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

The anhydrous ferric chloride can be prepared by (b) heating metallic iron in a stream of dry chlorine gas.

Step 1: Analyze Each Option

Let's consider each option to determine the feasibility of synthesizing anhydrous ferric chloride (FeCl₃): (a) Heating hydrated ferric chloride might lead to some dehydration, but it typically results in the formation of the hydrated form again upon cooling. Not an effective method for the anhydrous form. (b) Heating metallic iron in a stream of dry chlorine has the potential to directly form FeCl₃, as iron reacts with chlorine gas to form the anhydrous chloride. (c) The reaction between ferric oxide (Fe₂O₃) and hydrochloric acid (HCl) generally produces FeCl₃ and water, but it usually yields a hydrated form due to the presence of water. (d) Reaction of metallic iron with hydrochloric acid produces ferrous chloride (FeCl₂) and hydrogen gas, not ferric chloride.

Step 2: Evaluate the Correct Option

Focusing on option (b), we see that metallic iron reacts with dry chlorine gas: Fe (solid) + rac{3}{2} Cl₂ (gas) → FeCl₃ (solid) In this reaction, the absence of water ensures that the product formed is anhydrous ferric chloride. This is a direct pathway to obtaining the desired compound without any contamination or formation of hydrates.

Step 3: Confirm the Infeasibility of Other Options

To solidify our conclusion, consider the reactions: - Option (a) does not yield anhydrous FeCl₃ due to the difficulty of completely eliminating water in hydrated forms by heat alone. - Option (c) involves water and likely results in hydrates. - Option (d) results in FeCl₂, not the ferric compound we seek. Thus, option (b) is the most appropriate method for synthesizing anhydrous ferric chloride.

Key Concepts

Chemical Reactions

Chemical reactions are transformations where substances, known as reactants, re-arrange their atoms to form new substances, called products. These reactions can involve a variety of processes such as oxidation, reduction, or acid-base reactions.
Understanding chemical reactions requires knowing how bonds between atoms need energy to break and how new bonds release energy when formed. This exchange often dictates whether a reaction will proceed spontaneously.
Some common indicators of chemical reactions include changes in temperature, color, the formation of precipitates, or the release of gases. For instance, when iron reacts with chlorine gas, we observe a distinct change as iron chloride, a new compound, forms.

Anhydrous Compounds

Anhydrous compounds are substances that contain no water molecules within their structure. They are typically generated either by removing water from hydrates or forming them without contact with water. This distinction is important because many compounds can exist in both hydrated and anhydrous forms, affecting their chemical properties and uses.
Achieving an anhydrous state often requires specific conditions to prevent water incorporation, particularly during synthesis or storage. Techniques such as heating in dry environments or using chemicals that absorb moisture can help achieve this goal. Anhydrous compounds are crucial in chemical applications where water would interfere with reactions or product stability.

Iron(III) Chloride Synthesis

Iron(III) chloride, commonly referred to as ferric chloride, is a chemical compound with multiple uses, including in sewage treatment and as an etching solution for electronics.
The synthesis of anhydrous Iron(III) chloride typically involves reacting iron with chlorine gas in the absence of moisture. The equation for this reaction is: \[ \text{Fe (solid) + } \frac{3}{2} \text{ Cl}_2 \text{ (gas) } \rightarrow \text{ FeCl}_3 \text{ (solid)} \]
Anhydrous preparation is preferred because the presence of water leads to less stable hydrates. Maintaining a dry environment prevents contamination, ensuring that the chemical properties are consistent for intended industrial use. Using pure, dry chlorine gas enhances the efficiency of the reaction since it promotes the direct synthesis of the anhydrous form without side products.