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? (a) ferric nitrate (b) copper nitrate (c) manganese nitrate (d) silver nitrate

Step-by-Step Solution

Answer

Silver nitrate leaves behind elemental silver on heating.

Step 1: Understanding the Stability

The decomposition of nitrates depends on the stability of the metal to form its respective oxide, nitrite, or elemental metal. Nitrates of noble metals decompose to leave the metal, while other metals may form oxides or different compounds.

Step 2: Decomposition of Ferric Nitrate

Ferric nitrate ( ext{Fe(NO}_3 ext{)}_3 ext{)} decomposes upon heating to form ferric oxide ( ext{Fe}_2 ext{O}_3 ext{)}, nitrogen dioxide ( ext{NO}_2 ext{)}, and oxygen ( ext{O}_2 ext{)}. No elemental iron is produced.

Step 3: Decomposition of Copper(II) Nitrate

Copper(II) nitrate ( ext{Cu(NO}_3 ext{)}_2 ext{)} decomposes to form copper(II) oxide ( ext{CuO} ext{)}, nitrogen dioxide ( ext{NO}_2 ext{)}, and oxygen ( ext{O}_2 ext{)}. Elemental copper is not left.

Step 4: Decomposition of Manganese(II) Nitrate

Manganese(II) nitrate ( ext{Mn(NO}_3 ext{)}_2 ext{)} decomposes into manganese(II) oxide ( ext{MnO} ext{)}, nitrogen dioxide ( ext{NO}_2 ext{)}, and oxygen ( ext{O}_2 ext{)}. No elemental manganese is formed.

Step 5: Decomposition of Silver Nitrate

Silver nitrate ( ext{AgNO}_3 ext{)} on strong heating decomposes directly to elemental silver ( ext{Ag} ext{)}, nitrogen dioxide ( ext{NO}_2 ext{)}, and oxygen ( ext{O}_2 ext{)}. Thus, silver is left behind as the metal.

Key Concepts

Noble Metals

Noble metals, known for their excellent resistance to corrosion and oxidation, stand out in the periodic table. These metals include silver, gold, palladium, and platinum, among others. The unique quality of noble metals is their minimal reactivity compared to other elements. They often remain unaffected by environmental factors, making them apposite for several industrial and ornamental uses.
When it comes to chemical reactions, noble metals exhibit noteworthy behaviors. Unlike most other metals, they do not oxidize easily. This makes their nitrates interesting to study. When noble metal nitrates decompose, they tend to leave behind the pure metal. For example, when silver nitrate is heated, it decomposes to elemental silver rather than forming an oxide.

Thermal Decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into simpler substances when heated. This process is crucial in understanding how various compounds behave under rising temperatures. A classic instance of thermal decomposition is the breakdown of nitrates.
The majority of nitrates will decompose into metal oxides, nitrogen dioxide (\( ext{NO}_2\)), and oxygen (\( ext{O}_2\)). However, noble metal nitrates behave differently because of their decreased tendency to oxidize. In the case of silver nitrate, thermal decomposition results in the formation of elemental silver. This is due to its intrinsic heat stability and chemical structure.

Chemical Stability

Chemical stability refers to a substance's ability to remain unchanged under various conditions. This property can significantly influence how a substance behaves during reactions. In the context of nitrate decomposition, the stability of the formed compounds during heating is a crucial factor.
Noble metals, like silver, have high chemical stability, which means they are less likely to react with other elements to form new compounds. As a result, during the decomposition of silver nitrate, the tendency to form silver oxides is low compared to less stable metal nitrates, leading instead to the formation of elemental silver.

Elemental Metal Formation

Elemental metal formation occurs when a metal is left in its purest, uncombined state after a reaction. This typically happens when compounds decompose, and is a distinguishing feature when dealing with noble metals.
In the process of nitrate decomposition, not all nitrates lead to elemental metal formation. Many, especially those of metals other than noble ones, result in oxides or other compounds. Silver nitrate is one that decomposes directly to leave elemental silver. This phenomenon is significant when identifying the outcomes of thermally-induced reactions, offering insights into the practical applications of noble metals in creating pure metal products through decomposition.