Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement is true about ideal solutions? (a) the volume of mixing is zero (b) the enthalpy of mixing is zero (c) both \(\mathrm{A}\) and \(\mathrm{B}\) (d) none of these

Step-by-Step Solution

Answer

Option (c) is correct; both volume and enthalpy of mixing are zero in ideal solutions.

Step 1: Understanding Ideal Solution

In chemistry, an ideal solution refers to a solution in which the components mix completely while maintaining the characteristics of perfect behavior. This means the intermolecular forces between unlike molecules are similar to those between like molecules.

Step 2: Exploring Volume of Mixing

The volume of mixing refers to the change in volume when two substances are mixed. In an ideal solution, the volume of mixing is zero because there is no volume change when components are mixed due to uniform intermolecular forces.

Step 3: Exploring Enthalpy of Mixing

The enthalpy of mixing discusses the thermal energy change when mixing two substances. In an ideal solution, the enthalpy of mixing is zero, indicating that no energy is absorbed or released in the process due to the similarity of intermolecular interactions.

Step 4: Evaluating the Choices

Options (a) and (b) both individually describe correct characteristics of an ideal solution. Therefore, option (c), which states both (a) and (b), is the complete correct statement. Consequently, option (d), stating 'none of these', is incorrect.

Key Concepts

Volume of Mixing

The concept of the volume of mixing is crucial when discussing ideal solutions. When two substances are combined to form a solution, there might be a change in total volume. However, in an ideal solution, the volume of mixing is zero. This implies that there is no volume change upon mixing the components.

Why does this happen? It’s because the molecules interact with each other in a way where their combined volumes stay consistent. The intermolecular forces—which are the attractions or repulsions between molecules—are so uniform that no expansion or contraction occurs. This is particularly important because it suggests that the molecules fit together perfectly without altering their spatial arrangement.

For example, if you mix 1 liter of substance A with 1 liter of substance B, you end up with exactly 2 liters in a perfect ideal solution. This characteristic is a direct result of consistent intermolecular forces and is a hallmark of ideality in chemistry.

Enthalpy of Mixing

Enthalpy of mixing is a measure of thermal energy change involved when two different substances are mixed. The enthalpy change can show whether energy is absorbed or released during the mixing process.

In the case of an ideal solution, the enthalpy of mixing equals zero. This implies that there is no heat exchange required to mix the substances; neither is energy released nor absorbed. Why does this energy neutrality happen? The key lies in the nature of the intermolecular forces. Since the attractive forces between unlike molecules are equal to those between like molecules, there’s no net energy change. The interactions remain uniform throughout the mixture, assuring that no energy shifts occur as the substances mix.

Imagine mixing two liquids that don’t require heating or cooling to achieve a perfect homogenous state—that’s the essence of zero enthalpy of mixing in ideal solutions.

Intermolecular Forces

Intermolecular forces play a pivotal role in defining the behavior of ideal solutions. These forces are attractions and repulsions between the molecules of substances. In an ideal solution, the nature and strength of these forces remain consistent both within the same type of molecules and between different types.

This constancy is why properties like the volume and enthalpy of mixing become zero. The forces between molecules are balanced perfectly, so when substances are mixed, they neither push apart nor pull together more tightly than before.

Ideal solutions essentially mimic the behavior of pure components because the molecules do not interact differently when mixed. This perfect balance means that no additional energy is needed to maintain the solution, and the structure stays uniform throughout. This concept is foundational in understanding solution chemistry and is a critical aspect of why ideal solutions demonstrate their unique properties.