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: Understand Raoult's Law

Raoult's Law states that the partial vapor pressure of each component in a solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution. It assumes ideal behavior, meaning that intermolecular forces are similar between all molecules.

Step 2: Identify the Type of Deviation

When the actual interactions between different components in a mixture deviate from ideal interactions, we see deviations from Raoult's Law. If attraction between different molecules (A-B) is stronger than between like molecules (A-A or B-B), the molecules will stay closer together, leading to a lower vapor pressure than predicted by Raoult's law.

Step 3: Determine the Deviation Type

A lower than expected vapor pressure indicates negative deviation from Raoult's Law. This is because stronger interactions between A and B mean the molecules are less likely to escape into the gas phase, reducing vapor pressure.

Step 4: Answer the Question

Given that the -B attraction is stronger, the solution will show a negative deviation from Raoult's Law according to our understanding from previous steps.

Key Concepts

Ideal Solutions

An ideal solution is a theoretical concept where two or more components are mixed, and their interactions perfectly follow Raoult's Law. In such solutions, the intermolecular forces among unlike molecules (like A-B) are identical to those among like molecules (A-A or B-B). This means that the interactions in the solution are homogeneous and uniform. Therefore, the enthalpy of solution is zero, implying no heat is absorbed or released during the mixing process.

For ideal solutions, the behavior of the mixture can be accurately predicted by Raoult's Law. This law states that the partial vapor pressure of each component is directly proportional to the mole fraction of that component in the solution. However, this ideal scenario is rarely achieved in real life, as most solutions exhibit some deviations due to differing intermolecular forces.

In practice, ideal solutions act as a benchmark to compare and understand the behavior of real solutions. When a solution deviates from this ideal behavior, it's important to investigate the reasons, which often involve complex intermolecular forces.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring atoms, molecules, or ions. They are crucial in determining many physical properties of substances, such as boiling points, melting points, and vapor pressures.

There are several types of intermolecular forces, including:

• Dipole-dipole interactions: Occur between polar molecules where the positive end of one molecule is attracted to the negative end of another.
• Hydrogen bonding: A strong type of dipole-dipole interaction, which occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.
• Van der Waals forces (London dispersion forces): Occur between all molecules, regardless of polarity, and are the result of transient dipoles forming as electron clouds fluctuate.

When comparing a mixture to an ideal solution, deviations occur when these intermolecular forces between unlike molecules (A-B) differ significantly from those between like molecules (A-A and B-B).

If A-B attractions are stronger than A-A and B-B, the solution might show negative deviation from ideality, as the molecules are more tightly bound, reducing vapor pressure.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is a measure of the tendency of particles to escape from the liquid (or solid) to the gas phase.

The magnitude of vapor pressure depends on several factors, including:

• Temperature: Higher temperatures provide more energy to molecules, increasing their tendency to escape into the gas phase, hence increasing vapor pressure.
• Intermolecular forces: Substances with strong intermolecular forces will have lower vapor pressures because molecules are held more tightly together, making it harder for them to break away into vapor.

Raoult's Law correlates vapor pressure within an ideal solution by stating that the vapor pressure of a component is proportional to its mole fraction in the solution. When the actual intermolecular forces cause the vapor pressure to deviate from this prediction, we observe deviations from Raoult's Law.

For example, if A-B interactions are stronger and create negative deviation, the solution's vapor pressure is less than what Raoult's Law would predict because fewer molecules escape into the vapor phase.