Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

pKa values of three acids \(\mathrm{A}, \mathrm{B}\) and \(\mathrm{C}\) are \(4.3,3.3\) and 5.5 respectively. Which of the following represents the correct order of acid strength? (a) \(\mathrm{A}>\mathrm{B}>\mathrm{C}\) (b) \(C>A>B\) (c) \(\mathrm{B}>\mathrm{A}>\mathrm{C}\) (d) \(\mathrm{C}>\mathrm{B}>\mathrm{A}\)

Step-by-Step Solution

Answer

Option (c) B > A > C is the correct order of acid strength.

Step 1: Understand the relationship between pKa and acid strength

The strength of an acid is inversely related to its pKa value. A lower pKa value indicates a stronger acid. This is because pKa is a measure of the acidity of a molecule, with lower values meaning the acid is more likely to donate a proton.

Step 2: Compare pKa values of each acid

Identify the pKa values given for each acid: - Acid A: 4.3 - Acid B: 3.3 - Acid C: 5.5. Since lower pKa indicates stronger acid, arrange them in ascending order of pKa.

Step 3: Determine the order of acid strength based on pKa

Arrange the acids based on their pKa values: - Acid B with a pKa of 3.3 is the strongest. - Acid A with a pKa of 4.3 is next. - Acid C with a pKa of 5.5 is the weakest. Hence, the correct order based on decreasing acid strength is B > A > C.

Step 4: Match the order with given choices

Compare the order B > A > C to the options provided: (a) A > B > C (b) C > A > B (c) B > A > C (d) C > B > A. The correct choice corresponding to B > A > C is (c) B > A > C.

Key Concepts

Acid Strength Comparison

When we talk about acid strength, we're referring to how easily an acid can donate a proton (a hydrogen ion). The easier it is for an acid to give away its proton, the stronger the acid is. In the problem with acids A, B, and C, the strength comparison revolves around their ability to release protons.
To determine which acid out of several is the strongest, we look at each acid's pKa value, which serves as a critical indicator. The general rule of thumb is:

• A lower pKa value signifies a stronger acid.
• A higher pKa value indicates a weaker acid.

For acids A, B, and C, the pKa values are 4.3, 3.3, and 5.5 respectively. Following the principle, acid B, with the lowest pKa, is the strongest. Acid A comes next, followed by C, making B > A > C the correct order of acid strength.

Relationship Between pKa and Acidity

Understanding the relationship between pKa and acidity is essential in evaluating how acidic a compound is. pKa provides a quantifiable measure of an acid's tendency to donate protons. Low pKa values indicate more acidic substances due to their stronger propensity for proton donation. Think of pKa as a scale – the lower the number, the more it «tips» toward strong acidity.
To put it simply:

• A low pKa means more acidic because it's easier for the acid to donate hydrogen ions compared to those with higher pKa.
• A high pKa suggests a less acidic, and consequently, a weaker acid.

With acids A, B, and C, their respective pKa values allow us to rank them by strength. Acid B's notably low pKa of 3.3 reveals it is more acidic compared to A (4.3) and C (5.5), thus making it the strongest among them.

Proton Donation in Acids

Proton donation is the process by which an acid gives up a proton to a base. It's the core concept defining an acid's strength. Strong acids, which have lower pKa values, readily donate protons, while weak acids, with higher pKa values, do not.
In practical terms:

• Stronger acids have a lower energy barrier for releasing protons, making the proton donation quick and robust.
• Weaker acids hold onto their protons, making them less willing to donate as easily.

For example, Acid B is the strongest of the acids because its pKa of 3.3 means it can donate protons more effectively than Acids A and C. Identifying the ability of acids to give up protons through their pKa values gives us a clear picture of their relative strengths.