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.

Short answer

The solution is non-ideal, showing +ve deviation from Raoult's Law (option a).

Step-by-step solution

Understanding Raoult's Law and Deviations

Raoult's Law is used to describe the vapor pressure of an ideal solution. According to Raoult's Law, the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction in the solution. Deviations from Raoult's Law occur when solutions do not behave ideally, showing either positive or negative deviations. Positive deviations occur when intermolecular forces between different molecules (e.g., n-heptane and ethanol) are weaker than those between similar molecules, resulting in a higher vapor pressure. Negative deviations occur when forces between different molecules are stronger, resulting in a lower vapor pressure.

Analyzing the Intermolecular Forces

n-Heptane is a non-polar molecule, whereas ethanol is a polar molecule that can form hydrogen bonds. When mixed, the interaction between n-heptane and ethanol is weaker than the hydrogen bonding present between ethanol molecules or the interactions between n-heptane molecules alone. This weaker interaction often leads to positive deviation from Raoult's Law because the components do not hold each other as strongly, increasing vapor pressure.

Determining the Behavior of the Solution

Since n-heptane and ethanol have weaker intermolecular interactions when mixed compared to their pure states, the solution is non-ideal and exhibits positive deviation from Raoult's Law. This occurs because the solution's vapor pressure will be higher than predicted by Raoult's Law.

Key concepts

Non-Ideal Solutions

When we talk about solutions, particularly those involving liquids, we often reference Raoult's Law to predict vapor pressures. However, not all solutions behave perfectly according to this law. These are called non-ideal solutions. A solution is considered non-ideal when the interactions between its molecules differ significantly from those in the pure components.

Non-ideal behavior can be due to differences in intermolecular forces between the different kinds of molecules in a solution. Such variations lead to deviations from Raoult's Law, which assumes that all intermolecular forces in a solution are identical to those in its pure components.

• In non-ideal solutions, these forces are not identical, causing more complex behavior.
• This is common when mixing polar and non-polar substances, such as ethanol (polar) and n-heptane (non-polar).

Understanding non-ideal solutions helps us predict how different substances interact and whether they show positive or negative deviations from Raoult's Law.

Positive Deviation

Positive deviation from Raoult's Law occurs in a solution when the vapor pressure is higher than what the law predicts. This happens when the attraction between the different molecules in the solution is weaker than the attraction between identical molecules in their pure forms.

Consider the mixture of n-heptane and ethanol. Their intermolecular forces in mixed states are weaker due to the difference in their polarity and the specific types of intermolecular forces each can form. This leads to positive deviation:

• The weaker attraction allows molecules to escape more easily into the vapor phase.
• The resulting vapor pressure is greater than would be expected based on Raoult's Law.

Such behavior is typical in mixtures of non-polar and polar substances, where one can experience weaker interactions compared to their pure components.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles. They are crucial in determining the physical properties of a substance, such as boiling point, melting point, and vapor pressure.

In the context of n-heptane and ethanol:

• n-Heptane is non-polar, primarily exhibiting London dispersion forces.
• Ethanol is polar and can form hydrogen bonds, which are stronger than London dispersion forces.

When these two are mixed, the resulting intermolecular forces between them are weaker than the hydrogen bonds between ethanol molecules or the dispersion forces between n-heptane molecules alone. As a result:

• The mixture experiences weaker interactions, contributing to a higher vapor pressure compared to expected values from Raoult's Law.
• This contrast in intermolecular forces is a key reason for the positive deviation observed in such mixtures.

Understanding these forces helps predict solution behaviors and why deviations from ideal laws are observed.