Raoult's Law

French Chemist François-Marie Raoult found out that when substances were mixed in a solution, the vapour pressure of the solution decreased at the same time, while conducting an experiment in 1877. To explain this phenomenon Raoult proposed a law, named Raoult’s law, which also became one of the laws of thermodynamics. Let us learn more about Raoult’s law and understand its principles and applications and study about its limitations in this article.

What is Raoult’s Law?

Raoult’s law states that a solvent’s partial vapour pressure in a solution is equal or the same as the vapour pressure of the pure solvent multiplied by its mole fraction in the solution. This means that the freezing and boiling points of an ideal solution are respectively depressed and elevated relative to that of the pure solvent by an amount proportional to the mole fraction of the solute.

Mathematically, Raoult’s law equation is given by the following formula.

Psolution = Xsolvent.P0Solvent

Where,

Psolution = Vapour pressure of the solution

Xsolvent = Mole fraction of the solvent

P0Solvent = Vapour pressure of the pure solvent

Importance of Raoult’s Law

Suppose a closed container is filled with a volatile liquid A. After some time, due to evaporation, vapour particles of A will start to form. As time passes, the vapour particles of A are going to be in dynamic equilibrium with the liquid particles on the surface. So, the pressure exerted by the vapour particles of A at any particular temperature is called the vapour pressure of A at that temperature. Vapour pressure is exhibited by all solids and liquids and depends only on the sort of liquid and temperature.

Now, if another liquid B is added to this container, the B particles will occupy the space between A particles on the surface of the solution.

For any given liquid there are a fraction of molecules on the surface having sufficient energy to escape to the vapour phase.

Since we have a fewer number of A particles on the surface at this point, the number of vapour particles of A in the vapour phase will be lesser. This is going to result in lower vapour pressure of A.

Now if B is also volatile, we will have a fewer number of B particles in the vapour phase as compared to pure liquid B.

This new pressure is the partial pressure of each A and B, and is given by Raoult’s law and depends on the concentration of each component in the liquid phase.

PA ∝ XA , PB ∝ XB  = XAP’A= XBP’B

Where P’ is the mole fractions of the components

Limitations of Raoult’s Law

The limitations of Raoult’s Law are as follows.

• Raoult’s law is apt for describing ideal solutions, that is, the solutions in which the gas phase exhibits thermodynamic properties analogous to those of a mixture of ideal gases. However, they are not easily found and are rare. Different chemical components have to be chemically identical equally.

• Since many of the liquids in the mixture do not have the same uniformity in terms of attractive forces, these types of solutions tend to deviate away from the law postulated by Raoult or it does not follow Raoult's law appropriately.

Deviations from Raoult’s Law

Deviations from Raoult’s Law can be done if there are adhesive or cohesive forces between two liquids.

Negative Deviation

When the vapour pressure is lower than expected from the law, this results in a negative deviation. It occurs when forces between particles are stronger than those between particles in pure liquids. For instance, this behaviour is observed in a mixture of chloroform and acetone. In this case, hydrogen bonds cause deviation. A solution of hydrochloric acid and water is another example of this.

Positive Deviation

The positive deviation occurs when the cohesion between similar molecules exceeds adhesion between unlike molecules. The result is higher-than-expected vapour pressure. Both components of the mixture escape the solution more readily than if the components were pure. This behaviour is observed in mixtures of benzene and methanol, and mixtures of chloroform and ethanol.

Ideal vs. Non-ideal Solutions

Ideal solutions are best for Raoult’s law. "Perfect solutions have intermolecular interactions that are similar to those of pure components and show thermodynamics blending properties similar to those of perfect different gasses." There are numerous options for other concepts in Chemistry, this law is only applicable for ideal solutions. It still works well, however, for the dilute solutions insolvent. In actuality though, the calculation of Raoult's law for the extremely dilute solution will be greater with the decrease in pressure.

Why did Raoult's Law Work?

The concept or process of colligative properties if we look through the reviews of it we will come across that additional solute will fill the gaps between solvent particles to take up space while adding a solute lower vapor pressure to it. This results in lowering vapor pressure as less of the solvent will be able to break free to enter the gas phase and will be on the surface of the solvent. Raoult's Principle starts with a basic visual approach and progresses to a more comprehensive one on the basis of entropy. Let’s look below at the given approach in a simple way.

The number sticking onto the surface again is the same as an equilibrium here the set-up of the number of particles breaking away from the surface. When a liquid is in a sealed container remember that saturated vapor pressure is what you get.

To escape from the surface (e.g., 1 in 1000 or 1 in a million) a certain ratio of the solvent molecules will have sufficient energy. You are going to reduce the number which can run away at any point of given time if you reduce the solvent molecules on the surface. The vapor to stick to the surface again will not make any difference to the ability of molecules.  If the vapor comes into contact with such a portion of the interface that is covered by the solutes, it may cling to a solvent molecule. Otherwise, you would not have a solution in the first place if there is no obvious attraction between solvent and solute.

In the vapor, there will be less liquid particles. The net effect of this is that when an equilibrium is established it's hardly that they are going to break down. There is nothing like returning problems to them. The saturated vapor pressure is lower if there are fewer particles in the vapor at parity.

Raoult’s Law and its Relationship with Other Laws

We all know the ideal gas law is similar to that. The exception of this law is that it’s only applicable for ideal solutions. All have read ideal gas law and we know that it takes the assumption of gasses of ideal behavior that present between different molecules is zero or non-existent in intermolecular forces. Here in Raoult's Law, we assume that the existing intermolecular forces between similar and dissimilar molecules are equal.

It is sometimes applicable to non-ideal solutions too. This is however done by several incorporating factors where the interactions between several are considered of different substances.

If the proper ideal vapor and an ideal liquid consist of the perfectly ideal system we can further find a vital equation by combining Dalton's Law and Raoult's Law.