Problem 272 - A Complete Resource Book in Chemistry for JEE Main

Question

Four species are listed below [2008] (I) \(\mathrm{HCO}_{3}^{-}\) (II) \(\mathrm{H}_{3} \mathrm{O}^{+}\) (III) \(\mathrm{HSO}_{4}^{-}\) (IV) \(\mathrm{HSO}_{3} \mathrm{~F}\) Which one of the following is the correct sequence of their acid strength? (a) IV

Step-by-Step Solution

Answer

(c) I < III < II < IV

Step 1: Analyze the Species

We need to determine the relative acid strength of the given species: (I) \( \mathrm{HCO}_{3}^{-} \), (II) \( \mathrm{H}_{3} \mathrm{O}^{+} \), (III) \( \mathrm{HSO}_{4}^{-} \), and (IV) \( \mathrm{HSO}_{3} \mathrm{~F} \). In general, acid strength is influenced by the stability of the resulting conjugate base after donating a proton.

Step 2: Identify Strong Acids

\( \mathrm{H}_{3} \mathrm{O}^{+} \) is known to not be a strong acid, even though it plays an important role in acid-base chemistry as a strong acid relative to water. \( \mathrm{HSO}_{4}^{-} \) is a stronger acid than \( \mathrm{H}_{3} \mathrm{O}^{+} \) because its conjugate base, \( \mathrm{SO}_{4}^{2-} \), is very stable.

Step 3: Analyze Other Species

\( \mathrm{HSO}_{3} \mathrm{~F} \) is a very strong acid (fluorosulfonic acid); thus, it should have the highest acidity among all the given species. \( \mathrm{HCO}_{3}^{-} \) is a weaker acid than \( \mathrm{HSO}_{4}^{-} \), considering \( \mathrm{HSO}_{4}^{-} \)'s greater stability as a conjugate base.

Step 4: Determine Correct Acid Strength Order

By comparing their acid strengths and conjugate base stability, the correct sequence of acid strength is: \( \mathrm{I} < \mathrm{III} < \mathrm{II} < \mathrm{IV} \). This aligns with choice (c).

Key Concepts

Conjugate Base

In acid-base chemistry, the concept of a conjugate base is fundamental to understanding acid strength. When an acid donates a proton (p^+ ext{H}^+), it forms a conjugate base. The stability of this conjugate base heavily influences the acid's strength.

• If a conjugate base is stable, the original acid tends to be stronger.
• If a conjugate base is unstable, the acid tends to be weaker.

For example, when \( ext{HSO}_4^-\), derived from sulfuric acid, loses a proton, it forms \( ext{SO}_4^{2-}\). The \( ext{SO}_4^{2-}\) ion is very stable due to resonance stabilization, making \( ext{HSO}_4^-\) a relatively strong acid. Conversely, \( ext{HCO}_3^-\)'s conjugate base, \( ext{CO}_3^{2-}\), is less stable, resulting in \( ext{HCO}_3^-\) being a weaker acid.

Fluorosulfonic Acid

Fluorosulfonic acid (\( ext{HSO}_3 ext{F}\)) is one of the strongest acids known. Its high acidity is due to its ability to donate a proton very efficiently and form a stable conjugate base. This compound is stronger than even sulfuric acid.

• Fluorosulfonic acid has a strong electron-withdrawing fluorine atom.
• This increases the positive charge on the hydrogen atom, making it more likely to dissociate as \( ext{H}^+\).

The molecule's structure plays a critical role in this behavior, where the electronegativity of fluorine and the resonance stabilization of the resulting conjugate base lead to a very high acid strength.

Acid-Base Chemistry

At its core, acid-base chemistry explores the interactions between acids and bases, emphasizing the transfer of protons.

• Acids are species that donate protons.
• Bases are species that accept protons.

The compatibility and relative strengths of these substances dictate reaction outcomes. An important metric in this field is the acid dissociation constant (\(K_a\)), which quantifies a species' acidity. Higher \(K_a\) values correspond to stronger acids. Understanding these interactions helps in predicting product formation in chemical reactions involving acids and bases. This knowledge is particularly useful in determining which species will dominate in a mixed solution environment.

Strong Acids

Strong acids are characterized by their complete ionization in solution, meaning they donate protons fully in aqueous environments. Some of the common strong acids include hydrochloric acid (\( ext{HCl}\)), sulfuric acid (\( ext{H}_2 ext{SO}_4\) in its first dissociation), and nitric acid (\( ext{HNO}_3\)).What makes an acid strong?

• A strong acid forms a stable conjugate base after donating a proton.
• The molecular structure supports efficient dissociation of the proton.

In the context of the exercise, fluorosulfonic acid (\( ext{HSO}_3 ext{F}\)) is a strong acid due to its effective proton donation and stable conjugate base formation. The presence of strong electron-withdrawing groups within the molecule aids in stabilizing the conjugate base, enhancing its acid character.