Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

Absolute alcohol is prepared by (a) fractional distillation (b) Kolbe's method (c) vacuum distillation (d) azeotropic distillation

Step-by-Step Solution

Answer

(d) Azeotropic distillation.

Step 1: Definition of Absolute Alcohol

Absolute alcohol, also known as anhydrous ethanol, is ethanol that contains less than 1% water.

Step 2: Evaluate Options

Review each method to determine which is used to prepare absolute alcohol. Fractional distillation and Kolbe's method are not typically used for producing absolute alcohol. Vacuum distillation can be used, but it is not the standard method for this task.

Step 3: Understand Azeotropic Distillation

Azeotropic distillation involves adding another substance to the ethanol-water mixture to break the azeotrope. This allows for the separation of ethanol from water, resulting in the production of absolute alcohol.

Step 4: Select the Correct Option

Since azeotropic distillation is specifically designed to remove the last traces of water from ethanol, it is the correct method for preparing absolute alcohol.

Key Concepts

Azeotropic Distillation

Azeotropic distillation is a specialized technique used to separate components of a mixture that form an azeotrope. An azeotrope is a unique blend of two or more liquids that maintains the same composition in both liquid and vapor phases in a distillation process. This attribute makes them difficult to separate by simple distillation. To overcome this, azeotropic distillation introduces an additional substance, known as an entrainer. The entrainer alters the properties of the original azeotropic mixture. - It affects the boiling point and relative volatility, leading to a break in the azeotropic behavior. - Typical entrainers include benzene, cyclohexane, or pentane, which facilitate the removal of water from ethanol by forming a new azeotrope with it. In simple terms, the entrainer shifts the boiling points, allowing the separation of ethanol from water effectively. Thus, azeotropic distillation is invaluable in producing absolute alcohol, by removing the trace level of water that remains after conventional distillation methods.

Anhydrous Ethanol

Anhydrous ethanol, also known as absolute alcohol, is ethanol that contains less than 1% water. This extremely low water content is crucial in many applications where water presence can be undesirable. Some key points about anhydrous ethanol include: - It is used extensively in the fuel industry, serving as a crucial component in the production of gasohol—a cleaner-burning fuel. - In laboratories, it serves as an essential solvent for chemical reactions, where water can affect the reaction course. - It also finds use in the spirits industry for the formulation of high-purity alcoholic beverages. The meticulous removal of water from ethanol using methods like azeotropic distillation ensures that anhydrous ethanol meets the required standards for various industrial applications. In the absence of sufficient water, it acts as a more controlled medium for many chemical processes.

Chemical Separation Techniques

Chemical separation techniques are fundamental methods in both laboratory and industrial settings to separate components from mixtures. These techniques leverage differences in physical or chemical properties, such as boiling point, polarity, solubility, or molecular size. Some common chemical separation techniques include: - **Distillation**: Based on differences in boiling points, this technique allows separation of components through phase change, utilized effectively in producing initial ethanol from fermentation broths. - **Fractional Distillation**: A form of distillation that uses a column to enhance separation efficiency, suitable for mixtures with closer boiling points. - **Azeotropic Distillation**: As previously discussed, it is ideal for breaking azeotropic mixtures by adding an entrainer. Each method is chosen based on the specific properties of the mixture components. Techniques such as azeotropic distillation display how complex mixtures require advanced methods to achieve high purity levels, crucial for scientific, pharmaceutical, and industrial applications.