Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Anhydrous zinc chloride is prepared by heating of (a) \(\mathrm{ZnCl}_{2} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (b) \(\mathrm{Zn}+\mathrm{HgCl}_{2}\) (c) \(\mathrm{Zn}+\mathrm{Cl}\) (d) both \((\mathrm{b})\) and \((\mathrm{c})\)

Step-by-Step Solution

Answer

(c) Zn + Cl₂ forms anhydrous zinc chloride.

Step 1: Understand the term 'anhydrous'

The term 'anhydrous' means without water. Therefore, to prepare anhydrous zinc chloride, which is fzinc chloride (ZfCl_2 ) without water, we must avoid compounds containing water.

Step 2: Analyze Option (a)

Option (a) is ZnCl_2
0ff2H_2 . Since it has water molecules, it can not be directly used to prepare anhydrous zinc chloride, as the objective is to obtain the form of zinc chloride that contains no water.

Step 3: Analyze Option (b)

Option (b) involves the reaction of zinc (Zn) with mercuric chloride (HgCl_2). This combination does not produce zinc chloride in its anhydrous form. It is not the correct choice for preparing anhydrous zinc chloride.

Step 4: Analyze Option (c)

Option (c) involves the reaction of zinc (Zn) with chlorine gas (Cl_2), which would yield anhydrous zinc chloride (ZnCl_2) directly, as there are no water molecules involved in the reaction. This is a valid method to prepare anhydrous zinc chloride.

Step 5: Conclude with Option (d)

Option (d) suggests both (b) and (c). Since only option (c) results in the formation of anhydrous zinc chloride and option (b) does not, option (d) is not suitable, as not both reactions work. Therefore, (d) cannot be correct.

Key Concepts

Zinc Chloride

Zinc chloride, represented chemically as \( \mathrm{ZnCl}_2 \), is a compound formed by the combination of zinc and chlorine. It is a significant chemical with various applications across industries.

Some of its notable characteristics include:

• It is hygroscopic, meaning it can absorb water from the atmosphere.
• It is used in textile processing, metallurgical industries, and as a flux in soldering.
• Available in both anhydrous (without water) and hydrated (with water) forms.

The anhydrous form is particularly useful in reactions where water can interfere with the process. This makes knowing how to prepare and use anhydrous zinc chloride essential for various chemical operations.

Anhydrous Chemistry

In chemistry, 'anhydrous' refers to substances that contain no water. This is critical in reactions where water can alter the outcome or where the presence of water is detrimental.

The preparation and use of anhydrous compounds are typical in laboratories and industrial settings:

• Anhydrous zinc chloride can be prepared by directly combining zinc and chlorine gas, which avoids introducing water into the reaction.
• These substances are stored carefully to prevent them from absorbing water from the environment.
• Anhydrous compounds often have distinct properties compared to their hydrated forms, which is why specific procedures are used to obtain them.

Understanding the principles of anhydrous chemistry is crucial for conducting precise and accurate chemical analyses and syntheses.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds. The preparation of anhydrous zinc chloride exemplifies a type of synthesis reaction.

Key points on this type of reaction include:

• Synthesis reactions are some of the simplest, where two or more simple substances combine to form a more complex product.
• In this particular case, zinc (\( \mathrm{Zn} \)) reacts with chlorine gas (\( \mathrm{Cl}_2 \)) to form anhydrous zinc chloride (\( \mathrm{ZnCl}_2 \)).
• The reaction must be carefully controlled to ensure the absence of water, which can be a contaminant.

These reactions are fundamental in producing new compounds, and understanding how they occur is a foundational aspect of chemistry.