Problem 48 - A Complete Resource Book in Chemistry for JEE Main

Question

A salt which on heating with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) gives violet vapours is (a) bromide (b) nitrate (c) iodide (d) sulphate

Step-by-Step Solution

Answer

The salt is iodide (c).

Step 1: Identify the Released Vapors

When a salt is heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), the chemical reaction can release gases or vapors depending on the type of salt involved. In this problem, the vapors released are violet, which is a typical characteristic of iodine vapors.

Step 2: Determine the Compound Responsible

Given the information that violet vapors are released, we associate the color with iodine. The salt that contains iodine is iodide. Therefore, when iodide is heated with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), it forms iodine vapors.

Step 3: Choose the Correct Answer

Based on the characteristic of forming violet vapors, iodide is the salt that matches the description. Therefore, the correct answer is (c) iodide.

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, are transformed into new substances, called products. In the scenario provided, a salt reacts with concentrated sulfuric acid. This interaction is an example of a decomposition reaction, where the iodide ions in the salt interact with \( \mathrm{H}_{2} \mathrm{SO}_{4} \), resulting in the release of iodine gas. Chemical reactions are often characterized by observable changes such as the release of gases, color changes, or energy changes like heat or light being emitted. Understanding these reactions involves recognizing the type of reaction, identifying the reactants and products, and predicting the outcome based on properties of the substances involved.

• Decomposition reactions: Breakdown of a compound into simpler substances.
• Gas release: Often identified by a change in color or odor in the vapors produced.
• Indicators of a chemical reaction: include the production of gas (bubbles), color change, and temperature change.

Properties of Iodine

Iodine is a non-metal located in Group 17 of the periodic table, known as halogens. It is commonly recognized by its distinctive violet vapor, especially visible when iodine is heated. Iodine's properties influence how it behaves in chemical reactions. It is less reactive than other halogens, yet it readily forms compounds with many elements.

• Violet vapors: Iodine is uniquely identified by the violet color of its vapors when sublimated.
• Sublimation: Iodine can transition directly from solid to gas without passing through a liquid state.
• Reactivity: While iodine is less reactive than chlorine and bromine, it still engages in reactions with various elements.

These properties are crucial in predicting iodide salt behavior in chemical reactions, such as those involving sulfuric acid.

Sulfuric Acid Reactions

Sulfuric acid, \( \mathrm{H}_{2} \mathrm{SO}_{4} \), is one of the strongest acids known. It acts as a powerful dehydrating agent, capable of reacting with a variety of substances. When concentrated sulfuric acid reacts with iodide salts, it facilitates the release of iodine gas through an oxidation-reduction reaction. In this process, iodide ions are oxidized, resulting in elemental iodine and the release of violet vapors.

• Strong acid properties: Concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \) can decompose and oxidize materials, releasing gases or altering compounds.
• Dehydrating agent: It can remove water molecules, which are sometimes produced in reactions.
• Oxidation process: Iodide is oxidized to iodine, illustrating sulfuric acid's oxidizing potential.

Understanding the behavior of sulfuric acid in reactions is crucial for identifying the outcomes in inorganic chemistry, especially in determining how specific salts will react when exposed to the acid.