Pressure deviation ii

Last time we talked about the first deviation from Raoult's Law.
This time we will discuss the second one. The deviation from Raoult's Law is important to know because some mixtures form an azeotrope. We will talk about that one in the next entry.

Another type of pressure deviation is:

1) Negative deviation: This deviation occurs when the intermolecular forces between component X and Y are stronger than those intramolecular forces in pure liquid X and pure liquid Y. In other words the resulting vapour pressure is lower than that of ideal mixtures.

Below are some of the mixtures that show this type of pressure deviation:

1) Nitric acid-Water mixture
2) Hydrochloric acid-Water mixture
3) Acetone-Chloroform mixture

In order to identify the difference between these two mixtures (an ideal mixture and a mixture with negative deviation), let us have a look at the P-x diagram.

P-x diagram for an ideal mixture:

P-x diagram for a mixture that shows a negative deviation:

If the deviation from Raoult's law is too strong, we will then have a situation which is not desired (sometimes it cannot be avoided as well) in the field of distillation. Stay tune!