Name: Non-ideal Solutions
Uploaded: 2019-07-19
Duration: 8 min 43 s
Description: We know that an ideal solution is a theoretical concept. So, what are non-ideal solutions? Why do they show a deviation from Raoult's law? Let's explore!

Question

Explain with the help of a diagram why some of the non-ideal solutions show positive deviations while some others show negative deviations.

Accepted Answer

(a) Non-Ideal solutions showing positive deviation: The positive deviation from Raoult's law shows that the molecules have more tendency to escape from the solution as compared to the tendency to escape from pure liquids. This is because the intermolecular forces of attraction in the solution $(A - B)$ are weaker than those of the pure components $(A - A, B - B)$ .

Consider an example of a solution obtained by mixing ethyl alcohol and acetone. The alcohol molecules are held together by strong hydrogen bonds in the pure solute.

When acetone is added to alcohol, the molecules of acetone tend to adjust between the molecules of alcohol. This results in the breakage of some of the hydrogen bond and thus the intermolecular forces of attraction get weaker in the solution. The escaping tendency of the molecules in the solution increases. As a result, vapour pressure of the solution is more than expected for an ideal solution and shows positive deviation from Raoult's law.

(b) Non-Ideal solutions showing negative deviation: The negative deviation from Raoult's law shows that the molecules have less tendency to escape from the solution as compared to the tendency to escape from pure liquids. This is because the intermolecular forces of attraction in the solution $(A - B)$ are stronger than those of the pure components $(A - A, B - B)$ .

Consider an example of a solution obtained by mixing water and sulphuric acid. In this solution, new attraction forces arise due to the formation of hydrogen bonds between solute and solvent. As a result, the escaping tendency of $H_{2} O$ and $H_{2} {SO}_{4}$ molecules from the solution decreases. The vapour pressure is less than expected for ideal behaviour and thus, it shows negative deviation from Raoult's law.

Note: The dotted lines in both figures indicate the ideal behaviour while the thick lines shows the actual behaviour.

Explain with the help of a diagram why some of the non-ideal solutions show positive deviations while some others show negative deviations.

Important Questions on Solutions

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