Problem 127 - A Complete Resource Book in Chemistry for JEE Main

Question

When brine solution is saturated with \(\mathrm{NH}_{3}\) and \(\mathrm{CO}_{2}\) a slightly alkaline white sodium salt (A) is formed which has \(\mathrm{pH}\) of nearly \(8.4\). (A) on heating liberates a gas (B) leaving a highly alkaline residue (C) of \(\mathrm{pH}\) nearly 10 to 11. Gas (B) is colourless and turns a solution of \(\mathrm{Ca}(\mathrm{OH})_{2}\) milky. Identify \((\mathrm{B})\). (a) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (b) \(\mathrm{NaHCO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{~S}\) (d) \(\mathrm{Na}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

(B) is carbon dioxide \((\mathrm{CO}_2)\), so the answer is (b) \(\mathrm{NaHCO}_3\).

Step 1: Understand the Reaction

When a brine solution is saturated with ammonia (\(\mathrm{NH}_3\)) and carbon dioxide (\(\mathrm{CO}_2\)), a reaction occurs that forms a white sodium salt (A). This salt has a slightly alkaline pH of about 8.4.

Step 2: Determine the Na Salt (A)

The slightly alkaline white sodium salt (A) with a pH of nearly 8.4 is likely to be sodium bicarbonate (\(\mathrm{NaHCO}_3\)). This compound is known for having a pH in the range of 8.4 when dissolved in water.

Step 3: Analyze Heating Reaction

Upon heating, the sodium bicarbonate (\(\mathrm{NaHCO}_3\)) decomposes to form sodium carbonate (\(\mathrm{Na}_2\mathrm{CO}_3\)), liberating a gas (B) with the reaction: \(2 \mathrm{NaHCO}_3 \rightarrow \mathrm{Na}_2\mathrm{CO}_3 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2\). This process leaves a highly alkaline residue (C) of sodium carbonate with a pH around 10 to 11.

Step 4: Identify Gas (B)

The liberated gas (B) is carbon dioxide (\(\mathrm{CO}_2\)). This gas is colorless and when bubbled through lime water \(\mathrm{Ca(OH)_2}\), it turns it milky due to the formation of calcium carbonate (\(\mathrm{CaCO}_3\)).

Step 5: Match with Options Provided

The gas (B) is identified as carbon dioxide (\(\mathrm{CO}_2\)). Among the given options, the reaction involving \(\mathrm{NaHCO}_3\) releasing \(\mathrm{CO}_2\) matches correctly. Therefore, the answer is option (b) \(\mathrm{NaHCO}_3\).

Key Concepts

Brine Solution

Brine solution is essentially a high concentration solution of salt (sodium chloride, NaCl) in water. It's widely used in various chemical processes due to its ability to dissolve a range of substances efficiently.

• Typically prepared by dissolving common salt in water to obtain a saturated solution.
• This solution is known for its ability to act as a solvent and participate in chemical reactions.

In the context of the exercise, the brine solution acts as a medium for the reaction between ammonia and carbon dioxide. The saturation of the brine solution with these gases is crucial for the formation of sodium bicarbonate. This is a common industrial process, used for producing a variety of compounds including baking soda and soda ash. Understanding how brine functions as a part of chemical processes is fundamental to grasping the production and transformation of chemical compounds in industrial settings.

Sodium Bicarbonate

Sodium bicarbonate, also known as baking soda, is represented by the formula \( \mathrm{NaHCO}_3 \). It is a white, crystalline powder that presents a slight alkaline property when dissolved in water.

• Its pH is typically around 8.4, making it a mild base.
• When heated, it decomposes to produce sodium carbonate, water, and carbon dioxide gas.

This characteristic decomposition reaction is expressed as:
\[ 2 \mathrm{NaHCO}_3 \rightarrow \mathrm{Na}_2\mathrm{CO}_3 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \]
This reaction not only explains its use in baking (where \( \mathrm{CO}_2 \) helps dough rise) but also shows its utility in various chemical and industrial applications. Sodium bicarbonate's ability to release carbon dioxide upon heating was pivotal in identifying the gas in the exercise, showcasing its role in understanding chemical reaction processes.

Carbon Dioxide

Carbon dioxide, \( \mathrm{CO}_2 \), is a colorless, odorless gas that plays an integral role in Earth's life cycle and industrial processes. In the chemical reactions described in the exercise, it is generated as a result of the decomposition of sodium bicarbonate.

• The formation of \( \mathrm{CO}_2 \) is vital in many processes including photosynthesis, respiration, and the carbonation of beverages.
• In the lab, it's known for its property of turning lime water (calcium hydroxide) milky, which is due to the formation of calcium carbonate:

\[ \mathrm{Ca(OH)_2} + \mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3 + \mathrm{H}_2\mathrm{O} \]
This reaction is a classic way to test for the presence of carbon dioxide in a system. It demonstrates the gas's reactive properties and its broad application in both laboratory and industrial contexts. Understanding the behavior of \( \mathrm{CO}_2 \) not only helps in analytical chemistry but also in appreciating its environmental impacts and uses in technology and industry.