Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(16.8 \mathrm{~g}\) of white solid \(\mathrm{X}\) were heated, \(4.4 \mathrm{~g}\) of acid gas A that turned lime water milky was driven off together with \(1.8 \mathrm{~g}\) of a gas \(\mathrm{B}\) which condensed to a colourless liquid. The solid that remained, \(Y\), dissolved in water to give an alkaline solution, which with excess barium chloride solution gave a white precipitate \(Z\). The precipitate effervesced with acid giving off carbon dioxide. Identify A, B and Y and write down the equation for the thermal decomposition of \(\mathrm{X}\). Here \((\mathrm{Y})\) and \((\mathrm{Z})\) are respectively: (a) \(\mathrm{NaHCO}_{3}\) and \(\mathrm{BaHCO}_{3}\) (b) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and \(\mathrm{BaCO}_{3}\) (c) \(\mathrm{NaOH}\) and \(\mathrm{BaCO}_{3}\) (d) \(\mathrm{K}_{2} \mathrm{CO}_{3}\) and \(\mathrm{BaCO}_{3}\)

Step-by-Step Solution

Answer

A is CO₂, B is H₂O, Y is Na₂CO₃. Equation: 2 NaHCO₃ → Na₂CO₃ + CO₂ + H₂O.

Step 1: Analyze Gas A

The gas A turns lime water milky, which indicates that it is carbon dioxide (CO₂). Therefore, A is CO₂.

Step 2: Analyze Gas B

Gas B condenses into a colorless liquid, which is likely water (H₂O). Therefore, B is H₂O.

Step 3: Analyze Solid Y

Gas A (CO₂) and gas B (H₂O) are products of the thermal decomposition of a carbonate compound, which means compound X is likely a metal bicarbonate. Since Y gives an alkaline solution, Y is likely sodium carbonate (Na₂CO₃), as it dissolves to form such a solution.

Step 4: Analyze Precipitate Z

Y (Na₂CO₃) reacts with excess barium chloride solution to form a white precipitate Z, which effervesces with acid to form carbon dioxide. This implies Z is barium carbonate (BaCO₃).

Step 5: Write the Decomposition Reaction

Based on the information, compound X before decomposition is sodium bicarbonate (NaHCO₃). The thermal decomposition reaction for NaHCO₃ is:\[2 ext{ NaHCO}_3 ightarrow ext{Na}_2 ext{CO}_3 + ext{CO}_2 + ext{H}_2 ext{O}\]

Key Concepts

Carbon Dioxide

Carbon dioxide is a colorless and odorless gas that plays a crucial role in various chemical reactions and natural processes. When carbon dioxide is passed through lime water, it turns milky. This result is due to the formation of calcium carbonate, which serves as a key indicator of the presence of carbon dioxide.
It is pervasive in nature as a greenhouse gas, affecting environmental conditions. But in chemistry, it often acts as a valuable reactant and product, especially in decomposition reactions.
In our case, carbon dioxide is a product of the thermal decomposition of sodium bicarbonate (\(\text{NaHCO}_3\)). It acts as a tell-tale sign when identifying chemical compounds through these kinds of reactions.

Bicarbonate Compounds

Bicarbonate compounds, like sodium bicarbonate (\(\text{NaHCO}_3\)), are versatile in numerous ways. They possess an ion containing one hydrogen, one carbon, and a trio of oxygen atoms.
Upon heating, these compounds undergo thermal decomposition, resulting in a few essential products: carbon dioxide (\(\text{CO}_2\)), water (\(\text{H}_2\text{O}\)), and the respective carbonate salt.
Sodium bicarbonate, commonly known as baking soda, is notable for its role in both culinary and chemical lab settings, as seen in our thermal decomposition example. This decomposition is straightforward yet illustrates the dynamic chemistry of bicarbonates, leading to the production of an alkaline solution.

Alkaline Solutions

Alkaline solutions are characterized by a pH greater than 7, meaning they are basic in nature. Such solutions have the ability to neutralize acids, making them important in a range of chemical processes.
In the exercise context, the solid Y, after decomposition, turns into sodium carbonate (\(\text{Na}_2\text{CO}_3\)), which dissolves in water to create an alkaline solution.
Sodium carbonate's basic nature is helpful in various industries, from glassmaking to softening water. The color change of an indicator from yellow to purple in an alkali solution signifies its basicity, allowing chemists to easily identify and confirm reactions are taking place.

Chemical Reactions

Chemical reactions are the transformations where substances convert into new products. They involve breaking and forming chemical bonds, resulting in products with different properties.
The thermal decomposition of sodium bicarbonate serves as a textbook example of a chemical reaction wherein heating results in the generation of carbon dioxide, water, and sodium carbonate.

• Reactants: \(\text{NaHCO}_3\)
• Products: \(\text{Na}_2\text{CO}_3\), \(\text{CO}_2\), and \(\text{H}_2\text{O}\)

Understanding these reactions is essential in predicting the behavior of compounds under heat, illustrating the core principles of chemistry like conservation of mass and transition of energy.