Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

The decreasing order of acidic nature of \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (I), \(\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{II})\), and \(\mathrm{HClO}_{4}(\mathrm{III})\) is (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\) (c) \(\mathrm{III}>\mathrm{I}>\mathrm{II}\) (d) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Step-by-Step Solution

Answer

The correct order is (c) \( \mathrm{III} > \mathrm{I} > \mathrm{II} \).

Step 1: Understand Acid Strength

The strength of an acid is measured by its ability to donate a proton (H extsuperscript{+}). A key factor affecting this is the stability of the conjugate base formed after the proton is donated. More stable conjugate bases correspond to stronger acids.

Step 2: Compare Acidic Nature of Each Acid

\( \mathrm{HClO}_4 \) (Perchloric acid) is a very strong acid because its conjugate base, \( \mathrm{ClO}_4^- \), is highly stable due to resonance. \( \mathrm{H}_2\mathrm{SO}_4 \) (Sulfuric acid) is also a strong acid, but slightly weaker than \( \mathrm{HClO}_4 \). \( \mathrm{H}_3\mathrm{PO}_4 \) (Phosphoric acid) is weaker than both, as its ability to donate protons is less and its conjugate base is less stable.

Step 3: Establish Order of Acidity

Order the acids by their ability to donate protons: \( \mathrm{HClO}_4 > \mathrm{H}_2\mathrm{SO}_4 > \mathrm{H}_3\mathrm{PO}_4 \). This reflects the trend noticed in Step 2, with \( \mathrm{HClO}_4 \) being the strongest acid, followed by \( \mathrm{H}_2\mathrm{SO}_4 \), and finally \( \mathrm{H}_3\mathrm{PO}_4 \).

Step 4: Choose the Correct Option

Among the given options, compare them with the order established in Step 3: \( \mathrm{III} > \mathrm{I} > \mathrm{II} \). The correct order of acidity is option (c).

Key Concepts

Conjugate Base Stability

When an acid donates a proton, it leaves behind a residual species known as the conjugate base. The stability of this conjugate base plays a crucial role in determining the acid's strength. The more stable the conjugate base, the stronger the acid. This is because a stable conjugate base is less likely to recombine with a proton.

Conjugate base stability can be attributed to several factors including:

• Resonance: This is the delocalization of electrons in the conjugate base. If the base can spread out its charge over multiple atoms, it becomes more stable. For example, the conjugate base of perchloric acid (\( \mathrm{ClO}_4^- \)) showcases excellent resonance stability, making \( \mathrm{HClO}_4 \) a very strong acid.
• Electronegativity: A more electronegative atom holding onto the negative charge will be more stable. This concept helps explain why \( \mathrm{H}_2\mathrm{SO}_4 \), with its highly electronegative oxygen atoms, is a strong acid.
• Inductive Effect: Electronegative atoms can stabilize the conjugate base by pulling electron density away from the charged center, which stabilizes the negative charge.

In essence, stronger conjugate bases emerge from systems where the negative charge can be effectively stabilized through these mechanisms.

Proton Donation Ability

Acid strength is fundamentally about how well an acid can donate its proton. Think of it as how easily an acid can "let go" of its hydrogen ion \( \mathrm{H}^{+} \). This ability is directly connected to how stable the remaining conjugate base is.

For instance:

• Perchloric acid (\( \mathrm{HClO}_4 \)) is superb at giving away its proton because, once it does, the resulting \( \mathrm{ClO}_4^- \) ion is very stable due to strong resonance.
• Sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)) also readily donates protons, albeit slightly less than perchloric acid, reflecting \( \mathrm{H}_2\mathrm{SO}_4 \)'s slightly lower acidity.
• Phosphoric acid (\( \mathrm{H}_3\mathrm{PO}_4 \)), however, finds it relatively harder to part with its protons. This is due to its conjugate base lacking the same degree of stability from resonance and electronegativity.

Thus, the order of acids from strongest to weakest, in terms of proton donation ability, follows from their respective conjugate base stabilities.

Order of Acidity

Ordering acids by their acid strength helps us arrange them in a decreasing order based on their intense ability to donate protons. In the case of \( \mathrm{HClO}_4 \), \( \mathrm{H}_2\mathrm{SO}_4 \), and \( \mathrm{H}_3\mathrm{PO}_4 \), it is important to compare their conjugate base stabilities and understand their effectiveness in giving away a proton.

After analyzing each acid:

• Perchloric acid (\( \mathrm{HClO}_4 \)): Due to strong resonance stabilization of its conjugate base, it tops the list as the strongest acid.
• Sulfuric acid (\( \mathrm{H}_2\mathrm{SO}_4 \)): While very strong, it ranks second with slightly less stability compared to perchloric acid's conjugate base.
• Phosphoric acid (\( \mathrm{H}_3\mathrm{PO}_4 \)): Not being able to donate protons as easily as the others, it ranks last due to less stable conjugate base.

This understanding aligns us with the order: \( \mathrm{HClO}_4 > \mathrm{H}_2\mathrm{SO}_4 > \mathrm{H}_3\mathrm{PO}_4 \) which reflects their decreasing order of acidity.