Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

Absolute alcohol cannot be obtained by simple fractionation of a solution of water and alcohol because (a) ethanol molecules are solvated (b) boiling points are very near (c) ethanol remains dissolved in water (d) they form a constant boiling mixture

Step-by-Step Solution

Answer

(d) they form a constant boiling mixture

Step 1: Understanding the mixture

A mixture of alcohol and water forms an azeotrope. An azeotrope is a constant boiling mixture that boils at a lower temperature than either of the pure components.

Step 2: Explain why simple fractionation fails

In simple fractionation, components separate based on their boiling points. Since an azeotrope behaves as a single component with a constant boiling point, standard distillation techniques cannot separate them completely.

Step 3: Identify the property of the ethanol-water mixture

The ethanol-water azeotrope contains about 95.6% ethanol and 4.4% water by volume. This mixture boils at approximately 78.1°C, which is close to the boiling point of pure ethanol (78.3°C), adding difficulty to separation via distillation.

Key Concepts

Absolute Alcohol

Absolute alcohol is another term for pure ethanol. It refers to ethanol that contains very little to no water. In practical terms, absolute alcohol is usually 99% or more pure ethanol. Achieving this level of purity can be quite challenging because ethanol tends to form azeotropes with water. This high-purity ethanol is important for scientific and industrial applications where even trace amounts of water can interfere with chemical reactions or processes. To obtain absolute alcohol, special techniques like chemical drying agents are often used since simple distillation won't suffice due to the azeotropic relationship between ethanol and water.

Ethanol-Water Mixture

The ethanol-water mixture is a classic example of an azeotrope. Azeotropes are mixtures of two or more liquids that behave as a single compound when boiled. This means their boiling point does not change during distillation, making separation difficult. In the case of ethanol and water, the mixture contains approximately 95.6% ethanol and 4.4% water by volume. This specific proportion boils at 78.1°C, which is slightly lower than the boiling point of pure ethanol. The unique interaction between ethanol and water molecules causes this specific boiling behavior. To achieve purer ethanol beyond this azeotropic composition, additional methods such as chemical desiccants or molecular sieves are required to break the azeotropic behavior.

Fractional Distillation

Fractional distillation is a method used to separate components of a liquid mixture based on differences in boiling points. This process involves heating a liquid mixture to form vapor and then cooling the vapor to form a liquid. As the mixture boils, components with lower boiling points vaporize first and are collected separately. However, fractional distillation has its limitations. Azeotropic mixtures, like the ethanol-water mixture, challenge this method. This is because azeotropes behave like a single substance with a fixed boiling point, making it difficult to separate the constituents by simple distillation techniques. To deal with azeotropes, more complex distillation techniques such as azeotropic distillation or pressure-swing distillation might be necessary.

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. It is a key property in separating mixtures through distillation. For pure ethanol, the boiling point is 78.3°C, while for water, it is 100°C under standard atmospheric pressure. However, in an ethanol-water mixture, these distinct boiling points cannot assist in separation due to the formation of an azeotrope. The ethanol-water azeotrope has a constant boiling point of 78.1°C. Such properties of azeotropes complicate the separation process, especially in standard distillation setups, as they boil at a lower temperature than either of their components would alone.