Question

What is the strongest type of intermolecular force between solute and solvent in each solution? (a) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}(g)\) in \(\mathrm{H}_{2} \mathrm{O}(l)\) (b) \(\operatorname{Ne}(g)\) in \(\mathrm{H}_{2} \mathrm{O}(l)\) (c) \(\mathrm{N}_{2}(g)\) in \(\mathrm{C}_{4} \mathrm{H}_{10}(g)\)

Short answer

a) Dipole-dipole, b) Dipole-induced dipole, c) London dispersion forces.

Step-by-step solution

Identify the Types of Molecules

Determine whether the solute and solvent molecules are polar or nonpolar. This will help in understanding the possible intermolecular forces between them.

Determine Intermolecular Forces for (a)

For \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)(g) in \(\mathrm{H}_{2} \mathrm{O}(l)\): \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (dimethyl ether) is a polar molecule, and \(\mathrm{H}_{2} \mathrm{O}\) (water) is also a polar molecule. Thus, the strongest intermolecular force between them would be hydrogen bonding, although \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) cannot form hydrogen bonds itself, it can interact through dipole-dipole interactions.

Determine Intermolecular Forces for (b)

For \(\operatorname{Ne}(g)\) in \(\mathrm{H}_{2} \mathrm{O}(l)\): Neon is a nonpolar gas and water is polar. The interaction between nonpolar and polar molecules is typically governed by dipole-induced dipole interactions, which are relatively weak forces.

Determine Intermolecular Forces for (c)

For \(\mathrm{N}_{2}(g)\) in \(\mathrm{C}_{4} \mathrm{H}_{10}(g)\): Both nitrogen gas \(\mathrm{N}_2\) and butane \(\mathrm{C}_4 \mathrm{H}_{10}\) are nonpolar. The intermolecular force between them would be London dispersion forces, which are the weakest type of intermolecular forces.

Key concepts

hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole interaction which is particularly strong. It occurs when a hydrogen atom is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. This leaves the hydrogen atom with a significant positive charge, allowing it to interact strongly with negative regions on neighboring molecules. Examples of hydrogen bonding can be observed in water (H₂O) and between amines (NH₂ groups). These bonds are crucial for many biological molecules, such as DNA and proteins, where hydrogen bonding helps to stabilize structures.

Key points to remember about hydrogen bonding:

• Occurs only with N, O, or F attached to H.
• Responsible for the high boiling point of water.
• Influences the unique properties of substances like proteins and nucleic acids.

dipole-dipole interactions

Dipole-dipole interactions occur between molecules that have permanent dipole moments, i.e., molecules with polar covalent bonds. The positive end of one dipole is attracted to the negative end of another dipole. These interactions are generally stronger than London dispersion forces but weaker than hydrogen bonds.

Examples of dipole-dipole interactions include the attraction between acetone (CH₃COCH₃) molecules and formaldehyde (CH₂O) molecules.

Important details about dipole-dipole interactions:

• Only occur between polar molecules.
• Influence properties like boiling and melting points.
• Strength depends on the polarity of the molecules involved.

London dispersion forces

London dispersion forces, also known simply as dispersion forces or van der Waals forces, are the weakest intermolecular forces. These forces arise from momentary fluctuations in electron density within molecules, which create temporary dipoles. This temporary dipole can then induce a similar dipole in a neighboring molecule, leading to an attraction between them. This type of force is the only intermolecular force acting in nonpolar molecules.

Key points about London dispersion forces:

• Present in all molecules, but are the only force in nonpolar molecules.
• Increase in strength with the size and shape of the molecules.
• Important in large, nonpolar molecules such as gases (e.g., N₂, O₂) and long hydrocarbons (e.g., butane, C₄H₁₀).