Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An azeotropic solution of two liquids has boiling point lower than either of them when it (a) shows negative deviation from Raoult's law (b) shows no deviation from Raoult's law (c) shows positive deviation from Raoult's law (d) is saturated

Step-by-Step Solution

Answer

An azeotropic solution has a lower boiling point when it shows positive deviation from Raoult's law.

Step 1: Understanding Azeotropes

An azeotropic mixture is a mixture of two or more liquids whose proportions cannot be altered by simple distillation. This happens when the boiling point of the mixture is either lower or higher than the boiling points of the individual components. Azeotropes can show either positive or negative deviations from Raoult's law, but when boiling point is lowered, it typically indicates positive deviation.

Step 2: Defining Raoult's Law and Deviations

Raoult's law states that the partial vapor pressure of each component in a solution is proportional to its mole fraction. Deviations occur when interactions between different molecules (A-B) are stronger or weaker than interactions between similar molecules (A-A or B-B). Positive deviation occurs when A-B interactions are weaker, resulting in a higher vapor pressure and lower boiling point.

Step 3: Identifying the Correct Condition

To determine when an azeotropic mixture has a boiling point lower than either of the components, we identify the condition associated with such behavior. A positive deviation from Raoult's law leads to weaker interactions between different molecules, resulting in a higher vapor pressure and thus a lower boiling point.

Key Concepts

Azeotropic Mixtures

An azeotropic mixture, often referred to as an azeotrope, is a fascinating phenomenon in chemistry. This mixture occurs when two or more liquids form a homogeneous mixture where the boiling point of the mixture is either lower or higher than the boiling points of the individual components. They behave uniquely during distillation. This is because the composition of the vapor phase is the same as the liquid phase, making it impossible to separate the components by simple distillation. Azeotropes are divided into two main categories depending on how they deviate from Raoult's law: positive and negative azeotropes. The key aspect of an azeotropic mixture is its constant boiling nature, which can either enhance or reduce the boiling point below that of any of its constituent liquids.
When a mixture forms a minimum boiling azeotrope, it shows a lower boiling point than any of the pure components. In contrast, a maximum boiling azeotrope occurs when the mixture's boiling point is higher. Understanding azeotropic mixtures is crucial for industries involved in distillation and separation processes, as they present limitations and challenges in purifying substances.

Positive Deviation

Positive deviation from Raoult's Law is an interesting concept that often comes into play with certain azeotropic mixtures. Raoult's Law predicts that the total vapor pressure of an ideal solution is directly proportional to the mole fractions of its components. However, when the actual behavior of a solution deviates from this ideal prediction, we observe deviations. In cases of positive deviation, the interactions between different molecules (A-B) are weaker than those between similar molecules (A-A or B-B).
This condition leads to the component molecules escaping more easily into the vapor phase, resulting in increased vapor pressure and consequently a lower boiling point. For example, in mixtures that show positive deviation, like ethanol and water, these weaker interactions lead to a minimum boiling azeotrope. Understanding positive deviation helps us predict how different mixtures behave and how their boiling points change when mixed.

Vapor Pressure

Vapor pressure is a critical concept when studying liquid mixtures and their boiling points. It refers to the pressure exerted by a vapor in equilibrium with its liquid or solid form at a given temperature. For solutions, the total vapor pressure depends on the vapor pressures of the individual components and their mole fractions, which is described by Raoult's Law under ideal conditions.

• For ideal solutions, the intermolecular forces between the different components are similar, and the mixture's vapor pressure follows Raoult's Law predictably.
• However, in real-world applications, deviations occur. Positive deviation occurs when the vapor pressure is higher than predicted, due to weaker interactions between different molecules.
• This increase in vapor pressure usually leads to a reduction in the boiling point.

Understanding vapor pressure is crucial for practical applications like distillation and understanding how mixtures will behave under different conditions. It assists chemists and engineers in predicting how altering components will affect the physical properties like boiling points and volatility of the solution.