Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Fractional distillation is used to separate liquids which differ in their boiling points by (a) \(5^{\circ} \mathrm{C}\) (b) \(10^{\circ} \mathrm{C}\) (c) \(30^{\circ} \mathrm{C}-80^{\circ} \mathrm{C}\) (d) Less than \(20^{\circ} \mathrm{C}\)

Step-by-Step Solution

Answer

Fractional distillation is best for boiling point differences of 30°C-80°C.

Step 1: Understanding Fractional Distillation

Fractional distillation is a process used to separate a mixture of liquids that have different boiling points. This is achieved by heating the mixture so that one component boils and turns into vapor before the others, allowing it to be separated and collected.

Step 2: Key Boiling Point Difference

The effectiveness of fractional distillation depends on the difference in boiling points of the liquids being separated. A small difference in boiling points makes it more challenging to separate the liquids without them evaporating together.

Step 3: Standard Boiling Point Differences for Separation

Typically, fractional distillation is most effective when the boiling points of the liquids differ by at least 25°C, or ideally more, to ensure that each component can be boiled and condensed separately without much of the other components also turning into vapor.

Step 4: Evaluating the Given Options

The provided options include boiling point differences of 5°C, 10°C, 30°C-80°C, and less than 20°C. Based on standard procedures, fractional distillation is best suited for a boiling point difference of at least 25°C to 80°C, ensuring that each liquid can be separated accurately.

Step 5: Final Decision on Correct Option

Considering the above information, option (c) with a boiling point difference of 30°C-80°C is the most appropriate range for effective fractional distillation. It provides a sufficient difference that allows for the separation of liquids without significant overlap in evaporation.

Key Concepts

Boiling Point Difference

The key to successful fractional distillation lies in the difference in boiling points of the liquids in a mixture. When you have a set of liquids with different boiling points, the one with the lowest boiling point will evaporate first upon heating. This difference in boiling points is essential as it prevents the simultaneous evaporation of multiple components, making it easier to collect each substance separately.
A significant boiling point difference is crucial. If the boiling points are too close, within say 5°C or 10°C, it becomes challenging to ensure only one component evaporates without drawing along others. Conversely, when the difference is at least 30°C, or ideally more, it becomes much simpler to control the process.
This ensures that each stage of heating and condensation can effectively separate mixtures, leading to a pure substance collection from fractionating columns.

Separation of Mixtures

Fractional distillation is a powerful technique for separating mixtures when the components differ in boiling points. The process involves several steps:

• Heating the mixture to a temperature where at least one component will evaporate.
• Catching the vapor within a fractionating column where separation occurs more distinctly.
• Condensing the vapor into a pure liquid in a separate container.

The fractionating column plays an essential role. It's filled with materials that create surface area where condensation and evaporation happen continuously. This helps segregate the vapor into its respective components, thereby increasing the purity of the final product.
Fractional distillation is most effective with mixtures where the boiling points have a wider difference, as this allows for sequential separation of each component.

Chemical Process Efficiency

The efficiency of fractional distillation as a chemical process heavily depends on the careful management of heat and the design of the distillation apparatus. Efficient separation occurs due to:

• The fractionating column optimizing the separation through repeated vaporization-condensation cycles.
• Accurate control of temperature to ensure precise targeting of individual boiling points.
• The purity of the separated components, achieved through careful operation, reducing product overlap.

When performed under ideal conditions, fractional distillation can achieve high purity levels of each component in a mixture, which is crucial for applications in industries like chemical manufacturing and refining crude oil.
This efficiency not only makes fractional distillation valuable for scientific research and chemical production but also ensures cost-effectiveness by minimizing the loss of materials and maximizing yield.