Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following leaves no residue on heating? (a) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (b) \(\mathrm{NaNO}_{3}\) (c) \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) leaves no residue on heating.

Step 1: Understanding Residue Formation

When salts are heated, they decompose into different products. The key is to identify which substance, upon heating, breaks down entirely into gaseous products (no residue), or leaves a solid residue.

Step 2: Analyzing Ammonium Nitrate, \(\mathrm{NH}_{4} \mathrm{NO}_{3}\)

\(\mathrm{NH}_{4} \mathrm{NO}_{3}\) decomposes upon heating to produce \(\mathrm{N}_{2}\mathrm{O}\) (laughing gas) and \(\mathrm{H}_{2}\mathrm{O}\) vapor. Both are gaseous, meaning no solid residue is left.

Step 3: Analyzing Sodium Nitrate, \(\mathrm{NaNO}_{3}\)

\(\mathrm{NaNO}_{3}\) decomposes upon heating to \(\mathrm{NaNO}_{2}\) and \(\mathrm{O}_{2}\). \(\mathrm{NaNO}_{2}\) is a solid, so there is a residue.

Step 4: Analyzing Copper(II) Nitrate, \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\)

\(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) decomposes to form \(\mathrm{CuO}\) (a solid), \(\mathrm{NO}_2\), and \(\mathrm{O}_2\). \(\mathrm{CuO}\) is the solid residue.

Step 5: Analyzing Lead(II) Nitrate, \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\)

\(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) decomposes on heating to form \(\mathrm{PbO}\) (a solid), \(\mathrm{NO}_2\), and \(\mathrm{O}_2\). \(\mathrm{PbO}\) is the solid residue.

Step 6: Conclusion

Only \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) decomposes without leaving any solid residue since it breaks down into completely gaseous products.

Key Concepts

Residue Formation

When you heat a compound, it can undergo a process known as thermal decomposition. This involves breaking down into simpler components. One significant outcome of this process is residue formation. Here's what you should know about it:- **Residual Insight**: Residue is the solid material left after a compound has been thermally decomposed. Not all compounds leave residue after heating.- **Key Determinants**: The presence or absence of residue depends on the compounds formed during decomposition. If they are all gases, no solid residue remains.- **Practical Application**: Understanding residue formation is crucial in industries where clean decompositions are needed.- **Examples and Exceptions**: For instance, ammonium nitrate (\(\mathrm{NH}_{4} \mathrm{NO}_{3}\)) decomposes entirely to gases, while sodium nitrate (\(\mathrm{NaNO}_{3}\)) leaves a solid residue.

Chemical Decomposition

Chemical decomposition is a fascinating chemical reaction where a single compound breaks down into two or more elements or new compounds. This occurs under the influence of heat in a process known as thermal decomposition.- **Process Insight**: During thermal decomposition, heat provides energy to break chemical bonds within the compound.- **Types of Decomposition**: It can lead to different types of products: - Gaseous and solid - Entirely gaseous - Entirely solid- **Reaction Differentiation**: Each decomposition reaction is unique and results in different products. For example, when \(\mathrm{NH}_{4}\mathrm{NO}_{3}\) decomposes, it forms nitrogen (\(\mathrm{N}_{2}\)) and water vapor (\(\mathrm{H}_{2}\mathrm{O}\)), both gases, which makes the reaction leave no residue.

Gaseous Products Formation

Gaseous product formation is a vital aspect of thermal decomposition reactions and dictates whether or not a residue is left behind.- **Gaseous Predominance**: If a decomposition reaction primarily results in gaseous products, no residue will be left. For instance, \(\mathrm{NH}_{4}\mathrm{NO}_{3}\)) produces only gases.- **Analysis of Reactions**: Understanding what gaseous substances form can provide insights into the chemical reactions: - **\(\mathrm{NH}_{4}\mathrm{NO}_{3}\)) example**: Breaks into \(\mathrm{N}_{2}\) and \(\mathrm{H}_{2}\mathrm{O}\) vapor, filling the air with gas but keeping no solid trace.- **Significance**: The ability to predict gaseous products is crucial in fields like environmental science and engineering, where managing emissions and residues is vital.- **Industrial Application**: Selecting compounds that decompose primarily into gases can minimize solid waste.