Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following liquid pairs shows a positivedeviation from Raoult' law? (a) water-nitric acid (b) water-hydrochloric acid (c) benzene-methanol (d) acetone-chloroform

Step-by-Step Solution

Answer

The pair benzene-methanol (c) shows a positive deviation from Raoult's law.

Step 1: Understanding Deviations from Raoult's Law

Raoult's law predicts the vapor pressure of an ideal solution based on the vapor pressures of the pure components and their mole fractions. A positive deviation occurs when the interactions between different molecules are weaker than the interactions between like molecules. This results in a higher vapor pressure than expected.

Step 2: Evaluating Water-Nitric Acid Pair (Choice a)

Water and nitric acid form strong hydrogen bonds when mixed, causing the solution to have stronger interactions than the pure components do, typically leading to a negative deviation from Raoult's law.

Step 3: Evaluating Water-Hydrochloric Acid Pair (Choice b)

Similar to water-nitric acid, water and hydrochloric acid form strong interactions, such as hydrogen bonding and ion-dipole forces, resulting in stronger intermolecular forces in the mixture than in the pure substances, which typically results in a negative deviation.

Step 4: Evaluating Benzene-Methanol Pair (Choice c)

Benzene and methanol exhibit weaker interactions between the different molecules compared to the similar molecules because benzene is non-polar and methanol is polar. Such a combination tends to show positive deviation from Raoult's law due to decreased intermolecular forces in the mixture.

Step 5: Evaluating Acetone-Chloroform Pair (Choice d)

Acetone and chloroform form strong hydrogen bonds when mixed, which results in a negative deviation from Raoult's law as the interactions within the mixture are stronger compared to those in the pure components.

Step 6: Determining the Correct Choice

From the analysis, benzene and methanol have weaker interactions amongst different molecules than like molecules, leading to a positive deviation from Raoult's law. Thus, choice (c) is the correct answer.

Key Concepts

Positive Deviation

A positive deviation from Raoult's Law happens when the mixture's actual vapor pressure is higher than what would be predicted for an ideal solution. This phenomenon indicates weaker interactions between the different molecules in a solution compared to the interactions within the pure components.
When you have liquid pairs with differing strengths of intermolecular forces, the attraction between the separate substances can be less than what each substance would typically exert on its own kind. This lessened attraction allows more molecules to escape into the vapor phase, resulting in an increased pressure.
In simpler terms, for a solution to show a positive deviation, the solution must be 'less sticky' than the components on their own. This decreased 'stickiness', or reduced intermolecular forces, causes the vapor pressure to be higher than expected under Raoult's Law. Examples include mixtures like benzene and methanol, where different types (non-polar and polar) intermolecular forces reduce the overall attraction in the mixture.

Vapor Pressure

Vapor pressure is the force per unit area exerted by a vapor in equilibrium with its liquid or solid form. It's a direct indication of a liquid's evaporation rate; the higher the vapor pressure, the more volatile the liquid.
In the context of Raoult's Law, for an ideal solution, the total vapor pressure can be calculated by summing the products of the mole fractions and vapor pressures of each component. This equation assumes that each component in the solution exerts its vapor pressure independently of the others.
However, in solutions showing positive deviation, the actual vapor pressure is greater than what Raoult's Law would predict. The increased vapor pressure suggests a lesser degree of interaction between molecules in the mixture compared to pure liquids, allowing more molecules to escape into the vapor phase.

• This makes liquids like benzene and methanol a classic case of positive deviation, where their mixed vapor pressure overshoots the calculated ideal value.

Intermolecular Forces

Intermolecular forces are the invisible, attractive forces that act between molecules. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces, among others.
The strength and type of these forces greatly influence whether a solution will show a positive or negative deviation from Raoult's Law.
In mixtures with positive deviation, interactions between unlike molecules are weaker than those between like molecules. This is often due to differences in polarity or the presence of polar and non-polar entities that don't attract each other as strongly.

• A good example is benzene and methanol, where the non-polar benzene and polar methanol lead to reduced overall interactions in the solution.

The weakening of intermolecular forces permits easier evaporation, explaining the heightened vapor pressure in such mixtures.

Ideal Solutions

An ideal solution is a hypothetical mixture where the chemical components behave according to Raoult's Law over the entire composition range. In these mixtures, the intermolecular forces between different molecules are equal to those among like molecules.
This ideality ensures that vapor pressure above the solution linearly combines the individual vapor pressures of the components based on their mole fractions.

• These solutions don't exist perfectly in reality; they are theoretical constructs used to understand actual solutions better.

Generally, solutions of similar substances (like benzene and toluene) more closely approximate ideal behavior. For non-ideal solutions that exhibit positive deviation, the difference in intermolecular forces leads to a higher vapor pressure than calculated, due to weaker attractions between the mismatched molecules.
It’s important to grasp the distinction between ideal and non-ideal solutions because it provides insight into the fundamental nature of intermolecular interactions. Understanding how positive deviation arises from non-ideal mixtures sheds light on why vapor pressures can be unexpectedly high.