Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Absolute alcohol (100\% alcohol) is prepared by distilling rectified spirit over (a) \(\mathrm{Mg}\left(\mathrm{OC}_{2} \mathrm{H}_{5}\right)_{2}\) (b) \(\mathrm{CaCl}_{2}\) (c) \(\mathrm{Na}\) (d) \(\mathrm{Mg}\)

Step-by-Step Solution

Answer

Absolute alcohol is prepared by distilling rectified spirit over magnesium ethoxide (\\( \mathrm{Mg(OC_2H_5)_2} \\)).

Step 1: Understanding the Question

The question asks which substance should be used to distill rectified spirit in order to prepare absolute alcohol. Absolute alcohol means alcohol with no water content.

Step 2: Exploring Options

We are given four options: 1. \( \mathrm{Mg(OC_2H_5)_2} \) is magnesium ethoxide, which can react with water to form ethanol and magnesium hydroxide, thus removing water.2. \( \mathrm{CaCl_2} \) is a drying agent that can absorb water but is not suitable for use in alcohol drying.3. \( \mathrm{Na} \) is sodium, which reacts violently with water and cannot be safely used for distilling rectified spirit.4. \( \mathrm{Mg} \) is magnesium, which does not have the reactivity needed to remove water from alcohol effectively.

Step 3: Identifying the Correct Option

Rectified spirit can be turned into absolute alcohol by removing water content. \(\mathrm{Mg(OC_2H_5)_2} \) reacts with any remaining water to form ethanol and \( \mathrm{Mg(OH)_2} \), thus yielding absolute alcohol. This makes option (a) the correct choice.

Key Concepts

Rectified Spirit

Rectified spirit is a form of ethanol that contains about 95-96% alcohol by volume. This spirit contains a small percentage of water, which makes it different from absolute alcohol. Rectified spirit is typically used as a base for high-proof alcoholic beverages or as a raw material in industrial applications. The reason it is not 100% alcohol is due to an azeotrope formed with water at this concentration.
This azeotrope makes it difficult to remove further water using simple distillation methods, hence advanced techniques or chemical treatments are required to achieve absolute alcohol.

Distillation Process

The distillation process is a common method used to separate mixtures based on differences in boiling points. For alcohol purification, it involves heating the mixture to vaporize components at different temperatures and then condensing the vapor back into liquid.
The challenge with distilling rectified spirit to absolute alcohol arises because ethanol and water form an azeotrope. This means that the boiling point of the mixture doesn’t allow complete separation through standard distillation. Special techniques, such as chemical action, are needed to break this azeotrope to enhance alcohol purification beyond the natural limit of distillation.

Water Removal in Alcohol

Removing water from alcohol to make absolute alcohol requires breaking the azeotropic barrier present in rectified spirit. One effective method is using a chemical drying agent, like magnesium ethoxide, that reacts with water.
Magnesium ethoxide specifically interacts with water molecules, leading to the formation of ethanol and a byproduct, magnesium hydroxide, which doesn't mix with alcohol. This reaction eliminates the water content, allowing us to obtain alcohol that's effectively free of water. Other agents like calcium chloride and sodium aren't suitable due to their inability to safely or effectively remove water from alcohol.

Chemical Reactions in Alcohol Preparation

Chemical reactions play a pivotal role in preparing absolute alcohol from rectified spirit. The key reaction involves using magnesium ethoxide, which reacts with water in the rectified spirit.
This reaction forms ethanol and magnesium hydroxide, which precipitates out. The process thereby removes water, producing absolute alcohol. Understanding such chemical reactions is essential, as they are carefully selected based on their ability to remove water without introducing new impurities or causing unwanted side reactions.

• Magnesium Ethoxide reacts with water -> Formation of Ethanol + Magnesium Hydroxide
• Ensures purity without additional volatile reactions

Choosing the right chemical reaction facilitates achieving the desired purity and concentration levels in the final alcohol product.