Problem 124 - 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

(b) heating metallic iron in a stream of dry chlorine gas.

Step 1: Understanding Ferric Chloride Formation

To determine how anhydrous ferric chloride (FeCl₃) is prepared, we need to understand the reactions involved in each option given. Anhydrous ferric chloride is a compound formed by the direct reaction of iron and chlorine gas.

Step 2: Analyzing Option (a)

Option (a) suggests heating hydrated ferric chloride in air. This will likely result in water loss but does not involve chlorine gas, so it likely won't produce anhydrous FeCl₃.

Step 3: Analyzing Option (b)

Option (b) suggests heating metallic iron in a stream of dry chlorine gas. This reaction involves elemental iron (Fe) reacting with chlorine (Cl₂) gas, which will form FeCl₃. The balanced chemical reaction is:\[ 2Fe + 3Cl_2 ightarrow 2FeCl_3 \]

Step 4: Analyzing Option (c)

Option (c) suggests the reaction of ferric oxide with hydrochloric acid, which will produce ferric chloride and water, not the anhydrous form:\[ Fe_2O_3 + 6HCl ightarrow 2FeCl_3 + 3H_2O \]This produces a hydrated form of FeCl₃ due to water content.

Step 5: Analyzing Option (d)

Option (d) involves the reaction of metallic iron with hydrochloric acid producing ferrous chloride initially, which isn't the desired anhydrous ferric chloride:\[ Fe + 2HCl ightarrow FeCl_2 + H_2 \]

Key Concepts

Chemical Reactions

Chemical reactions are at the heart of creating compounds like anhydrous ferric chloride. A chemical reaction involves the transformation of substances through the breaking and forming of bonds. In our exercise, ferric chloride is formed by specific reactions involving iron and chlorine. Not all reactions produce the desired product. For example, heating hydrated ferric chloride (Option a) mainly results in water being driven off, not in forming anhydrous FeCl₃. It's crucial to understand the context and the elements involved to predict the reaction outcome accurately.

When discussing reactions, the reactants are the starting substances, and the products are what you end up with after the reaction. Equations like the reaction between metallic iron and chlorine gas show this transformation clearly:

• Reactants: Iron (Fe) and Chlorine gas (Cl₂)
• Product: Anhydrous ferric chloride (FeCl₃)

To form the anhydrous ferric chloride effectively, the reaction between iron and chlorine gas is essential, ensuring there is no water to avoid forming the hydrated version.

Chlorination Process

The chlorination process is a common method for preparing various compounds, including anhydrous ferric chloride. This process uses chlorine gas as a key reactant, and choosing the correct conditions is vital in forming products without unwanted side compounds.

In the specific chlorination reaction in our exercise (Option b), metallic iron is heated in a stream of dry chlorine gas. The lack of moisture ensures the FeCl₃ remains in its anhydrous form, unlike when water is present. Here's the balanced equation for reference:

• \[ 2Fe + 3Cl_2 \rightarrow 2FeCl_3 \]

During the chlorination, both temperature and the exclusion of moisture are crucial. Maintaining dry conditions prevents hydration of the product. This approach illustrates the importance of controlling the chlorination environment to achieve the desired chemical composition.

Anhydrous Compounds

Anhydrous compounds like ferric chloride are those which contain no water molecules bound to them. Creating anhydrous compounds requires careful preparation and handling to ensure no water remains.

The absence of water is important because it affects the properties and uses of the compound. In the preparation of anhydrous ferric chloride, using a dry chlorination process (as seen in Option b of the exercise) is advantageous. This process efficiently avoids the introduction of water. When ferric chloride is prepared using wet methods, like combining ferric oxide with hydrochloric acid (Option c), water as a byproduct leads to a hydrated ferric chloride:

• \[ Fe_2O_3 + 6HCl \rightarrow 2FeCl_3 + 3H_2O \]

Thus, ensuring anhydrous conditions involves diligent removal or prevention of moisture, often employing dry gases or desiccants to absorb water.