Ideal and Non-Ideal Solutions

An ideal solution is a mixture in which molecules of different species can be distinguished; however, unlike the molecules in an ideal gas, the molecules in an ideal solution exert forces on one another. A solution is said to be perfect when those forces are the same for all molecules regardless of species.

Ideal Solution

The simplest definition of an ideal solution is a homogeneous solution in which the interactions between molecules of components (solute and solvents) are exactly the same as the interactions between molecules of each component itself. At practically all concentrations and temperatures, these solutions obey Raoult’s Law.

Characteristics of Ideal Solution

A perfect solution can be created by combining a solute and a solvent with matching molecular structures and sizes. When we mix two substances, X and Y, we can see that there are various intermolecular forces between them. As an example

• Intermolecular forces of attraction exist between X and X.
• Intermolecular forces of attraction exist between Y and Y.
• Intramolecular forces of attraction exist between X and Y.

When the intermolecular forces of attraction are the same or equal in this case, we get an ideal solution.

Raoult’s Law

In 1986, a French chemist named Francois Marte Raoult proposed a relationship between partial pressure and the mole fraction of volatile liquids. ‘The mole fraction of the solute component is precisely proportional to its partial pressure,’ says the law. Liquid-liquid solutions can be classified into two categories according to Raoult’s Law. There are ideal and non-optimal solutions.

Thus, 

ΔP/PA_o = No. of moles of solute/ Total moles of solute and solvent

Properties of an Ideal Solution

The physical qualities of an ideal solution are usually closely comparable to the properties of the pure components. The following are some of its characteristics:

1. The solution’s enthalpy is zero. When the enthalpy of the solution approaches zero, it is more likely to exhibit perfect behavior.
2. The volume of mixing is also equal to zero. This means that the total volume of the solution equals the sum of the solute and solution volumes. Furthermore, it denotes that there will be a contraction or expansion of the volume when the two components are being mixed.
3. The solute-solute and solvent-solvent interactions are nearly identical to the solute-solvent interaction.

Examples of Ideal Solutions

Obtaining a well-balanced ideal solution might be difficult, however, some solutions can occasionally demonstrate ideal behavior. Here are several examples:

• Toluene and Benzene.
• Ethyl Iodide and Ethyl Bromide.
• Bromobenzene and Chlorobenzene.
• N-heptane and n-hexane.

Notably, when it comes to solution concentrations, most dilute solutions have or exhibit properties of an ideal solution.

Non-ideal Solutions

Non-Ideal Solutions are solutions that do not obey Raoult’s law at all concentrations and temperatures. Non-ideal solutions are those that stray from ideal solutions and are also referred to as Non-Ideal Solutions.

Margules functions or activity coefficients can be used to describe deviations from ideality. If the deviations from ideality are minor, a single Margules parameter may be adequate to define the features of the solution; such solutions are referred to be regular. Unlike ideal solutions, whose volumes are strictly additive and mixing is always complete, the volume of a non-ideal solution is not always the simple sum of the volumes of the component pure liquids, and solubility is not guaranteed across the whole composition range. The thermodynamic activity of components can be assessed by measuring their densities.

Characteristics of Non-ideal Solutions

Non-ideal solutions have the following characteristics:

1. Solute-solute and solvent-solvent interactions are distinct from solute-solvent interactions.
2. The enthalpy of mixing, that is, Δ_mixH≠0, indicates that heat may have been released if the enthalpy of mixing is negative (Δ_mixH < 0), or heat may have been seen if the enthalpy of mixing is positive (Δ_mixH>0).
3. The volume of mixing, i.e., Δ_mixV≠0, indicates that there will be some expansion or contraction in liquid dissolution.

Sample Questions

Question 1: What do you mean by an ideal solution?

Answer: 

A solution in which the interactions of component molecules do not differ from the interactions of the molecules of each component. The optimal solutions are those that obey Raoult’s law at all concentrations and temperatures.

Question 2: What is Raoult’s Law formula?

Answer: 

Raoult’s law is a chemical law that refers to the vapour pressure of a solution dependent on the mole fraction of a solution applied.

Question 3:  What makes an ideal gas?

Answer: 

An ideal gas is one in which all collisions between atoms or molecules are entirely elastic and no attractive intermolecular forces exist. One can visualise it as a succession of perfectly hard spheres colliding but not communicating with one another.

Question 4: What does Raoult’s law state?

Answer: 

Raoult’s law states that the solution’s vapour pressure is equal to the sum of the vapour pressures of each volatile component multiplied by the mole fraction of that component in the solution.

Question 5: What are the limitations of Raoult’s law?

Answer: 

1. Raoult’s law can be applied only to dilute solutions

2. The law is applicable only to those solutions in which the solute is non volatile.

3. This law is applicable to those solutions in which the solute must not dissociate(break into particles) or associate(combination to form larger particles)

Question 6: What are the characteristics of an ideal solution?

Answer: 

An ideal solution has the following characteristics: no change in mixing volume and no change in mixing heat.