Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

When attraction between \(\mathrm{A}-\mathrm{B}\) is more than that of \(\mathrm{A}-\mathrm{A}\) and \(\mathrm{B}-\mathrm{B}\), the solution will show deviation from Raoult's law (a) positive (b) negative (c) no (d) cannot be predicted

Step-by-Step Solution

Answer

(b) negative

Step 1: Understanding 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. In an ideal solution, the interactions between all kinds of particles are equal.

Step 2: Analyzing the Given Scenario

In the problem, the attraction between A-B is greater than that for A-A and B-B. This means that the intermolecular forces between the different components are stronger than those within the pure components.

Step 3: Identifying the Deviation Type

Stronger interactions between A-B than A-A and B-B lead to a negative deviation from Raoult's Law. In such cases, the vapor pressure is lower than predicted because the molecules hold onto each other more tightly, reducing their tendency to escape into the vapor phase.

Step 4: Conclusion

Since A-B interactions are stronger, the solution will exhibit a negative deviation from Raoult's Law.

Key Concepts

Negative Deviation

A negative deviation from Raoult's Law occurs when the interactions between different components in a mixture are stronger than those within the pure components themselves. This means that the attraction forces hold the molecules together more robustly than the average interactions anticipated by the law.
A classic telltale sign of negative deviation is a lower vapor pressure than expected. This happens because the tightly bonded molecules are less likely to escape into the surrounding atmosphere.
When studying such deviations, it’s crucial to acknowledge that Raoult's Law assumes ideal conditions where all intermolecular forces are equal. However, real-world scenarios often deviate from this ideal situation, particularly when strong specific interactions, such as hydrogen bonds or other chemical forces, exist between different types of molecules. These enhanced attractions cause the solution's vapor pressure to drop below the linear expectations of Raoult's Law.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles. These forces include hydrogen bonds, dipole-dipole interactions, and London dispersion forces.
Strong intermolecular forces lead to a high boiling point and low vapor pressure, as these forces hold the molecules together tightly. In the context of solutions, if the mixed molecules attract each other more strongly than they do themselves, it results in the negative deviation from Raoult's Law.
There are three main types of intermolecular forces to consider:

• Hydrogen bonds: Occur when hydrogen is bonded to a more electronegative atom like oxygen or nitrogen.
• Dipole-dipole interactions: These occur between polar molecules where partial positive and partial negative charges attract each other.
• London dispersion forces: Present in all molecules, these are weak forces due to temporary dipoles induced in atoms or molecules.

Recognizing the type and strength of intermolecular forces helps in predicting the behavior of the solutions, including deviations from Raoult's Law.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form. It's essentially a measure of the tendency of particles to leave the liquid phase and enter the gaseous phase.
The vapor pressure of a substance is influenced by temperature as well as the nature of intermolecular forces present. Stronger intermolecular forces result in a lower vapor pressure because molecules are less able to escape the liquid's surface.
In the context of a solution adhering to Raoult's Law, the vapor pressure of a solution is proportional to the mole fraction of the solvent and its inherent vapor pressure. But when you have stronger interactions between the components compared to their pure states, the actual vapor pressure will be lower than the predicted value—this is where the negative deviation arises.
Understanding vapor pressure and how it relates to intermolecular forces can help predict whether a mixture will deviate from Raoult's Law. It is a key factor in evaluating whether the bonds in a solution are strong enough to give rise to lower observed vapor pressure than expected.