Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

Anhydrous ferric chloride is prepared by (a) heating hydrated ferric chloride at a high temperature in a stream of air (b) heating metallic iron in a stream of dry chlorine gas (c) reaction of ferric oxide with hydrochloric acid (d) reaction of metallic iron with hydrochloric acid

Step-by-Step Solution

Answer

Option (b): Heating metallic iron in a stream of dry chlorine gas is the correct method.

Step 1: Analyze Option (a)

Hydrated ferric chloride when heated loses its water content and would form anhydrous ferric chloride. However, this process is not typically done in a stream of air as ferric chloride can react with the oxygen in the air. Thus, this option is not ideal for preparing anhydrous ferric chloride.

Step 2: Analyze Option (b)

Heating metallic iron in a stream of dry chlorine gas will cause the iron to react with the chlorine, forming anhydrous ferric chloride. This method is direct and effective for creating anhydrous compounds.

Step 3: Analyze Option (c)

The reaction of ferric oxide with hydrochloric acid will form ferric chloride, but this process involves water as a solvent and does not result in the anhydrous form.

Step 4: Analyze Option (d)

The reaction of metallic iron with hydrochloric acid produces ferric chloride in solution, which is not anhydrous. It requires further processing to remove water, making it an inefficient method for directly producing anhydrous ferric chloride.

Step 5: Select the Best Option

Based on the analysis, option (b) is the most suitable method for directly producing anhydrous ferric chloride as it uses dry chlorine gas and avoids introducing water.

Key Concepts

Ferric Chloride Preparation

The preparation of ferric chloride often involves choosing a method that ensures the purity and dryness of the final product. Ferric chloride, also known as iron (III) chloride, is an important chemical used in water treatment and electronics.
One efficient method involves reacting metallic iron with dry chlorine gas. This straightforward process ensures the conversion of solid iron into ferric chloride without the presence of water. It's necessary to avoid any moisture since water can lead to the formation of hydrated forms rather than the desired anhydrous form.
Alternatives like using ferric oxide with hydrochloric acid or metallic iron with hydrochloric acid are often less ideal for anhydrous forms. This is because they tend to produce ferric chloride in a solution, which includes water. Once water is introduced to the reaction, extra steps would be required to remove it, making the process less efficient. Always aim for methods that maintain an anhydrous environment.

Anhydrous Compounds

Anhydrous compounds are chemicals that do not contain water. They are specifically made to avoid any incorporation of water during their creation or application.
These compounds are crucial in various industrial and laboratory processes where water can interfere with the chemical reactions or the integrity of the product.
Anhydrous conditions can be maintained by several methods:

• Using dry reagents and apparatus during chemical reactions.
• Storing chemicals in airtight containers to prevent moisture absorption from the air.
• Employing drying agents or desiccators to remove traces of water.

These approaches prevent the transition from anhydrous to hydrated forms, which could affect the reactivity or properties of the compound.

Chemical Reactions with Chlorine

Chlorine is a highly reactive element, particularly with metals. When in its gaseous state, chlorine can directly react with many metals to form metal chlorides.
The reaction between chlorine gas and metallic iron, for example, results in anhydrous ferric chloride. This kind of reaction showcases chlorine's ability to form stable ionic compounds with metals.
Some key characteristics of reactions with chlorine include:

• The need for dry conditions to avoid forming hydrated products.
• The exothermic nature, where heat is released, sometimes requiring temperature control.
• The production of byproducts, which must be managed to prevent unwanted reactions or hazardous conditions.

Such reactions underscore the importance of safety precautions and understanding the dynamics of chlorine as a reactant.