Problem 140 - A Complete Resource Book in Chemistry for JEE Main

Question

Nitrate of which of the following elements can be converted into their oxides on heating? (a) \(\mathrm{Na}\) (b) \(\mathrm{Li}\) (c) \(\mathrm{Mg}\) (d) \(\mathrm{Rb}\)

Step-by-Step Solution

Answer

Lithium and magnesium nitrates decompose to oxides upon heating.

Step 1: Understanding the Question

The question asks which element's nitrate converts to its oxide on heating. We need to know about the thermal decomposition of nitrates.

Step 2: Review of Nitrate Decomposition

When nitrates are heated, they decompose into different products depending on the metal involved. For nitrates of alkali metals like sodium (Na) and rubidium (Rb), the products are different from those of lithium (Li) and magnesium (Mg). Lithium and magnesium nitrates decompose to their oxides, nitrogen dioxide, and oxygen.

Step 3: Recognizing the Pattern

Alkali metals except lithium generally form nitrites and oxygen on heating, while lithium and other metals form oxides and nitrogen dioxide, along with oxygen.

Step 4: Identifying the Correct Options

Given the choices—(a) Na, (b) Li, (c) Mg, and (d) Rb—lithium and magnesium convert their nitrates into oxides. Therefore, the correct answers are (b) Li and (c) Mg as their nitrates decompose to give oxides when heated.

Key Concepts

Thermal Decomposition of Nitrates

Thermal decomposition is a process where a chemical compound breaks down into simpler substances when heated. Nitrates, which are compounds containing the nitrate ion \((\text{NO}_3^-)\), undergo thermal decomposition to form different products depending on the metal present. This process is important in understanding how different nitrates behave when heat is applied.

When nitrates decompose under heat, they can yield different substances such as oxides, nitrites, or gases like oxygen and nitrogen dioxide. Typically, nitrates of more reactive metals, like those of alkali metals, decompose to form metal nitrites and oxygen, while nitrates of less reactive metals, like lithium, decompose to form metal oxides, nitrogen dioxide, and oxygen.

Understanding these decomposition reactions is crucial in various applications, including manufacturing and chemical synthesis, where precise control over the outcomes is needed. This is why learning about thermal decomposition is an essential part of chemistry studies.

Oxide Formation in Metals

Metal oxides are compounds formed between metals and oxygen. During the thermal decomposition of some metal nitrates, oxides are one of the primary products. The type of oxide formed can vary significantly based on the metal involved.

For instance, lithium and magnesium nitrates decompose to form lithium oxide (Li₂O) and magnesium oxide (MgO) respectively. These oxides are typically solid and features of each oxide depend on the metal's characteristics.

• **Lithium oxide (Li₂O):** A white solid that reacts with water to form lithium hydroxide.
• **Magnesium oxide (MgO):** A hygroscopic solid, widely used as a refractory material due to its high melting point.

The formation of oxides in metals through thermal decomposition is vital in industries where metal refining and purification are necessary.

Overall, metal oxide formation is consistent with the specific decomposition patterns influenced by the thermal stability of the metal involved.

Alkali Metal Nitrates Decomposition

Alkali metals, found on the far left of the periodic table, have unique properties reflected in their nitrate's behavior during decomposition. Most alkali metal nitrates, except lithium, decompose differently compared to nitrates of other metals.

• **Sodium (Na), Rubidium (Rb):** These alkali metal nitrates decompose primarily to form metal nitrites and oxygen gas. The reaction can be represented as: \[ 2\text{NaNO}_3 \rightarrow 2\text{NaNO}_2 + \text{O}_2 \] Similar reactions occur for rubidium nitrates.
• **Lithium (Li):** Unlike other alkali metals, lithium nitrate decomposes to lithium oxide, nitrogen dioxide, and oxygen. The reaction is: \[ 4\text{LiNO}_3 \rightarrow 2\text{Li}_2\text{O} + 4\text{NO}_2 + \text{O}_2 \]

Because of this distinction, lithium is sometimes regarded as an anomaly when grouped with other alkali metals in decomposition reactions.

This knowledge is essential not only for academic purposes but also for practical applications like the synthesis of metal oxides or during the refinement of alkali metals.