Problem 209 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements is true? (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is a stronger acid than \(\mathrm{H}_{2} \mathrm{SO}_{3}\) (b) in aqueous medium HF is a stronger acid than \(\mathrm{HCl}\) (c) \(\mathrm{HClO}_{4}\) is a weaker acid than \(\mathrm{HClO}_{3}\) (d) \(\mathrm{HNO}_{3}\) is a stronger acid than \(\mathrm{HNO}_{2}\)

Step-by-Step Solution

Answer

Statement (d) is true: \(\mathrm{HNO}_{3}\) is a stronger acid than \(\mathrm{HNO}_{2}\).

Step 1: Understanding Acid Strength

Acid strength is determined by the ability of an acid to donate protons (H⁺ ions) in an aqueous solution. Stronger acids dissociate more completely in water and are generally more likely to donate protons than weaker acids.

Step 2: Comparing H3PO3 and H2SO3

- \(\mathrm{H}_{3} \mathrm{PO}_{3}\) is phosphorous acid, while \(\mathrm{H}_{2} \mathrm{SO}_{3}\) is sulfurous acid.- \text{Phosphorous acid (H}_3\text{PO}_3) \text{ is weaker than sulfurous acid (H}_2\text{SO}_3) \text{ because it has fewer oxygens to pull electron density from the central atom, weakening the O-H bond.}- \text{Thus, statement (a) is false.}\

Step 3: Examining HF and HCl in Aqueous Medium

- \(\mathrm{HF}\) is hydrofluoric acid, and \(\mathrm{HCl}\) is hydrochloric acid.- \(\text{Hydrochloric acid (HCl) is stronger than hydrofluoric acid (HF) in aqueous solution. HF is a weak acid, meaning it does not dissociate completely in water, compared to the strong acid HCl.}\)- \text{Thus, statement (b) is false.}\

Step 4: Analyzing HClO4 and HClO3

- \(\mathrm{HClO}_{4}\) is perchloric acid, and \(\mathrm{HClO}_{3}\) is chloric acid.- Perchloric acid is stronger than chloric acid due to having a higher oxidation state of chlorine and more electronegative oxygen atoms that stabilize the negative charge.- \text{Therefore, statement (c) is false.}\

Step 5: Comparing HNO3 and HNO2

- \(\mathrm{HNO}_{3}\) is nitric acid, and \(\mathrm{HNO}_{2}\) is nitrous acid.- Nitric acid is a stronger acid than nitrous acid because it dissociates more completely in solution, having more oxygens to stabilize charge.- \text{Therefore, statement (d) is true.}\

Key Concepts

Oxidation State

The oxidation state is an important concept in chemistry that helps us determine how oxidized or reduced a compound is. It can be thought of as a theoretical charge on an atom if all bonds were completely ionic. The oxidation state is crucial when analyzing the strength of acids like \(\text{HClO}_4\) (perchloric acid) and \(\text{HClO}_3\) (chloric acid).

• The higher the oxidation state of the central atom, the stronger the acid typically is. This is because more electronegative atoms can better stabilize the negative charge after dissociation.
• In \(\text{HClO}_4\), the chlorine has an oxidation state of +7, making it a very strong acid.
• For \(\text{HClO}_3\), the chlorine has an oxidation state of +5, which makes it weaker than its +7 counterpart.

Understanding oxidation states is key to predicting and comparing the strength of different acids, especially those containing central atoms bonded to oxygen.

Acid Dissociation

Acid dissociation refers to the process by which an acid donates a proton (\(\text{H}^+\)) in aqueous solution. This is a fundamental aspect of determining acid strength.

• Stronger acids dissociate more completely, releasing more \(\text{H}^+\) ions into the solution.
• For example, hydrochloric acid (\(\text{HCl}\)) is considered a strong acid because it fully dissociates in water.
• In contrast, hydrofluoric acid (\(\text{HF}\)) only partially dissociates, making it a weak acid in aqueous solutions.
• This partial dissociation means that fewer protons are donated, resulting in less acidic strength.

Understanding acid dissociation is vital for predicting how an acid will behave in water and how it will affect chemical reactions.

Phosphorous Acid

Phosphorous acid (\(\text{H}_3\text{PO}_3\)) is a weaker acid compared to its counterparts like sulfurous acid (\(\text{H}_2\text{SO}_3\)). Here's why:

• This acid has fewer oxygen atoms connected to the phosphorous, leading to less deviation of electron density.
• This reduces the overall acidity as the electron-withdrawing effect isn't as strong, thus the O-H bond is not as polarized.
• Less polarization in the O-H bond means it's less likely to release a proton, making it a weaker acid.

Phosphorous acid is an important compound to understand when learning about different acids and their strengths based on their structures.

Sulfurous Acid

Sulfurous acid (\(\text{H}_2\text{SO}_3\)) showcases the typical characteristics of a stronger acidic compound than phosphorous acid.

• It has a molecular structure that involves more oxygens attached to the sulfur atom, which increases its electronegativity and ability to stabilize the conjugate base after the loss of a proton.
• This results in a stronger acid because of the better distribution and stability of the negative charge upon dissociation.
• It is an acid often discussed in the context of its dissociation ability and the role of oxygen atoms in acid strength.

Understanding sulfurous acid gives insight into how the presence of oxygen in acid compounds affects their strength and behavior in chemical reactions.