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 silver metal on strong heating.

Step 1: Understanding the Problem

We need to determine which metal nitrate, when strongly heated, will decompose to leave the pure metal as one of the products.

Step 2: Decomposition of Nitrates

Metal nitrates decompose upon heating, typically leaving behind metal oxides, nitrogen dioxide, and oxygen. However, for noble metals and some other specific cases, the decomposition may lead to free metal.

Step 3: Evaluating Each Option

(a) Ferric nitrate decomposes to iron(III) oxide, so no metal is left. (b) Copper nitrate decomposes to copper(II) oxide, so no metal is left. (c) Manganese nitrate decomposes to manganese(II, III) oxide, so no metal is left. (d) Silver nitrate decomposes directly to silver metal and nitrogen dioxide.

Step 4: Conclusion

Based on the decomposition process for each nitrate, silver nitrate (d) is the only one that yields the pure metal, silver, when heated.

Key Concepts

Metal Nitrates

Metal nitrates are an important group of compounds in chemistry, known for their nitrate ion bound to a metal ion. These compounds share a common chemical formula pattern of M(NO₃)ₓ, where M represents the metal, and x indicates the valency of the metal.
These solid compounds often appear as crystalline solids and can dissolve in water, forming aqueous solutions. A significant characteristic of metal nitrates is their behavior upon heating. Generally, they undergo thermal decomposition, a process where heat causes the compound to break down.
Due to their stable nitrate ions, most metal nitrates release gases, such as nitrogen dioxide, upon decomposition. These fumes are often visible as brown smoke. The thermal stability varies among different metal nitrates, leading to different decomposition products depending on the metal involved.

Thermal Decomposition

Thermal decomposition is a process where a chemical compound breaks down into simpler substances under the influence of heat. In the case of metal nitrates, thermal decomposition typically leads to several products:

• Metal oxides
• Nitrogen dioxide gas (NO₂)
• Oxygen gas (O₂)

Depending on the specific nitrate and the nature of the metal, the decomposition might yield slightly different products.
This process is very temperature dependent, and the specific temperature at which decomposition occurs can vary widely between different nitrates. The more reactive the metal, the more likely it is to form an oxide rather than the pure metal upon decomposition. However, noble metals and several other exceptions can directly form pure metals when their nitrates decompose.

Silver Nitrate

Silver nitrate, represented by the formula AgNO₃, is a fascinating example of a metal nitrate known for its unique thermal decomposition process. Unlike many metal nitrates, upon heating, silver nitrate decomposes to directly yield metallic silver (Ag).
During this reaction, silver nitrate breaks down to form three primary products:

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

This behavior is due to the noble character of silver, which allows it to separate from its nitrate group upon decomposition. The compound is widely used in laboratories and industries due to its reactivity and ability to form pure metal easily under thermal conditions.

Noble Metals

Noble metals, such as gold, silver, and platinum, are a group of metals known for their resistance to corrosion and oxidation. They are termed 'noble' because of these stable properties, which also influence their chemical behavior.
One striking feature of noble metals is their unique behavior during the decomposition of their nitrates. Unlike many other metals, noble metals do not tend to react with oxygen to form oxides as easily. Therefore, the decomposition of their nitrates often results directly in the metal itself rather than forming metal oxides.
This intrinsic property is why silver, a noble metal, directly produces silver metal when silver nitrate is heated, differing significantly from other metal nitrates where metal oxides are typically the main products.