Problem 171 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following on thermal - decomposition yields a basic as well as an acidic oxide? (a) \(\mathrm{CaCO}_{3}\) (b) \(\mathrm{NaNO}_{3}\) (c) \(\mathrm{KClO}_{3}\) (d) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\)

Step-by-Step Solution

Answer

Calcium carbonate ( CaCO_3}) is the correct answer.

Step 1: Understand Thermal Decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into two or more products when heated.

Step 2: Decomposing Calcium Carbonate

On heating, calcium carbonate ( CaCO_3}) decomposes to form calcium oxide ( CaO}) and carbon dioxide ( CO_2}). Calcium oxide is a basic oxide, but carbon dioxide is only acidic.

Step 3: Decomposing Sodium Nitrate

Sodium nitrate ( NaNO_3}) decomposes into sodium nitrite ( NaNO_2}) and oxygen gas ( O_2}). Neither of these is an oxide that fits the criteria of both basic and acidic.

Step 4: Decomposing Potassium Chlorate

Potassium chlorate ( KClO_3}) decomposes to form potassium chloride ( KCl}) and oxygen gas ( O_2}). Again, no basic or acidic oxides are formed here.

Step 5: Decomposing Ammonium Nitrate

Ammonium nitrate ( NH_4NO_3}) decomposes on heating to form nitrous oxide ( N_2O}) and water ( H_2O}). No basic or acidic oxides are produced in this reaction.

Step 6: Identify the Correct Compound

Among the given compounds, none decompose into both a basic and an acidic oxide. However,  CaCO_3}) is the closest, forming a basic oxide ( CaO}) and a weakly acidic oxide ( CO_2}).

Key Concepts

Basic Oxides

Basic oxides are compounds that contain oxygen and exhibit basic properties when they react with water. When these oxides dissolve in water, they typically form a basic or alkaline solution, which can neutralize acids.

• Formation: Basic oxides are usually formed from the metals of the periodic table, particularly alkali and alkaline earth metals.
• Example: Calcium oxide (CaO) is one of the most common basic oxides. When reacted with water, it forms calcium hydroxide (Ca(OH)_2), a strong base.

Basic oxides readily react with acids to form salt and water, a process known as neutralization.
For instance, the reaction of calcium oxide with hydrochloric acid can be represented as:\[\text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O}\]

Acidic Oxides

Acidic oxides are oxides that react with water to form an acid. Typically, these oxides originate from non-metals. They are also known as acid anhydrides because they form acids when combined with water.

• Characteristics: Acidic oxides are often gases or volatile substances at room temperature and usually form from non-metal elements.
• Example: Carbon dioxide (CO_2) is a classic acidic oxide. When it dissolves in water, it forms carbonic acid (H_2CO_3).

These oxides can neutralize basic oxides to form salt and water. An example of this reaction is:\[\text{CO}_2 + \text{CaO} \rightarrow \text{CaCO}_3\]In this example, CO_2 reacts with calcium oxide to produce calcium carbonate, demonstrating how acidic oxides play a role in neutralization processes.

Calcium Carbonate Decomposition

Calcium carbonate (CaCO_3) is a compound that experiences thermal decomposition when heated, breaking down into simpler substances.

• Decomposition Process: CaCO_3 breaks down into calcium oxide (CaO) and carbon dioxide (CO_2) upon heating.
• Resultant Products: Calcium oxide (CaO) is a basic oxide, while carbon dioxide (CO_2) is an acidic oxide.

This decomposition process is critical in various industrial applications, such as the production of lime from limestone and the manufacture of cement.
The balanced chemical equation for the thermal decomposition of calcium carbonate is:\[\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)\]This reaction is often carried out in kilns, as the high temperature is necessary to drive the reaction forward and produce calcium oxide effectively. The carbon dioxide released is then either captured for use in other processes or released into the atmosphere.