Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

A binary liquid solution is prepared by mixing \(n\)-heptane and ethanol. Which on of the following statement is correct regarding the behavior of the solution? (a) The solution in non-ideal, showing +ve deviation from Raoult's Law. (b) The solution in non-ideal, showing \(-\) ve deviation from Raoult's Law. (c) n-heptane shows tre deviation while ethanol shows -ve deviation from Raoult's Law. (d) The solution formed is an ideal solution.

Step-by-Step Solution

Answer

The solution shows positive deviation from Raoult's Law (option a).

Step 1: Understanding Raoult's Law

Raoult's Law states that in an ideal solution, the partial vapor pressure of a component in a solution is directly proportional to its mole fraction. Deviations from this law indicate non-ideality due to different intermolecular forces.

Step 2: Identifying Intermolecular Forces

n-heptane is a nonpolar compound, while ethanol is polar due to its hydroxyl group. When mixed, their distinct polarities suggest a lack of strong intermolecular attractions, leading to deviations from Raoult's Law.

Step 3: Determining the Type of Deviation

A positive deviation from Raoult's Law occurs when the interactions between different molecules in the mixture are weaker than the interactions in the pure components. This results in a higher vapor pressure than predicted by Raoult's Law.

Step 4: Conclusion Based on Interactions

As n-heptane and ethanol have weaker interactions in the mixture compared to their pure forms due to polarity differences, the solution exhibits a positive deviation from Raoult's Law.

Key Concepts

Ideal Solutions

Ideal solutions are a fascinating concept in chemistry where the interactions between the molecules in a solution are perfectly balanced. This balance means that the forces between unlike molecules (the ones of different components in the solution) are exactly the same as those between like molecules (the ones of the same component). Hence, neither component has more attraction or repulsion than the other. When this perfect balance exists, Raoult's Law is obeyed completely.
According to Raoult's Law, the partial vapor pressure of each component in an ideal solution is proportional to its mole fraction. Mathematically, it is represented as:

1. For component A in solution: \( P_A = P^0_A \cdot x_A \)
2. For component B in solution: \( P_B = P^0_B \cdot x_B \)

Here, \( P^0_A \) and \( P^0_B \) are the vapor pressures of pure components A and B, and \( x_A \) and \( x_B \) are their respective mole fractions in the solution. When two substances form an ideal solution, they must have similar physical and chemical properties. Examples include mixtures of benzene and toluene.

Non-Ideal Solutions

Non-ideal solutions deviate from the predictions of Raoult's Law due to the differing interactions between the molecules. These solutions can either show a positive or negative deviation. A positive deviation is observed when the intermolecular forces between different molecules in the mixture are weaker than those in the pure substances themselves.
For instance, consider a solution formed by mixing \(n\)-heptane (a nonpolar compound) and ethanol (a polar compound). In this case, the intermolecular attractions in the mixture are weaker than those in the pure components, which causes the total vapor pressure to be higher than expected by Raoult's Law. On the other hand, a negative deviation occurs when the interactions between differing molecules are stronger than in the pure components, leading to a lower total vapor pressure.
Understanding these deviations is crucial, as it helps predict and manipulate solution behaviors, essential for applications in industries such as pharmaceuticals and materials science.

Intermolecular Forces

Intermolecular forces play a critical role in determining the properties of solutions, whether ideal or non-ideal. These forces are the interactions that occur between molecules and can vary in strength and type. Some common types of intermolecular forces include:

• Van der Waals forces: Present in all molecules, stronger in larger or more polarizable molecules.
• Dipole-dipole interactions: Occur between polar molecules, stronger than Van der Waals forces.
• Hydrogen bonding: A particularly strong type of dipole-dipole interaction, occurring between molecules containing hydrogen attached to electronegative atoms like oxygen or nitrogen.

In the case of \(n\)-heptane and ethanol, the difference in their polarities affects the balance of these forces in the mixture. Ethanol, with its hydroxyl group, can engage in hydrogen bonding. However, when mixed with \(n\)-heptane, these strong interactions are disrupted, resulting in weaker overall interactions in the solution compared to the pure substances. This disruption leads to deviations from Raoult's Law, specifically a positive deviation in this example, as the vapor pressure is higher than predicted.