Problem 168 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following nitrates will leave behind a metal on strong heating? [2003] (a) ferric nitrate (b) copper nitrate (c) manganese nitrate (d) silver nitrate

Step-by-Step Solution

Answer

Silver nitrate leaves behind a metal (silver) upon strong heating.

Step 1: Identify Nitrate Decomposition

When nitrates decompose upon heating, they may leave behind different products depending on the metal involved. Some nitrates decompose to give a metal, while others give metal oxide or remain unchanged.

Step 2: Analyze Given Options

Analyze each option: (a) Ferric nitrate ( ext{Fe(NO}_3 ext{)}_3) - decomposes to iron oxide and nitrogen dioxide. (b) Copper nitrate ( ext{Cu(NO}_3 ext{)}_2) - decomposes to copper oxide and nitrogen dioxide. (c) Manganese nitrate ( ext{Mn(NO}_3 ext{)}_2) - decomposes to manganese dioxide and nitrogen dioxide. (d) Silver nitrate ( ext{AgNO}_3) - decomposes to silver and oxygen.

Step 3: Determine Which Leaves Metal

Based on the analysis, silver nitrate is the only compound that leaves a metal (silver) upon decomposition. The reaction is ext{2AgNO}_3 ightarrow ext{2Ag} + ext{2NO}_2 + ext{O}_2.

Key Concepts

Metal Nitrates Decomposition

When we talk about thermal decomposition of metal nitrates, we are discussing how these compounds break down when exposed to heat. This process is different depending on the specific metal involved in the nitrate. Generally, when metal nitrates decompose, they can yield several products:

• Metal Oxides
• Oxygen Gas
• Nitrogen Dioxide

The specific products formed depend greatly on the metal that is part of the nitrate structure. For metals like silver, the decomposition is unique because it results in the formation of metallic silver rather than a metal oxide. This is not the case for other metals such as iron, copper, or manganese, where their nitrates typically form metal oxides as by-products.

Silver Nitrate Heating

When heating silver nitrate, a fascinating reaction occurs. Silver nitrate (\[ \text{AgNO}_3 \]) decomposes on strong heating to form metallic silver, along with nitrogen dioxide and oxygen gases:

• \[ 2\text{AgNO}_3 \rightarrow 2\text{Ag} + 2\text{NO}_2 + \text{O}_2 \]

This reaction is special because instead of forming a metal oxide, metallic silver is directly obtained. The release of nitrogen dioxide is evident by a brown gas seen during the heating. This process can be observed in a laboratory setting and defines a characteristic chemical property of silver nitrate. Understanding this reaction is crucial for comprehending the thermal behavior of nitrates containing less reactive metals like silver.

Metal Oxides Formation

In the decomposition of most metal nitrates, the typical end product includes a metal oxide. When metal nitrates like ferric nitrate, copper nitrate, or manganese nitrate are heated, metal oxides are formed rather than pure metals. For example:

• Ferric nitrate (\(\text{Fe(NO}_3)_3\)) decomposes to iron oxide (\(\text{Fe}_2\text{O}_3\))
• Copper nitrate (\(\text{Cu(NO}_3)_2\)) decomposes to copper oxide (\(\text{CuO}\))
• Manganese nitrate (\(\text{Mn(NO}_3)_2\)) decomposes to manganese dioxide (\(\text{MnO}_2\))

These reactions also produce nitrogen dioxide and oxygen gases as byproducts. The formation of metal oxides is an important part of understanding metal reactivity and compounds in many industrial and chemical processes. They are fundamental in processes such as ore refining and manufacturing catalysts.