Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

An aqueous solution of sodium carbonate is alkaline because sodium carbonate is a salt of (a) weak acid and weak base (b) weak acid and strong base (c) strong acid and strong base (d) strong acid and weak base

Step-by-Step Solution

Answer

(b) weak acid and strong base

Step 1: Identifying Sodium Carbonate's Components

Sodium carbonate ( Na_2CO_3 ) is a salt that forms from the combination of a base and an acid. Sodium ( Na^+ ) comes from sodium hydroxide ( NaOH ), which is a strong base. The carbonate ( CO_3^{2-} ) ion comes from carbonic acid ( H_2CO_3 ), which is a weak acid.

Step 2: Analyzing the Nature of the Acid and Base

Sodium hydroxide is known to dissociate completely in water, making it a strong base. Carbonic acid only partially dissociates in water, making it a weak acid. Together, this means sodium carbonate is formed from a strong base and a weak acid.

Step 3: Determining the Solution's Alkalinity

When a salt is formed from a strong base and a weak acid, the resultant aqueous solution tends to be alkaline. This is because the strong base contributes more hydroxide ions ( OH^- ) to the solution than the weak acid contributes hydrogen ions ( H^+ ).

Key Concepts

Understanding Sodium Carbonate

Sodium carbonate, often referred to as soda ash or washing soda, is quite fascinating in the world of chemistry. Chemically, it is represented as \( Na_2CO_3 \). This compound plays a significant role not just in chemical equations, but also in various industrial and household applications. Sodium carbonate is a salt produced from the reaction of a base and an acid.
To dig deeper, the "sodium" in sodium carbonate comes from sodium hydroxide \( (NaOH) \), a strong base. On the other hand, the "carbonate" part comes from carbonic acid \( (H_2CO_3) \), a weak acid.
- When dissolved in water, sodium carbonate dissociates into sodium ions \( (Na^+) \) and carbonate ions \( (CO_3^{2-}) \).
- It is the interplay of these ions that leads to the alkaline nature of the solution.
Understanding the properties of these individual components is crucial for fully grasping why sodium carbonate solutions behave the way they do.

Why Strong Bases Matter

Strong bases like sodium hydroxide are characterized by their complete dissociation in water. This means that they break down entirely into their constituent ions, giving off a large number of hydroxide ions \( (OH^-) \).
In the case of sodium carbonate, sodium hydroxide \( (NaOH) \) is the strong base involved. Here's what makes it important:

• Complete Dissociation: In water, a strong base dissociates fully, releasing all its hydroxide ions and significantly increasing the pH of the solution.


• Alkalinity Contribution: Strong bases like sodium hydroxide provide a substantial quantity of \( OH^- \) ions, which leads to increased alkalinity in solutions.

As a result, solutions containing strong bases tend to be more alkaline, meaning they have higher pH values and can neutralize acids effectively.

Characteristics of Weak Acids

Weak acids are quite different from their strong counterparts. Unlike strong acids, weak acids do not fully dissociate in water. Instead, they only partially break down into their ions, resulting in lesser but existing hydrogen ions \( (H^+) \) in the solution.
Carbonic acid \( (H_2CO_3) \) is a classic example of a weak acid, contributing to the composition of sodium carbonate.

• Partial Dissociation: In water, weak acids only partially release their hydrogen ions. This results in a low concentration of \( H^+ \), thus lowering the overall acidity.


• Equilibrium in Solutions: Weak acids often exist in a state of equilibrium between the undissociated acid and its dissociated ions, influencing the reactivity and pH level.

Understanding weak acids is crucial in acid-base chemistry. They play a nuanced role, balancing the effects of strong bases, such as sodium hydroxide, when forming salts like sodium carbonate.