- What does it consist of?
- Positive and negative deviations
- Positive deviations
- Negative deviations
- Examples
- Basic mix
- Binary mixture with non-volatile solute
- References
The Raoult was proposed by the French chemist François-Marie Raoult in 1887, and serves to explain the behavior of the vapor pressure of a solution of two (typically ideal) immiscible substances according to the partial vapor pressure of each component present in this.
There are laws of chemistry that are used to describe the behavior of substances under different conditions and explain the phenomena in which they are involved, making use of scientifically proven mathematical models. Raoult's law is one of these.
François-Marie Raoult
Using an explanation based on the interactions between the molecules of gases (or liquids) to predict the behavior of vapor pressures, this law is used to study non-ideal or real solutions, provided that the coefficients necessary to correct the model are considered. mathematical and adjust it to non-ideal conditions.
What does it consist of?
Raoult's law is based on the assumption that the solutions involved behave in an ideal way: this happens because this law is based on the idea that the intermolecular forces between different molecules are equal to those that exist between similar molecules (which not so accurate in reality).
In fact, the closer a solution approaches ideality, the more opportunity it will have of complying with the characteristics proposed by this law.
This law relates the vapor pressure of a solution with a non-volatile solute, stating that it will be equal to the vapor pressure of that pure solute at that temperature, multiplied by its mole fraction. This is expressed in mathematical terms for a single component as follows:
P i = Pº i. X i
In this expression P i is equal to the partial vapor pressure of component i in the gas mixture, Pº i is the vapor pressure of pure component i, and X i is the mole fraction of component i in the mixture.
In the same way, when there are several components in a solution and they have reached a state of equilibrium, the total vapor pressure of the solution can be calculated by combining Raoult's law with Dalton's:
P = Pº A X A + Pº B X B + Pº C X c …
Likewise, in those solutions where only one solute and the solvent are present, the law can be formulated as shown below:
P A = (1-X B) x Pº A
Positive and negative deviations
The solutions that can be studied with this law should normally behave in an ideal way, since the interactions between their molecules are small and allow the same properties to be assumed throughout the entire solution without exception.
However, ideal solutions are practically non-existent in reality, so two coefficients must be incorporated into the calculations that represent intermolecular interactions. These are the fugacity coefficient and the activity coefficient.
In this sense, deviations with respect to Raoult's law are defined as positive or negative, depending on the results obtained at the time.
Positive deviations
Positive deviations with respect to Raoult's law occur when the vapor pressure of the solution is greater than that calculated with Raoult's law.
This happens when the cohesion forces between similar molecules are greater than the same forces between different molecules. In this case, both components vaporize more easily.
This deviation is seen in the vapor pressure curve as a maximum point in a particular composition, forming a positive azeotrope.
The azeotrope is a liquid mixture of two or more chemical compounds that behaves as if it were made up of a single component and that evaporates without changing its composition.
Negative deviations
Negative deviations with respect to Raoult's law occur when the vapor pressure of the mixture is lower than expected after calculation with the law.
These deviations appear when the cohesion forces between the molecules of the mixture are greater than the average forces between the particles of the liquids in their pure state.
This type of deviation generates a retention of each component in its liquid state by attractive forces greater than those of the substance in its pure state, so that the partial pressure of vapor in the system is reduced.
The negative azeotropes in the vapor pressure curves represent a minimum point, and demonstrate an affinity between the two or more components involved in the mixture.
Examples
Raoult's law is commonly used to calculate the pressure of a solution based on its intermolecular forces, comparing the calculated values with real values to conclude if there is any deviation and if it should be positive or negative. Below are two examples of uses of Raoult's law:
Basic mix
The following mixture, made up of propane and butane, represents an approximation of the vapor pressure, and we can assume that both components are found in equal proportions within it (50-50), at a temperature of 40 ºC:
X propane = 0.5
Pº propane = 1352.1 kPa
X butane = 0.5
Pº butane = 377.6 kPa
It is calculated with Raoult's law:
P mixture = (0.5 x 377.6 kPa) + (0.5 x 1352.1 kPa)
So that:
P mixture = 864.8 kPa
Binary mixture with non-volatile solute
Sometimes it happens that the solute in the mixture is non-volatile, so the law is used to understand the behavior of vapor pressure.
Given a mixture of water and sugar in proportions of 95% and 5%, respectively, and under normal temperature conditions:
X water = 0.95
Pº water = 2.34 kPa
X sugar = 0.05
Pº sugar = 0 kPa
It is calculated with Raoult's law:
P mixture = (0.95 x 2.34 kPa) + (0.05 x 0 kPa)
So that:
P mixture = 2.22 kPa
Clearly there has been a depression in the vapor pressure of the water due to the effects of intermolecular forces.
References
- Anne Marie Helmenstine, P. (nd). Raoult's Law Definition. Retrieved from thoughtco.com
- ChemGuide. (sf). Raoult's Law and Non-Volatile Solutes. Retrieved from chemguide.co.uk
- LibreTexts. (sf). Raoult's Law and Ideal Mixtures of Liquids. Retrieved from chem.libretexts.org
- Neutrium. (sf). Raoult's Law. Retrieved from neutrium.net
- Wikipedia. (sf). Raoult's Law. Retrieved from en.wikipedia.org