Question

Absolute alcohol is prepared by: (a) Fractional distillation (b) Kolbe's method (c) Vacuum distillation (d) Azeotropic distillation

Short answer

Absolute alcohol is prepared by azeotropic distillation.

Step-by-step solution

Understanding Absolute Alcohol

Absolute alcohol is the term used to describe pure ethanol, with no water content. It is often required in certain industrial and laboratory settings.

Identifying the Preparation Methods

We need to determine how pure ethanol is prepared from the options provided. The common methods used are fractional distillation, Kolbe's method, vacuum distillation, and azeotropic distillation.

Reviewing Distillation Techniques

Fractional distillation and vacuum distillation are general methods for separating components based on boiling points. However, they are not efficient in completely removing water from ethanol to achieve an anhydrous state.

Analyzing Azeotropic Distillation

Azeotropic distillation involves the use of an additional substance to break the azeotrope formed by ethanol and water mixtures. This allows for the removal of water, achieving pure ethanol (absolute alcohol).

Conclusion: Choosing the Correct Method

Among the given options, azeotropic distillation is specifically designed to break the ethanol-water azeotrope to produce absolute alcohol. Therefore, it is the correct answer.

Key concepts

Azeotropic Distillation

Azeotropic distillation is a fascinating process used when separating substances that have close boiling points, like ethanol and water. When mixed, ethanol and water form an azeotrope, which is a constant boiling mixture that cannot be separated by simple distillation. This presents a challenge if we aim to obtain absolute alcohol, or pure ethanol, as water is inherently bound to ethanol in this azeotropic mixture.

• To overcome this, an additional substance (called an "entrainer") is introduced into the mixture. This substance interacts selectively with one of the components in the azeotrope.
• This modifies the boiling point relation, allowing separation of ethanol from water.

The entrainer is carefully chosen to ensure it facilitates breaking the azeotrope effectively. As a result, azeotropic distillation successfully produces absolute alcohol, freeing ethanol from the persistent traces of water.

Ethanol Purification

Ethanol purification is crucial when obtaining absolute alcohol for industrial or laboratory use. The main goal is to remove water and other impurities, resulting in an anhydrous form. Simple distillation is insufficient because ethanol and water form an azeotrope, meaning they cannot be fully separated using traditional methods.

• Azeotropic distillation, as explained, improves the separation efficiency by breaking the ethanol-water azeotrope through chemical means.
• Another method employed in purification is drying agents or chemical additives that absorb water content, achieving higher purity levels in ethanol.

Using these techniques, we can purify ethanol effectively, supporting its use in applications where absolute alcohol is essential.

Anhydrous Ethanol

Anhydrous ethanol, also known as absolute alcohol, is ethanol with all water content removed. This form of ethanol is particularly significant in industries and laboratories where purity is essential. The challenge in obtaining anhydrous ethanol lies in its natural tendency to retain water due to the azeotropic nature of ethanol-water mixtures.

• The process to achieve anhydrous ethanol often involves azeotropic distillation, where additional substances help in breaking the azeotrope to free ethanol from water.
• Another approach is the use of dehydrating agents that absorb moisture, thus producing ethanol with a purity level higher than 99.5%.

Such high-purity ethanol serves vital roles in manufacturing, research, and pharmaceuticals, where even small amounts of water can affect the outcomes.