Problem 220 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following (a) \(\mathrm{FeCl}_{3}\) solution (aqueous) (p) \(\mathrm{pH}<7\) (b) \(\mathrm{CH}_{3}\) COONa solution (aqueous) (q) \(\mathrm{pH}>7\) (c) Mixture of \(0.1 \mathrm{M}\) acetic acid and (r) \(\mathrm{pH}=7\) \(0.1 \mathrm{M}\) sodium acetate (aqueous) (d) \(0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COONH}_{4}\) (aqueous) (s) acidic (t) basic

Step-by-Step Solution

Answer

(a) matches with (p,s); (b) with (q,t); (c) with (p,s); (d) with (r).

Step 1: Understanding FeCl3 Solution

The solution of FeCl3 (ferric chloride) in water is acidic because Fe3+ ions hydrolyze to form hydronium ions. Hydrolysis of the salt tends to lower the pH below 7. This matches with both (p) \(\mathrm{pH} < 7\) and (s) acidic.

Step 2: Analyzing CH3COONa Solution

Sodium acetate (CH3COONa) is formed from a weak acid (acetic acid) and a strong base (NaOH). In an aqueous solution, the acetate ion (CH3COO-) can accept a proton forming some OH-, making the solution basic with \(\mathrm{pH} > 7\). This matches with (q) \(\mathrm{pH} > 7\) and (t) basic.

Step 3: Evaluating the Mixture of Acetic Acid and Sodium Acetate

This is a classic example of a buffer solution. A buffer containing equal concentrations of a weak acid (acetic acid) and its conjugate base (sodium acetate) will have a pH close to the pKa of acetic acid, which is around 4.76. Therefore, it will be \(\mathrm{pH} < 7\), matching (p) \(\mathrm{pH} < 7\) and (s) acidic.

Step 4: Considering CH3COONH4 Solution

Ammonium acetate (CH3COONH4) is a salt derived from a weak acid and a weak base. This solution will be nearly neutral, typically with \(\mathrm{pH} \approx 7\). Thus, it matches with (r) \(\mathrm{pH} = 7\).

Key Concepts

Acid-Base Reaction

Acid-base reactions are fundamental chemical reactions that involve the transfer of protons (H⁺ ions) between species. These reactions are crucial in understanding chemical equilibrium, especially when considering the products formed and their impact on the solution's pH. In a typical acid-base reaction, an acid donates a proton to a base.

• Acids: Substances that release H⁺ ions in solution, thereby lowering the pH.
• Bases: Substances that accept H⁺ ions or release OH⁻ (hydroxide) ions in solution, raising the pH.

Acid and base strength is a critical factor determining the reaction's outcome. Strong acids like HCl completely dissociate in water, while weak acids like acetic acid do not dissociate fully, forming an equilibrium between the acid and its ions. This equilibrium is represented by the acid dissociation constant (Ka), which provides insight into the acidity of the solution. Similarly, bases have a base dissociation constant (Kb) indicating their strength.

One must carefully balance these reactions to achieve the desired chemical equilibrium, influencing various biological and industrial processes. Understanding the concept of acid-base reactions is essential for predicting the behavior of solutions and designing systems to maintain specific pH levels.

Buffer Solution

A buffer solution is a special type of system that resists significant pH changes when small amounts of acids or bases are added. Buffers are essential in maintaining the stability of pH in biological and chemical systems where precise pH control is required, such as in blood.

Buffers work based on the presence of a weak acid and its conjugate base, or a weak base and its conjugate acid. The classic example is a mixture of acetic acid (CH₃COOH) and sodium acetate (CH₃COONa). In this combination:

• The weak acid (acetic acid) can donate H⁺ ions to neutralize added bases.
• The conjugate base (acetate ion) can accept H⁺ ions to neutralize added acids.

This dual action keeps the pH within a narrow range. The pH of a buffer is primarily determined by the ratio of the concentration of the acid to that of the base and is approximated by the Henderson-Hasselbalch equation:\[\mathrm{pH} = \mathrm{pK_a} + \log\left(\frac{[\text{base}]}{[\text{acid}]}\right)\]Buffers are widely used in chemistry labs, biology, and industry to stabilize solutions, products, and reactions.

Hydrolysis

Hydrolysis is a chemical process where a molecule reacts with water, resulting in the breakdown of that molecule. In the context of salts in aqueous solutions, hydrolysis often leads to changes in pH due to the formation of acidic or basic ions.

Consider the hydrolysis of ferric chloride (FeCl₃) in water:

• Fe³⁺ ions interact with water, causing the release of H⁺ ions.
• This increases the acidity of the solution, resulting in a lower pH.

Certain salts formed from weak acids and strong bases, or vice versa, undergo hydrolysis causing the solution to deviate from neutral (pH=7). For example, sodium acetate (CH₃COONa), a salt of a weak acid and a strong base, forms a basic solution due to acetate ions accepting protons from water, generating hydroxide ions (OH⁻).

Understanding hydrolysis is essential for predicting the behavior of various salts in solution and manipulating pH levels for research and industrial applications. It highlights the dynamic interplay between chemical species and solvent, emphasizing the complexities of maintaining chemical equilibrium.