Understanding Positive and Negative Deviations in Real Solutions

In the realm of chemistry, understanding the properties of mixtures is crucial. Two key concepts in this are positive and negative deviations in real solutions. This article will delve into what these deviations mean and how to remember them using simple tricks and examples.

What are Positive and Negative Deviations?

Positive deviation and negative deviation in real solutions refer to the behavior of the vapor pressure in a mixture compared to that of its pure components. A positive deviation occurs when the actual vapor pressure of the solution is higher than expected due to weaker intermolecular interactions. Conversely, a negative deviation occurs when the actual vapor pressure is lower than expected due to stronger intermolecular interactions.

Positive Deviations in Real Solutions

The term "positive deviation" is used to describe a situation where the actual vapor pressure of a mixture exceeds the predicted vapor pressure based on Raoult's Law. This typically happens when the intermolecular attractive forces between the solute and solvent molecules are weaker than those between the pure components.

Examples of Positive Deviation

Acetone and Carbon Disulphide Acetone and Benzene Carbon Tetrachloride and Toluene or Chloroform Methyl Alcohol and Water Acetone and Ethanol Ethanol and Water

To remember the examples of positive deviation, think of it as the actual vapor pressure exceeding the expected due to weaker intermolecular forces. For instance, in a mixture of ethanol and acetone, the hydrogen bonds between ethanol molecules are weakening due to the presence of acetone molecules, leading to a positive deviation.

Negative Deviations in Real Solutions

On the other hand, a negative deviation in real solutions means the actual vapor pressure is lower than the predicted vapor pressure based on Raoult's Law. This happens when the intermolecular attractive forces between solute and solvent molecules are stronger than those between the pure components.

Examples of Negative Deviation

Chloroform and Benzene Chloroform and Diether Acetone and Aniline Nitric Acid (HNO3) and Water Acetic Acid and Pyridine Hydrochloric Acid (HCl) and Water

To remember the examples of negative deviation, use the mnemonic NABI SIB CA PA:

N: Negative deviation AB: A and B (two liquids) I: Interaction S: Stronger I: Increases B: Boiling point CA: Chloroform and Acetone PA: Phenol and Aniline

In a mixture with negative deviation, the intermolecular attractive forces are stronger. For example, in a mixture of chloroform and acetone, the chloroform molecules form hydrogen bonds with acetone molecules, decreasing the escaping tendency of each component and leading to a negative deviation from Raoult's Law.

Remembering the Differences

Think of positive deviation as "actual exceeds expected” and negative deviation as "actual falls short of expected”. Applying this principle to real-world examples like revenue forecasting can help solidify this understanding. Additionally, the mnemonic NABI SIB CA PA can serve as a handy tool for remembering which mixtures exhibit positive or negative deviations.

Conclusion

By understanding the principles of positive and negative deviations in real solutions and applying the appropriate examples, chemists and enthusiasts can better predict and explain the behavior of solutions. Memorizing the examples using mnemonics and relating them to familiar concepts can make these complex ideas more accessible.