electronegative atoms such as oxygen alongside a central non-metal element like chlorine profoundly impacts this proton donation capability.

• The more likely an acid is to release a proton, the more acidic it is.
• ClO, ClO_2, ClO_3, and ClO_4 groups are different due to their oxygen content, which affects the acid's behavior.
• More oxygen generally means a higher oxidation state for chlorine, which enhances the aggregate negative charge stabilization when the proton is donated.

Thus, stronger oxyacids have central atoms with higher oxidation states, enabling them to give up protons more easily and efficiently, resulting in a more substantial acidic property.