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: Identify the Components of Sodium Carbonate

Sodium carbonate, also known as Na2CO3, is composed of sodium ions (Na+) and carbonate ions (CO₃²⁻). Now we need to understand which acid and base give rise to these ions.

Step 2: Determine the Type of Acid and Base

Carbonate ions (CO₃²⁻) come from carbonic acid (H₂CO₃), which is a weak acid, while sodium ions (Na+) come from NaOH, which is a strong base.

Step 3: Evaluate the Nature of the Salt

Sodium carbonate is the salt formed between a weak acid (carbonic acid) and a strong base (sodium hydroxide). This type of salt leads to an alkaline solution when dissolved in water.

Key Concepts

Alkalinity of Salts

Salts can have different effects on the pH of a solution, typically making it either acidic or alkaline. Sodium carbonate is a perfect example of an alkaline salt. When it dissolves in water, it increases the pH of the solution, making it basic. This is because sodium carbonate is a salt resulting from the reaction between a weak acid and a strong base.
The alkalinity of sodium carbonate comes from the dissociation of the carbonate ions in water. These ions react with water to produce hydroxide ions (OH⁻), which increases the solution's pH.

• The dissociation reaction of carbonate ions: \[ \text{CO}_3^{2-} + \text{H}_2\text{O} \rightarrow \text{HCO}_3^{-} + \text{OH}^{-} \]
• This reaction liberates hydroxide ions, which are primarily responsible for the solution's alkalinity.
• The process also partially reverses, but the net effect is an increase in hydroxide ions, boosting pH above 7.

Understanding how salts influence alkalinity helps in predicting the behavior of solutions in various chemical reactions and processes.

Weak Acid Strong Base Interaction

Weak acids and strong bases create a particular type of salt that often results in an alkaline solution. When a strong base like sodium hydroxide (NaOH) interacts with a weak acid such as carbonic acid (H₂CO₃), it produces sodium carbonate (Na₂CO₃), a salt.

• The strong base, NaOH, fully dissociates in water to Na⁺ and OH⁻ ions.
• The weak acid, H₂CO₃, only partially dissociates, providing very few H⁺ ions in solution.
• In this interaction, the strong base dominates, and the resultant salt tends to create an alkaline environment when dissolved.

When sodium carbonate dissolves in water, it forms Na⁺, CO₃²⁻, and some OH⁻ ions, which collectively increase the solution's pH above neutral. Therefore, understanding the interaction between weak acids and strong bases is crucial in predicting solution characteristics and their practical applications.

Carbonate Ions

Carbonate ions, represented as CO₃²⁻, are central to the chemistry of sodium carbonate. They originate from carbonic acid, H₂CO₃, which is a weak acid. Because it is weak, it does not completely dissociate in water. This partial dissociation is crucial for understanding the behavior of carbonate ions in solution.
Carbonate ions have several roles and properties in chemical reactions:

• They exhibit a basic nature upon reacting with water, forming bicarbonate (HCO₃⁻) and hydroxide ions (OH⁻), raising the pH of the solution:
• Reaction: \[ \text{CO}_3^{2-} + \text{H}_2\text{O} \leftrightarrow \text{HCO}_3^{-} + \text{OH}^{-} \]
• Carbonate ions also serve as buffer agents, helping to maintain the pH stability in various chemical and biological processes by neutralizing small amounts of added acids or bases.

Understanding carbonate ions facilitates a deeper comprehension of many natural phenomena, such as ocean chemistry and the formation of cave stalactites and stalagmites. So, their role is as essential in lab experiments as they are in natural processes.