Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following acid-base titration, \(\mathrm{pH}\) is greater than 8 at equivalence point? (a) acetic acid vs ammonia (b) acetic acid vs sodium hydroxide (c) hydrochloric acid vs ammonia (d) hydrochloric acid vs sodium hydroxide

Step-by-Step Solution

Answer

The titration of acetic acid vs sodium hydroxide ( (b)) has a pH greater than 8 at equivalence.

Step 1: Understanding the Equivalence Point

In an acid-base titration, the equivalence point is reached when the amount of acid equals the amount of base. The nature of the salt formed at this point influences the pH. For a pH greater than 8, the salt should be the conjugate base of a weak acid strong enough to significantly increase the pH.

Step 2: Analyzing Each Titration Pair

(a) Acetic acid vs ammonia results in a weak acid and a weak base, typically producing a pH close to neutral at equivalence. (b) Acetic acid vs sodium hydroxide results in the formation of sodium acetate, which is a weak base that will hydrolyze to form a basic solution. (c) Hydrochloric acid vs ammonia forms a salt that does not affect pH strongly, generally slightly below 7. (d) Hydrochloric acid vs sodium hydroxide results in table salt and water, where neither affects the pH strongly, leading to a neutral pH.

Step 3: Determining the Best Option

Among the options, the reaction between acetic acid and sodium hydroxide results in a solution containing sodium acetate, the salt of a weak acid (acetic acid) and a strong base (sodium hydroxide). The acetate ion undergoes hydrolysis, creating a basic environment with a pH greater than 8 at equivalence.

Key Concepts

Equivalence Point

The concept of the equivalence point is crucial in understanding acid-base titrations. It marks the moment in the titration process when the quantity of acid is exactly balanced by the amount of base. This doesn't necessarily mean the pH is neutral. At the equivalence point, the chemical composition of the solution changes significantly, depending on the strengths of the acids and bases in the reaction.

• In cases where a strong acid meets a strong base, the equivalence point is neutral, with a pH close to 7.
• However, when a weak acid reacts with a strong base, the equivalence point results in a basic pH, often above 7.
• Conversely, a strong acid and a weak base combination typically yields an acidic equivalence point.

Understanding what compounds are formed at the equivalence point, such as salts, is essential. These compounds, like sodium acetate from acetic acid and sodium hydroxide, determine the pH through their properties.

Weak Acid-Strong Base Titration

A weak acid-strong base titration involves a weak acid, such as acetic acid, titrated with a strong base, like sodium hydroxide. This kind of titration is distinct from others because of the behavior of the acid and base:

• Weak acids partially ionize in solution, meaning they don't fully donate all their hydrogen ions (H⁺).
• Strong bases, on the other hand, fully dissociate in solution, releasing all the hydroxide ions (OH⁻).

The interesting part comes during the titration curve. When titrating a weak acid with a strong base, the resulting pH at the equivalence point is greater than 7. This happens because the conjugate base of the weak acid in the solution gets hydrolyzed, forming a basic solution. An example is titrating acetic acid with sodium hydroxide, which forms sodium acetate—a salt that increases the pH significantly.

pH at Equivalence Point

Determining the \(\mathrm{pH}\) at the equivalence point in a titration allows us to understand the chemical environment created by the reaction.For a titration involving a weak acid and a strong base, the resulting pH is basic. This occurs because the salt formed, specifically the conjugate base of the weak acid, undergoes hydrolysis. Hydrolysis is the chemical breakdown of a compound due to reaction with water.Let's consider sodium acetate, formed from acetic acid (a weak acid) and sodium hydroxide (a strong base). Sodium acetate dissociates to produce acetate ions, which can react with water:\[ \text{CH}_3\text{COONa} ightarrow \text{CH}_3\text{COO}^- + \text{Na}^+ \]The acetate ion reacts with water to form acetic acid and hydroxide ions, raising the \(\mathrm{pH}\):\[ \text{CH}_3\text{COO}^- + \text{H}_2\text{O} ightarrow \text{CH}_3\text{COOH} + \text{OH}^- \]This reaction tips the balance, resulting in a pH greater than 8 at the equivalence point. Recognizing this transformation helps students predict the properties and behavior of solutions in titration experiments.