Problem 56 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the weakest acid? (a) \(\mathrm{HCOOH}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\)

Step-by-Step Solution

Answer

The weakest acid is \( \mathrm{CH}_{3} \mathrm{COOH} \).

Step 1: Understanding Acid Strength

Acid strength is determined by the acid's ability to donate protons ( H^{+} ions) in solution. A weaker acid has a lower tendency to give away its H^{+} ions compared to a stronger acid.

Step 2: Evaluate Acids Provided

We have two acids: HCOOH (formic acid) and CH_{3}COOH (acetic acid). To compare their strength, we need to understand their dissociation in water.

Step 3: Refer to Relative pKa Values

The pKa value helps in determining acid strength. The lower the pKa value, the stronger the acid. Formic acid has a lower pKa (~3.75) than acetic acid (~4.76), indicating formic acid is stronger.

Step 4: Determine the Weakest Acid

Since acetic acid ( CH_{3}COOH ) has a higher pKa value, it is less willing to donate H^{+} ions than formic acid. Hence, it is the weaker acid.

Key Concepts

Proton Donation

In the context of acids, understanding proton donation is crucial. Acids have the ability to donate protons, represented as hydrogen ions (\(H^{+}\)). This process of losing a proton is the fundamental action that defines an acid's strength in a chemical sense.
Acids that donate protons readily in water are considered strong acids. They ionize completely, releasing their \(H^{+}\) ions into the solution, which is a sign of strong reactive behavior.

• Stronger acids = Higher tendency to donate protons.
• Weaker acids = Lower tendency to donate protons.

Whenever you encounter an acid and need to evaluate its strength, think about its willingness to let go of those \(H^{+}\) ions. The easier it loses protons, the stronger the acid. However, if an acid resists giving up protons, it is weaker. This concept is foundational when comparing acids like formic acid (\(HCOOH\)) and acetic acid (\(CH_{3}COOH\)). Understanding which acid is weaker or stronger revolves around how readily each can donate its proton.

pKa Values

The pKa value is a numerical representation that helps us easily compare the strength of different acids. It is derived from the acid dissociation constant (Ka) and calculated as the negative logarithm of the Ka value:\[ \text{pKa} = -\log_{10}(K_a) \] The lower the pKa value, the stronger the acid.

For example, with formic acid having a pKa of approximately 3.75 and acetic acid with a pKa around 4.76, we can conclude that formic acid is stronger.

• Lower pKa = stronger acid (more likely to donate protons).
• Higher pKa = weaker acid (less likely to donate protons).

Understanding pKa values provides a clear, numerical approach to grasping acid strength, helping us visualize how acids behave in different chemical environments.

Formic Acid vs Acetic Acid

When comparing formic (\(HCOOH\)) and acetic acid (\(CH_{3}COOH\)), the focus is on their ability to donate protons in solutions, which is directly related to their pKa values.
Formic acid, with a pKa of approximately 3.75, is stronger compared to acetic acid, which has a pKa of around 4.76. This means formic acid is more willing to release protons than acetic acid in solution.

• Formic Acid (\(HCOOH\)) = Stronger acid, lower pKa.
• Acetic Acid (\(CH_{3}COOH\)) = Weaker acid, higher pKa.

These differences in acidity can be influenced by the molecular structure of each acid. Formic acid has a hydrogen atom connected directly to the carboxyl group, while acetic acid has a methyl group that slightly reduces its ability to donate protons.
In practical terms, whenever dealing with solutions where acid strength is a consideration, knowing these distinctions helps in predicting and controlling chemical reactions effectively.