Problem 44 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

The addition of sodium acetate to acetic acid solution will cause (a) increase in its \(\mathrm{pH}\) value (b) decrease in its \(\mathrm{pH}\) value (c) no change in \(\mathrm{pH}\) value (d) change in \(\mathrm{pH}\) which cannot be predicted

Step-by-Step Solution

Answer

The addition of sodium acetate to acetic acid solution will cause an increase in its pH value, option (a).

Step 1: Understanding the Acetic Acid-Sodium Acetate System

This system is a typical weak acid (acetic acid, CH3COOH) - salt of its conjugate base (sodium acetate, CH3COONa) buffer. In solution, sodium acetate dissociates completely to produce acetate ions, CH3COO-, which are the conjugate base of acetic acid.

Step 2: Applying Le Châtelier's Principle

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. Here, adding sodium acetate increases the concentration of acetate ions.

Step 3: Result of Equilibrium Shift

According to Le Châtelier's Principle, the increased concentration of acetate ions will shift the equilibrium position of the acetic acid dissociation reaction to the left. This results in fewer hydrogen ions (H+) in solution, which corresponds to an increase in pH.

Step 4: Understanding pH Change

The pH is related to the concentration of hydrogen ions (H+) in solution. A decrease in H+ concentration means an increase in pH. Thus, adding sodium acetate to an acetic acid solution will cause an increase in pH.

Key Concepts

Le Châtelier's Principle in Buffer Systems

When studying buffer systems in chemistry, one critical concept to grasp is Le Châtelier's Principle. It's a fundamental theory that helps us predict how a chemical equilibrium will shift when it's subjected to external changes, such as pressure, temperature, or changes in concentration of components. A buffer system, which consists of a weak acid and its conjugate base, is in a state of dynamic equilibrium. When an outside substance, in our case sodium acetate, is added to the system, Le Châtelier's Principle comes into play.

pH Value Changes in Buffer Solutions

Understanding the pH scale is key to grasping why and how buffer solutions work. The pH value represents the acidity or basicity of a solution. Adding sodium acetate to an acetic acid solution increases the basicity, resulting in a higher pH value. This is because sodium acetate provides more acetate ions, which then combine with the hydrogen ions, reducing their overall number. Therefore, the pH moves towards the basic side of the scale (higher pH value).

Buffer Systems in Chemistry

Buffers are pivotal in maintaining pH levels in a range of chemical and biological systems. They are composed of a weak acid and its conjugate base and are used to resist changes in pH upon the addition of an acid or base. The acetic acid-sodium acetate buffer is a classic example. It resists changes in pH by shifting its equilibrium when components such as acetic acid (the weak acid) or sodium acetate (the salt of its conjugate base) are added or when the system is diluted.

Equilibrium Shift in Buffer Reactions

An equilibrium shift refers to the direction in which a reversible chemical reaction moves in response to a change in conditions. When extra sodium acetate is introduced to the acetic acid solution, it provides additional acetate ions, causing the equilibrium to shift to the left, as per Le Châtelier's Principle. This shift results in the formation of more neutral acetic acid molecules and a decrease in hydrogen ions, which in turn results in an increase in the pH of the solution.