Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

A mixture of sodium oxide and calcium oxide are dissolved in water and saturated with excess carbon dioxide gas. The resulting solution is ......It contains . (a) acidic \(\mathrm{NaOH}\) and \(\mathrm{CaCO}_{3}\) (b) basic \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{CaCO}_{3}\) (c) neutral \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{CaCO}_{3}\) (d) basic \(\mathrm{NaOH}\) and \(\mathrm{Ca}(\mathrm{OH})\).

Step-by-Step Solution

Answer

The solution contains basic \(\text{Na}_2\text{CO}_3\) and \(\text{CaCO}_3\); hence, option (b) is correct.

Step 1: Understand the Reactants and Process

Sodium oxide (\(\text{Na}_2\text{O}\)) and calcium oxide (\(\text{CaO}\)) are basic oxides. When they react with water, they form their respective hydroxides: \(\text{NaOH}\) and \(\text{Ca(OH)}_2\). Thus, the initial reaction with water produces basic hydroxides.

Step 2: Reaction with Carbon Dioxide

When excess carbon dioxide gas \((\text{CO}_2)\) is passed through the solution of \(\text{NaOH}\) and \(\text{Ca(OH)}_2\), \(\text{NaOH}\) reacts with \(\text{CO}_2\) to form \(\text{Na}_2\text{CO}_3\):\[2\text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O}\] \(\text{Ca(OH)}_2\) reacts with \(\text{CO}_2\) to form \(\text{CaCO}_3\):\[\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}\] This results in the formation of \(\text{Na}_2\text{CO}_3\) and \(\text{CaCO}_3\).

Step 3: Analyze the Resulting Solution

\(\text{Na}_2\text{CO}_3\) is a basic salt due to the presence of \(\text{Na}^+\) ions that do not neutralize the basic nature of the carbonate ions, making the solution basic. \(\text{CaCO}_3\) is largely insoluble in water and precipitates out of the solution.

Key Concepts

Sodium Oxide

Sodium oxide ( ext{Na}_2 ext{O} ) is a white, ionic compound composed of sodium and oxygen. It is considered a basic oxide. This means that when it reacts with water, it forms a basic solution. Specifically, sodium oxide reacts with water to form sodium hydroxide ( ext{NaOH} ), which is a strong base.

• The reaction is straightforward:  ext{Na}_2 ext{O} + ext{H}_2 ext{O} ightarrow 2 ext{NaOH} .
• This reaction results in the formation of an alkaline or basic solution of sodium hydroxide in water.

Sodium hydroxide itself is well-known for its use in many fields, including manufacturing and chemistry, due to its strong basic properties.
In the context of our exercise, sodium oxide is an important reactant, forming sodium hydroxide before reacting further with carbon dioxide.

Calcium Oxide

Calcium oxide ( ext{CaO} ), also known as quicklime, is another basic oxide. Like sodium oxide, calcium oxide also reacts with water to form a hydroxide. The product of this reaction is calcium hydroxide ( ext{Ca(OH)}_2 ), often referred to as slaked lime:

•  ext{CaO} + ext{H}_2 ext{O} ightarrow ext{Ca(OH)}_2 .
• Calcium hydroxide is moderately soluble in water, creating a weak basic solution.

This hydrated form of calcium is widely used in agriculture and construction.
In our exercise scenario, calcium oxide contributes to the solution by reacting with water to form a base, which then reacts further with carbon dioxide.

Carbon Dioxide Reaction

In our chemical scenario, carbon dioxide ( ext{CO}_2 ) plays a crucial role in transforming the basic hydroxides into carbonates. - When carbon dioxide is bubbled through a solution containing  ext{NaOH} and  ext{Ca(OH)}_2 , it reacts to form sodium carbonate ( ext{Na}_2 ext{CO}_3 ) and calcium carbonate ( ext{CaCO}_3 ). The reactions are as follows: -  ext{2NaOH} + ext{CO}_2 ightarrow ext{Na}_2 ext{CO}_3 + ext{H}_2 ext{O} -  ext{Ca(OH)}_2 + ext{CO}_2 ightarrow ext{CaCO}_3 + ext{H}_2 ext{O} These reactions are critical to understand as they show how carbon dioxide can neutralize bases to form new compounds.
The formation of  ext{CaCO}_3 is particularly notable because calcium carbonate is generally insoluble in water, leading it to precipitate out.

Basic Solutions

Solutions are termed basic (or alkaline) when they have a pH greater than 7. In our case, the initial solution is basic due to the presence of sodium and calcium hydroxides. These contribute hydroxide ions ( ext{OH}^- ), which result in a basic pH. -  ext{NaOH} and  ext{Ca(OH)}_2 both add hydroxide ions to the solution, increasing its pH. When carbon dioxide reacts within this basic arena, it forms a carbonate with sodium, and as a precipitate with calcium. -  ext{Na}_2 ext{CO}_3 remains in the solution, maintaining its basic nature due to sodium  ext{Na}^+ not affecting the carbonate ion's behavior. -  ext{CaCO}_3 precipitates out but does not alter the basic property of the remaining solution. Understanding the outcome of these reactions helps explain why, after the series of reactions, the resulting solution remains basic.