Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following nitrates will leave behind a metal on strong heating? (a) ferric nitrate (b) copper nitrate (c) manganese nitrate (d) silver nitrate

Step-by-Step Solution

Answer

Silver nitrate leaves behind metal (silver) on strong heating.

Step 1: Identify Reaction Type

When nitrates are strongly heated, they decompose. The metal nitrate decomposes to metal, nitrogen dioxide and oxygen gas in the case of certain nitrates.

Step 2: Nitrate Decomposition Analysis

Decomposition of some metal nitrates directly results in the formation of the metal itself. Not all metal nitrates decompose in this manner. Examples are silver nitrate, which decomposes to silver, nitrogen dioxide, and oxygen.

Step 3: Determine the Outcome for Options

Analyze the possible outcomes for the nitrates given in the options: (a) Ferric nitrate (b) Copper nitrate (c) Manganese nitrate (d) Silver nitrate. Silver nitrate is known to decompose into silver metal, while the decomposition of others results in metal oxides or differing products.

Step 4: Conclusion from Analysis

Based on the decomposition reactions, silver nitrate will leave behind silver metal, whereas the other nitrates will not decompose to leave a pure metal.

Key Concepts

Silver Nitrate Decomposition

Silver nitrate decomposition occurs when silver nitrate is heated to high temperatures. During this process, it breaks down into simpler substances. When you strongly heat silver nitrate, it splits into three distinct components:

• Silver metal (Ag)
• Nitrogen dioxide gas (NO₂)
• Oxygen gas (O₂)

This type of reaction is a classic example of a thermal decomposition reaction. Essentially, the compound disintegrates due to the application of heat. This is because the energy from the heat breaks the bonds holding the silver nitrate together, causing the individual components to form. It's crucial to note that not all metal nitrates decompose in this fashion, which makes silver nitrate unique in leaving behind metallic silver as a residue.

Metal Nitrate Reactions

Metal nitrate reactions can vary significantly depending on the metal involved. Generally, when metal nitrates are subjected to heat, they decompose but the products of these reactions can differ:

• Some metal nitrates, like silver nitrate, decompose to yield the metal and gases.
• Others, such as iron or copper nitrates, typically decompose into metal oxides.

This distinction is important because it determines the final substances left after the reaction. For instance, copper nitrate decomposes to form copper oxide (CuO), nitrogen dioxide (NO₂), and oxygen (O₂).
Each metal nitrate behaves differently because the stability and composition of a metal nitrate determine how it will break down during heating. The nature of the metal and the bonding in the nitrate complex play key roles here.

Metal Residue After Decomposition

Understanding the metal residue left after nitrate decomposition helps to determine the nature of the reaction products. When a metallic nitrate decomposes:

• The type of residue depends on the metal in the nitrate.
• Some metals will form a pure element, while others form metal oxides.

Take silver nitrate as an example. After decomposition, it leaves a residue of pure elemental silver. This is quite different from metals like ferric or manganese nitrate, which leave metal oxides instead.
The reason behind this difference is down to the stability and electron configuration of the metals involved. Reactive metals or those with multiple oxidation states tend to form oxides, while others like silver, decompose to the elemental form more readily. This distinction is an integral part of understanding metal nitrate reactions in chemistry.