Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

When sodium bicarbonate is heated, the product obtained is (a) \(\mathrm{Na}\) (b) \(\mathrm{NaCO}_{3}\) (c) \(\mathrm{NaHCO}_{3}\) (d) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

The product is (d) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\).

Step 1: Understand the Decomposition Reaction

Sodium bicarbonate (NaHCO₃) decomposes when heated. This is a common chemical reaction where a bicarbonate compound breaks down to form a metal carbonate, water, and carbon dioxide. The reaction can be expressed as: \(2 \mathrm{NaHCO}_{3} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3} + \mathrm{H}_{2}\mathrm{O} + \mathrm{CO}_{2}\).

Step 2: Identify the Products of the Reaction

According to the decomposition equation, heating sodium bicarbonate produces sodium carbonate (Na₂CO₃), along with water (H₂O) and carbon dioxide (CO₂). The focus in the options given is on the solid compound formed, which is sodium carbonate (Na₂CO₃).

Step 3: Match the Product with Given Options

Among the given options (a) Na, (b) NaCO₃, (c) NaHCO₃, and (d) Na₂CO₃, the compound that matches the expected product (sodium carbonate) from the decomposition is option (d) Na₂CO₃.

Key Concepts

Sodium Carbonate Formation

Sodium carbonate, also known as soda ash or washing soda, is formed when sodium bicarbonate is heated. This reaction is a classic example of a thermal decomposition process. When you heat sodium bicarbonate ( NaHCO₃), it breaks down into three products: sodium carbonate ( Na₂CO₃), water ( H₂O), and carbon dioxide ( CO₂).

• Heat causes the bicarbonate to lose a molecule of carbon dioxide and water, leaving behind sodium carbonate.
• The resulting sodium carbonate remains as a solid, which makes this reaction very useful in various applications such as glass manufacturing and in laundry detergents.

The importance of this reaction lies in obtaining sodium carbonate from the easily available and inexpensive sodium bicarbonate. Remember, sodium carbonate is more stable at higher temperatures compared to bicarbonate, making it a crucial compound in industrial processes.

Thermal Decomposition

Thermal decomposition is a type of chemical reaction where a compound breaks down into simpler substances when heat is applied. In the case of sodium bicarbonate, the decomposition occurs as the bicarbonate heats up.

• The process involves breaking chemical bonds in the sodium bicarbonate molecules, which requires energy in the form of heat.
• This energy input allows the bicarbonate ions to separate, releasing carbon dioxide gas and forming liquid water.

The thermal decomposition reaction of sodium bicarbonate is endothermic, meaning it absorbs heat from the surroundings. This is in contrast with exothermic reactions, which release heat.
Understanding thermal decomposition is crucial as it applies to numerous other chemical reactions and industries, beyond just the heating of sodium bicarbonate.

Bicarbonate Reactions

Bicarbonate reactions, such as the decomposition of sodium bicarbonate, are part of a broader class of chemical reactions involving bicarbonate ions ( HCO₃⁻).

• Bicarbonates are typically formed from the reaction between a base, such as sodium hydroxide, and carbonic acid.
• These reactions are reversible, meaning bicarbonates can decompose back into carbon dioxide and the original hydroxide compound.

In everyday life, bicarbonate reactions are essential. For example, the reaction of baking soda, a common bicarbonate, with acids like vinegar is widely used in cooking as a leavening agent. It helps dough rise by releasing carbon dioxide gas, which expands within the dough.
Understanding bicarbonate reactions extends to environmental science as well. In water bodies, bicarbonates play a role in maintaining pH balance as they react in equilibrium with carbon dioxide and carbonate.