Question

Among the following mixtures, dipole-dipole as the major interaction, is present in (a) benzene and ethanol (b) acetonitrile and acetone (c) \(\mathrm{KCl}\) and water (d) benzene and carbon tetrachloride

Short answer

Acetonitrile and acetone have major dipole-dipole interactions.

Step-by-step solution

Identify the Molecules

First, we need to identify the type of molecules in each mixture. Benzene is a nonpolar aromatic hydrocarbon, ethanol is a polar molecule due to its hydroxyl group. Acetonitrile and acetone are both polar molecules due to their cyano and carbonyl groups, respectively. KCl is an ionic compound, and water is a polar molecule. Carbon tetrachloride is a nonpolar molecule.

Identify Intermolecular Forces

We need to determine the types of intermolecular forces that can exist between the molecules in each pair. Dipole-dipole interactions occur between polar molecules. Ionic interactions occur between ions and polar molecules. London dispersion forces occur in nonpolar molecules.

Analyze Each Mixture for Dipole-Dipole Interactions

For the benzene and ethanol mixture, benzene is nonpolar, and ethanol is polar, so any interaction is not primarily dipole-dipole. In the acetonitrile and acetone mixture, both are polar molecules and can exhibit dipole-dipole interactions predominately. KCl and water consist of ionic interactions primarily, rather than dipole-dipole. Benzene and carbon tetrachloride are both nonpolar, so dipole-dipole interactions won't be present.

Conclusion Based on Analysis

From our analysis, the acetonitrile and acetone mixture is the only one where both components are polar, allowing for dipole-dipole interactions as the major interaction.

Key concepts

Dipole-Dipole Interactions

Dipole-dipole interactions are a type of intermolecular force that occurs between polar molecules. These interactions arise because molecules with permanent dipoles have regions of positive and negative charge. When these polar molecules come close to each other, the positive end of one molecule is attracted to the negative end of another. This creates a force that holds the molecules together.

Some important points about dipole-dipole interactions include:

• They are generally stronger than London dispersion forces but weaker than ionic bonds.
• They play a significant role in determining the boiling and melting points of substances.
• These interactions are only present when molecules are close together in liquids or solids.

For example, in the mixture of acetonitrile and acetone, both of these polar molecules interact through dipole-dipole forces due to their permanent dipoles.

Polar Molecules

Polar molecules are molecules that have an uneven distribution of charge. This means one end of the molecule is slightly positive while the other end is slightly negative. This charge difference is due to the difference in electronegativity between the atoms involved in the bond.

Characteristics of polar molecules include:

• They have an asymmetrical shape, leading to a permanent dipole.
• They can interact with other polar molecules through dipole-dipole interactions.
• Examples of polar molecules include water (H₂O), ethanol (C₂H₅OH), acetonitrile (CH₃CN), and acetone ((CH₃)₂CO).

Polar molecules are crucial for many chemical interactions and are often found in polar solvents, making them important in both industrial and biological processes.

Nonpolar Molecules

Nonpolar molecules have an even distribution of electrical charge, meaning no permanent dipoles are present. The atoms in nonpolar molecules have similar or identical electronegativities, making the electron sharing in the bonds equal.

Key features of nonpolar molecules include:

• They generally have symmetrical shapes, causing any charge differences to cancel out.
• London dispersion forces are usually the only type of intermolecular force present.
• Common examples include benzene (C₆H₆) and carbon tetrachloride (CCl₄).

Nonpolar molecules tend to be gases or liquids with low boiling points at room temperature because London dispersion forces are relatively weak compared to other intermolecular forces.

Ionic Interactions

Ionic interactions occur between charged particles, typically between positively charged cations and negatively charged anions. These interactions are the result of electrostatic attraction and significantly influence the properties of ionic compounds.

Essentials of ionic interactions:

• They are much stronger than dipole-dipole and London dispersion forces.
• Ionic interactions lead to high melting and boiling points.
• Water is a common solvent for ionic compounds due to its polar nature, which helps dissolve ions.

An example of ionic interaction is found in a mixture of potassium chloride (KCl) and water, where KCl dissociates into potassium and chloride ions, which then interact with the polar water molecules.

London Dispersion Forces

London dispersion forces are weak intermolecular forces that arise in all molecules, whether polar or nonpolar, but are the only forces present in nonpolar molecules. They result from temporary fluctuations in the electron clouds of molecules, leading to the creation of instantaneous dipoles.

Characteristics of London dispersion forces include:

• These forces increase with increasing molecular size or mass because larger electron clouds are more polarizable.
• They are typically weaker than dipole-dipole interactions but can become significant in large atoms or molecules.
• These forces are present in substances like benzene and carbon tetrachloride, explaining their properties.

London dispersion forces are important for explaining why nonpolar substances can condense into liquids or solids under the right conditions, especially as the size of the molecules increases.