Predict which of the following mixtures will show a positive deviation from Raoul's Law?

Ethanol and acetone

The correct answer is option 2. Ethanol and acetone.

Raoult's Law states that the partial vapor pressure of each component in an ideal solution is proportional to its mole fraction. For an ideal solution, the total vapor pressure is the sum of the partial pressures of the individual components, and the vapor pressures should be proportional to the concentration of each component.

Deviation from Raoult's Law:

Positive Deviation: Occurs when the vapor pressure of the solution is higher than expected from Raoult's Law. This happens when the intermolecular forces between different components in the mixture are weaker than those between the molecules of the pure substances. In other words, the interactions in the mixture are less favorable compared to the interactions in the pure components, leading to an increased vapor pressure.

Negative Deviation: Occurs when the vapor pressure of the solution is lower than expected. This happens when the intermolecular forces in the mixture are stronger than those in the pure components. The mixture is more stable due to these stronger interactions, resulting in a lower vapor pressure.

Analyzing Each Mixture:

1. Nitric Acid and Water:

Nitric acid (\( \text{HNO}_3 \)) can ionize in water, leading to strong ion-dipole interactions and hydrogen bonding. The presence of strong ion-dipole interactions usually results in a lower vapor pressure compared to the ideal case because the solution is more stable, leading to a negative deviation from Raoult's Law.

2. Ethanol and Acetone:

Ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)) and acetone (\( \text{CH}_3\text{COCH}_3 \)) can form hydrogen bonds with each other. However, these hydrogen bonds are typically weaker compared to the hydrogen bonds found in pure ethanol or pure acetone. Thus, the intermolecular forces in the mixture are weaker than those in the pure substances, leading to a positive deviation from Raoult's Law. The vapor pressure of the solution is higher than predicted because the mixture has weaker interactions than the pure components.

3. Phenol and Aniline:

Both phenol (\( \text{C}_6\text{H}_5\text{OH} \)) and aniline (\( \text{C}_6\text{H}_5\text{NH}_2 \)) can form hydrogen bonds with each other. The hydrogen bonding between phenol and aniline can be quite strong. As a result, the vapor pressure of the mixture will be lower than expected due to the strong interactions stabilizing the mixture, leading to a negative deviation from Raoult's Law.

4. Chloroform and Acetone:

In the solution of chloroform and acetone, the hydrogen bond is formed with the acetone molecule by the chloroform molecule. Hydrogen bonding between acetone and chloroform reduces the possibility of molecules of the mixture escaping which leads to a decrease in vapour pressure. Therefore, the solution indicates a negative deviation from Raoult’s law.

Conclusion:

Ethanol and acetone show a positive deviation from Raoult's Law because the interactions between ethanol and acetone are weaker than the interactions present in the pure substances. This results in a higher vapor pressure for the mixture compared to the ideal value predicted by Raoult's Law.