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? [2009] (a) The solution in non-ideal, showing tre deviation from Raoult's Law. (b) The solution in non-ideal, showing - ve deviation from Raoult's Law. (c) n-heptane shows +ve 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 is non-ideal, showing +ve deviation from Raoult's Law.

Step 1: Understanding Raoult's Law

Raoult's Law states that in an ideal solution, the partial vapor pressure of each component is proportional to its mole fraction and the vapor pressure of the pure component. Mathematically, this is expressed as \( P_i = X_i \cdot P_i^0 \), where \( P_i \) is the partial vapor pressure, \( X_i \) is the mole fraction, and \( P_i^0 \) is the vapor pressure of the pure component.

Step 2: Determining Nature of Ethanol and n-Heptane Interaction

Ethanol, being a polar solvent due to its hydroxyl group, tends to form hydrogen bonds. n-Heptane, a non-polar hydrocarbon, does not form hydrogen bonds and is not capable of interacting with ethanol through these bonds.

Step 3: Analyzing Deviation Behavior

In non-ideal solutions, positive deviation (higher than expected vapor pressure) or negative deviation (lower than expected vapor pressure) from Raoult's Law occurs due to differences in intermolecular forces. When n-heptane and ethanol are mixed, the departure from ideal behavior typically results in positive deviation due to the inability of the non-polar heptane to effectively replace the hydrogen bonding of ethanol molecules, leading to weaker intermolecular forces on average.

Step 4: Evaluating the Provided Statements

Given the behavior of n-heptane and ethanol, where the result is usually a weakening of intermolecular forces across the mixture, we can determine that the solution does not form an ideal solution and exhibits positive deviation. Thus, none of the components show negative deviations specifically, dismissing some options.

Step 5: Conclusion

Option (a), 'The solution in non-ideal, showing +ve deviation from Raoult's Law,' directly describes our discussed outcome for the interaction between n-heptane and ethanol.

Key Concepts

Binary Liquid Solution

When two liquids are mixed to form a homogeneous mixture, such as n-heptane and ethanol, they create a binary liquid solution. These types of solutions contain two components. A great way to think about them is like making a cocktail – two different liquids come together to form one consistent drink.
Understanding binary liquid solutions is important because they are common in daily life, from the gasoline we use to fuel our cars to the beverages we consume. In our specific example, n-heptane and ethanol, each contributes its respective chemical properties to the resulting solution.

Ideal and Non-Ideal Solutions

An ideal solution perfectly follows Raoult's Law, meaning its components’ partial vapor pressures depend exactly on their mole fractions. This happens when the intermolecular forces between like and unlike molecules are similar, resulting in no deviation from expected behavior.
However, most real-life solutions are non-ideal. This means that the actual behavior of the mixture deviates from the predictions of Raoult's Law due to differing intermolecular forces between the mixed molecules.
In our case of n-heptane and ethanol, since these two chemicals interact differently than anticipated in an ideal state, they create a non-ideal solution.

Intermolecular Forces

Intermolecular forces are attractions that occur between molecules. They play a crucial role in determining the properties of a solution, including boiling points and vapor pressures.
For ethanol, these forces include hydrogen bonds, which are strong attractions caused by the presence of a hydroxyl group. Hydrogen bonding gives ethanol its unique physical properties. Meanwhile, n-heptane, a non-polar molecule, relies on weaker Van der Waals forces for attraction.
When mixed, the differing strengths of these forces can significantly affect the behavior of the resulting solution, determining whether it deviates from ideal behavior.

Positive and Negative Deviation

When a binary solution shows differing vapor pressure from the expected value, it exhibits a deviation. The deviations can be positive or negative.

• **Positive Deviation:** Higher vapor pressure than anticipated. This occurs when the intermolecular attractions in the solution are weaker than those in the pure components. It often indicates molecules are escaping into vapor more easily.
• **Negative Deviation:** Lower vapor pressure than anticipated. This happens when attractions in the solution are stronger than in pure components, making it harder for molecules to escape.

In the n-heptane and ethanol mixture, positive deviation is observed because the combined forces are weaker, allowing molecules to vaporize more readily.

Ethanol and n-Heptane Interaction

When ethanol and n-heptane are mixed, their different natures lead to interesting interactions. Ethanol, a polar molecule, loves to form hydrogen bonds because of its hydroxyl group. These strong forces are what give ethanol its distinctive characteristics. On the other hand, n-heptane is non-polar and can only muster weaker Van der Waals forces.
When they're combined, ethanol's ready hydrogen bonds can't partner well with n-heptane's more laid-back nature. This mismatch creates an overall reduction in partner bonding strength, manifesting as positive deviation from expected behavior according to Raoult's Law.
This means the solution doesn't follow the neat predictions of ideal solutions – the molecules are a bit freer to escape than expected, affecting the solution's vapor pressure.