Question

An azeotropic mixture of two liquids has a boiling point higher than either of them when it: (a) shows positive deviation from Raoult's law (b) shows negative deviation from Raoult's law (c) shows ideal behaviour (d) is saturated

Short answer

The boiling point of an azeotropic mixture is higher than either of its components when it shows positive deviation from Raoult's law (option a).

Step-by-step solution

Understanding Azeotropes

An azeotropic mixture is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This mixture exhibits a constant boiling point and composition during boiling. An azeotrope that boils at a higher temperature than either of its constituents is known as a positive azeotrope.

Identifying Azeotrope Type

A positive azeotrope shows a positive deviation from Raoult's law. This means that the vapor pressure of the mixture is higher than that predicted by Raoult's law. Consequently, the boiling point of the mixture must be higher to reach the ambient pressure, which is characteristic of a positive azeotrope.

Analyzing the Options

Option (a) shows a positive deviation from Raoult's law, which is associated with a boiling point higher than either of the pure components. Options (b), (c), and (d) are not associated with an azeotropic mixture that boils at a higher temperature than either of its components.

Key concepts

Understanding Positive Deviation from Raoult's Law

Have you ever wondered why some liquid mixtures don't behave as you'd expect when it comes to their boiling points? It turns out there's a concept in physical chemistry that explains this phenomenon, known as positive deviation from Raoult's law.

Raoult's law states that the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction. Ideally, the total vapor pressure of the solution would be the sum of the partial pressures of each component. However, some mixtures exhibit a higher vapor pressure than predicted, which is known as a positive deviation.

This occurs because the molecules of the different liquids interact less favorably compared to the interactions in the pure liquids. With weaker intermolecular forces at play, the molecules escape into the vapor phase more readily, resulting in a higher vapor pressure. As a result, such mixtures require a higher temperature to reach the ambient atmospheric pressure, hence they have an elevated boiling point, a characteristic property of an azeotrope exhibiting positive deviation.

• Less favorable intermolecular forces
• Increased vapor pressure over predicted
• Higher boiling temperature to reach ambient pressure

Boiling Point Elevation in Azeotropic Mixtures

Understanding boiling point elevation is crucial when you're diving into physical chemistry and studying for exams like the JEE. In the context of azeotropic mixtures, this term describes a scenario where the boiling point of a mixture is higher than that of the pure components.

For an azeotropic mixture demonstrating a positive deviation from Raoult's law, the boiling point elevation happens due to the weaker intermolecular forces between the different liquid molecules in the mixture. Since these forces are weaker than those in the pure components, more energy (in the form of heat) is needed to overcome the atmospheric pressure, thus elevating the boiling point of the mixture.

The practical relevance of this is seen in distillation processes where azeotropes can be stubborn, as their boiling points do not change with composition. This is why breaking an azeotropic mixture into its individual components through standard distillation is a challenge.

• Higher boiling point than pure components
• Weak intermolecular forces require more heat
• Significance in distillation processes

Navigating Physical Chemistry for JEE

For students preparing for competitive exams such as the JEE (Joint Entrance Examination), grasping the nuances of physical chemistry can make a significant difference. Physical chemistry often deals with the behavior of materials on an atomic or molecular scale, and understanding concepts like azeotropic mixtures and their boiling points is essential.

When tackling problems on azeotropic mixtures for JEE, it's important to recognize the characteristics of mixtures with positive deviation from Raoult's law, as such questions are commonly featured. Mastery of these topics not only helps in answering multiple-choice questions but also in explaining the underlying concepts in a descriptive format.

The JEE syllabus demands a conceptual understanding that goes beyond rote learning, and connecting phenomena such as boiling point elevation to concrete examples of azeotropes is a great way to solidify your grasp of the subject.

• Understanding atomic or molecular behavior
• Recognize characteristics of azeotropic mixtures
• Conceptual understanding for descriptive explanations