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 \(\ldots \ldots\) (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 is basic and contains Na_2CO_3 and CaCO_3 (b).

Step 1: Identify Reactions of Sodium and Calcium Oxides with Water

Sodium oxide (Na_2O) reacts with water to form sodium hydroxide (NaOH). Calcium oxide (CaO) reacts with water to form calcium hydroxide (Ca(OH)_2). The equations are: \[ \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \]\[ \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \]

Step 2: Identify Reactions with Carbon Dioxide

In the presence of excess carbon dioxide, sodium hydroxide reacts to form sodium carbonate:\[ 2\text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \] Calcium hydroxide reacts to form calcium carbonate:\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]

Step 3: Determine Resulting Compounds in the Solution

After both reactions, the resulting compounds in the solution are sodium carbonate ( Na_2CO_3) and calcium carbonate ( CaCO_3). Sodium carbonate forms a basic solution, while calcium carbonate is only slightly soluble in water but can remain in the solution.

Step 4: Identify the Nature of the Solution

The solution contains sodium carbonate ( Na_2CO_3) and calcium carbonate ( CaCO_3). Sodium carbonate is basic in nature, whereas calcium carbonate is neutral. The overall solution would be dominated by the basicity of sodium carbonate.

Key Concepts

Sodium Oxide Reaction

When sodium oxide (Na_2O) comes into contact with water, an interesting chemical reaction happens. The sodium oxide reacts with the water molecules to form sodium hydroxide, denoted by the chemical equation: \[ \text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \]Sodium hydroxide is a strong base, meaning it raises the pH of the solution substantially. Make sure to remember:

• This reaction is exothermic, meaning it releases heat as it occurs.
• Sodium hydroxide in solution leads to a basic, or alkaline, environment.

Because of these properties, the solution can have significant effects, like being corrosive to certain materials. In a laboratory setting, it's crucial to handle sodium hydroxide solutions with care to prevent chemical burns or material damage. This fundamental reaction sets the stage for interactions with other substances like carbon dioxide.

Calcium Oxide Reaction

Calcium oxide (CaO), also known as quicklime, undergoes a hydration reaction when mixed with water. This reaction forms calcium hydroxide, as shown in the equation below:\[ \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \]Calcium hydroxide, which is also called slaked lime, is a moderately strong base.

• It is less soluble in water compared to sodium hydroxide but forms a suspensions known as limewater.
• This reaction is also exothermic and generates heat upon formation.
• Calcium hydroxide can help raise the pH of the solution, but not as dramatically as sodium hydroxide.

When these calcium compounds are further exposed to carbon dioxide, they transform once again into compounds like calcium carbonate (CaCO_3), useful for various reactions and processes in nature and industry.

Carbon Dioxide Reaction

Carbon dioxide (CO_2) plays a vital role in chemical reactions involving both sodium and calcium hydroxides. When carbon dioxide is introduced into these solutions, it provides a neutralizing effect that transforms strong bases into more stable compounds. Let's look at how this works:

• For sodium hydroxide, the reaction is:\[ 2\text{NaOH} + \text{CO}_2 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} \]This results in sodium carbonate, a compound that is less basic.
• With calcium hydroxide, it reacts as follows:\[ \text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O} \]Calcium carbonate is formed and is only slightly soluble in water.

In both cases, carbon dioxide acts to moderate the basicity of the solution, transforming strong alkaline solutions into products that are more stable and less reactive. Understanding these reactions is crucial for fields such as environmental science and industrial processes, where controlling pH and managing compounds like carbonates are often required.