Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

pKa value of four acids are given below. The strongest acid is (I) \(4.0\) (II) \(3.5\) (III) \(2.5\) (IV) 2 (a) I (b) II (c) III (d) IV

Step-by-Step Solution

Answer

(d) IV

Step 1: Understanding pKa

The pKa value is an inverse indicator of the strength of an acid. The smaller the pKa value, the stronger the acid, because it means the acid dissociates more in water, thus releasing more protons (H+).

Step 2: Comparing pKa Values

We are given four pKa values: 4.0, 3.5, 2.5, and 2. We compare these values, taking into account that the lowest pKa corresponds to the strongest acid.

Step 3: Identifying the Strongest Acid

Among the given values, 2 is the lowest pKa. Therefore, the corresponding acid with pKa of 2 is the strongest acid.

Key Concepts

Understanding pKa Value

The pKa value is essential in understanding the strength of an acid in a solution. It serves as an inverse measure of the acid's potency. In simpler terms, the lower the pKa value, the stronger the acid. This inverse relationship is because a low pKa indicates that the acid dissociates better in water. This dissociation leads to more protons (H+) being released into the solution.

When comparing acids, always remember:

• A lower pKa means a stronger acid.
• A higher pKa signifies a weaker acid.

This is because the pKa value is logarithmic. Thus, a one-unit decrease in pKa corresponds approximately to a tenfold increase in acid strength. Use this concept to evaluate and compare different acids, and determine which one is the strongest.

Exploring Acid Dissociation

Acid dissociation refers to the process where an acid molecule releases protons into the surrounding solution. This process is fundamental to acid strength. When an acid dissociates, it breaks apart and forms ions, specifically hydrogen ions (H+). The extent to which this dissociation occurs varies with different acids.

Factors Affecting Acid Dissociation:

• Molecular Structure: Acids with weaker bonds to hydrogen are more likely to dissociate completely.
• Solvent Interaction: How the solvent interacts with the acid can influence dissociation, with water facilitating dissociation for many acids.
• Concentration of the Acid: Higher concentration might increase the degree of dissociation.

Comprehending this aspect of acid behavior helps in analyzing and predicting how effectively an acid can release protons in a solution.

The Process of Proton Release

The release of protons (H+) is central to what makes an acid "acidic". When an acid dissociates, the protons are liberated into the solution. This proton release impacts the acidity of the solution.

Why Proton Release Matters:

• pH Influence: The concentration of protons in a solution dictates the pH level, a critical measure of acidity.
• Reactivity: Protons can react with other substances in the solution, driving various chemical reactions.
• Electric Conductivity: More free-moving ions, like protons, improve the solution's conductivity.

Understanding how and why protons are released provides insights into the chemical behavior and reactivity of acids. This is pivotal for tasks ranging from laboratory experiments to industrial applications.