Question

A perfectly ideal solution is rare but some solutions behave nearly ideal. Which of the following does not fall in this category?

• A. n-hexane and n-heptane
• B. Ethanol and acetone
• C. Benzene and toluene
• D. Bromoethane and chloroethane

Answer 1

The Correct Option is B

In chemistry, an ideal solution is one where the enthalpy of mixing is zero, and the volume of mixing is also zero. This typically occurs when the components have similar molecular sizes and intermolecular forces. Let's analyze the given solutions: 

n-hexane and n-heptane: Both are alkanes with similar structures and intermolecular forces, likely to form an ideal solution.

Benzene and toluene: Both are aromatic compounds with similar sizes and van der Waals forces, likely to exhibit ideal behavior.

Bromoethane and chloroethane: Both are haloalkanes with similar structures, likely to form an ideal solution.

Ethanol and acetone: Ethanol is an alcohol with hydrogen bonding, while acetone is a ketone with dipole interactions. The difference in their intermolecular forces leads to non-ideal behavior.

Hence, the pair that does not behave like an ideal solution is ethanol and acetone.

Answer 2

Ethanol and acetone are two common organic compounds that exhibit significantly different polarity and hydrogen bonding capabilities, which results in their deviation from ideal behavior in certain conditions.

Ethanol (C₂H₅OH) is a highly polar molecule due to the presence of a hydroxyl group (–OH) that can form strong hydrogen bonds with other ethanol molecules and with water. This ability to form hydrogen bonds makes ethanol highly soluble in water and gives it relatively high boiling and melting points compared to non-polar molecules of similar size.

In contrast, acetone ((CH₃)₂CO) is a polar molecule, but it does not form hydrogen bonds as strongly as ethanol. Acetone has a carbonyl group (C=O) that is polar, but it lacks the –OH group necessary for hydrogen bonding with itself. As a result, acetone exhibits different intermolecular forces compared to ethanol and behaves differently in solutions, particularly when mixed with water or other solvents.

These differences in polarity and hydrogen bonding lead to deviations from ideal behavior in solutions, such as non-ideal mixing and a deviation from Raoult's Law, which describes the vapor pressure of ideal solutions. When ethanol and acetone are mixed, the interactions between the molecules cause deviations in their physical properties, such as vapor pressure, boiling points, and solubility. This is a result of the differing strengths of intermolecular forces between ethanol and acetone molecules.

Understanding these differences is important in various applications, including solvent behavior, chemical reactions, and industrial processes where ethanol and acetone are commonly used.