Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following acids (1) \(\mathrm{H}_{2} \mathrm{SO}_{3}\) (2) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (3) \(\mathrm{HClO}_{3}\), in the increasing order of acid strength. (a) \(2>3>1\) (b) \(3>2>1\) (c) \(1>3>2\) (d) \(1>2>3\)

Step-by-Step Solution

Answer

(b) \(3 > 2 > 1\)

Step 1: Understanding Acid Strength

Acid strength refers to an acid's ability to donate protons (H⁺ ions). Stronger acids dissociate more in solution, giving more H⁺.

Step 2: Comparing Acids by Structure and Electronegativity

Acid strength is influenced by the stability of its conjugate base and the electronegativity or oxidation state of the central atom. Typically, acids with more electronegative central atoms and those in higher oxidation states are stronger.

Step 3: Analyzing Each Acid

1. B\(\mathrm{H}_{2}\mathrm{SO}_{3}\): Sulfur is in a lower oxidation state here compared to sulfuric acid, and it's not very electronegative in this state.2. B\(\mathrm{H}_{3}\mathrm{PO}_{3}\): Phosphorous is less electronegative than sulfur or chlorine, making this a weaker acid.3. B\(\mathrm{HClO}_{3}\): Chlorine, in a high oxidation state and being highly electronegative, makes this a strong acid.

Step 4: Determining the Order

Considering the electronegativity and oxidation states: - B\(\mathrm{HClO}_{3}\) has the highest strength due to chlorine's high electronegativity and oxidation state.- B\(\mathrm{H}_{2}\mathrm{SO}_{3}\) is stronger than B\(\mathrm{H}_{3}\mathrm{PO}_{3}\) because sulfur, in an intermediate oxidation state, is more electronegative than phosphorous.

Step 5: Arranging in Increasing Order

Based on the analysis, the order from weakest to strongest is B\(\mathrm{H}_{3}\mathrm{PO}_{3}\) < \(\mathrm{H}_{2}\mathrm{SO}_{3}\) < \(\mathrm{HClO}_{3}\).

Key Concepts

Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself. In acids, the central atom's electronegativity plays a key role in influencing the acid's strength. The more electronegative the central atom, the stronger the pulling force it exerts on the shared electrons in the O-H bond of the acid. This causes an increased dissociation of the hydrogen ion (H⁺) in solution.

For instance, in the acids

• electronegativity of chlorine in
• electronegativity of phosphorus in
• electronegativity of sulfur in

\(\text{HClO}_3\), \(\text{H}_3\text{PO}_3\), \(\text{H}_2\text{SO}_3\), \chlorine is the most electronegative, making \(\text{HClO}_3\) the strongest of the three. \Phosphorus is less electronegative than both chlorine and sulfur, rendering \(\text{H}_3\text{PO}_3\) as the weakest of these acids. \The stronger electronegativity of sulfur compared to phosphorus makes \(\text{H}_2\text{SO}_3\) stronger than \(\text{H}_3\text{PO}_3\). This illustrates how electronegativity directly influences acid strength.

Oxidation State

The oxidation state of the central atom in an acid molecule significantly affects its acid strength. A higher oxidation state generally corresponds to a stronger acid, as it often results in stronger and more polar bonds that facilitate the release of H⁺ ions in solution.

When examining different acids, notice how:

• the oxidation state of chlorine in
• the oxidation state of sulfur in
• the oxidation state of phosphorus in

\(\text{HClO}_3\) is the highest among the three being compared, supporting its high acid strength. On the other hand, in \(\text{H}_2\text{SO}_3\), sulfur possesses a lower oxidation state but is higher than that of phosphorus in \(\text{H}_3\text{PO}_3\), making \(\text{H}_2\text{SO}_3\) stronger than \(\text{H}_3\text{PO}_3\). This helps underline how integral the oxidation state is in determining the hierarchical order of acid strength.

Conjugate Base Stability

The concept of conjugate base stability is crucial in understanding acid strength. When an acid donates its proton (H⁺), it forms a conjugate base. The more stable this conjugate base is, the stronger the original acid, because a stable conjugate base prefers to remain in its dissociated state rather than recombining with an H⁺ ion.

For the acids under consideration,

• \(\text{HClO}_3\) forms the conjugate base \(\text{ClO}_3^-\). This base is highly stable due to resonance structures that disperse negative charges effectively.
• \(\text{H}_2\text{SO}_3\) creates \(\text{HSO}_3^-\), which is also quite stable but less so compared to \(\text{ClO}_3^-\) because of fewer resonance options.
• \(\text{H}_3\text{PO}_3\) forms \(\text{HPO}_3^{2-}\). This conjugate base has limited resonance stabilization, rendering it the least stable of the three.
  This results in \(\text{H}_3\text{PO}_3\)’s lower acid strength.

Hence, the stability of the conjugate base significantly dictates the acid's ability to release H⁺, emphasizing its role in acid strength dynamics.