Question

Which type of intermolecular attractive force operates between (a) all molecules, (b) polar molecules, (c) the hydrogen atom of a polar bond and a nearby small electronegative atom?

Short answer

(a) The type of intermolecular attractive force operating between all molecules is Van der Waals force (London dispersion force). (b) The type of intermolecular attractive force operating between polar molecules is dipole-dipole interaction. (c) The type of intermolecular attractive force operating between the hydrogen atom of a polar bond and a nearby small electronegative atom is a hydrogen bond.

Step-by-step solution

Identify the force for all molecules

Van der Waals forces, also known as London dispersion forces, are the weakest type of intermolecular attractive force, and they operate between all types of molecules, polar or nonpolar. (a) The type of intermolecular attractive force operating between all molecules is Van der Waals force (London dispersion force).

Identify the force for polar molecules

Polar molecules have a permanent dipole moment due to the differences in the electronegativity of the atoms in the molecule. The intermolecular attractive force between polar molecules is called dipole-dipole interaction. (b) The type of intermolecular attractive force operating between polar molecules is dipole-dipole interaction.

Identify the force between the hydrogen atom of a polar bond and a nearby small electronegative atom

A hydrogen bond is a type of dipole-dipole interaction between the hydrogen atom of a polar bond (like O-H, N-H, or F-H) and a nearby small electronegative atom (such as O, N, or F). It is a relatively strong intermolecular force compared to the other two mentioned previously. (c) The type of intermolecular attractive force operating between the hydrogen atom of a polar bond and a nearby small electronegative atom is a hydrogen bond.

Key concepts

Van der Waals Forces

The intricate dance of particles in matter includes various interactions that hold molecules together, but one of the most universal is Van der Waals forces. Named after Dutch scientist Johannes Diderik van der Waals, these forces are the weakest of the intermolecular attractions, yet they play a crucial role in the behavior of molecules.

Van der Waals forces can be thought of as the background noise of intermolecular attractions, as they occur between all types of molecules, whether polar or nonpolar. To understand these forces, imagine two neighboring molecules. Even if they are nonpolar, the electrons in these molecules are always moving. Occasionally, by happenstance, electrons in one part of a molecule may be more concentrated, creating a temporary dipole. As a result, this can induce a corresponding dipole in the neighboring molecule. Think of this as two dancers momentarily leaning towards each other before swiftly drifting apart—this fleeting attraction is the essence of Van der Waals forces.

Van der Waals forces consist of attractions between temporary dipoles, permanent dipoles, and induced dipoles. However, these forces are generally weaker compared to other intermolecular forces because they rely on temporary, shifting electron clouds rather than permanent polarity or chemical bonds. They are significant in explaining why even nonpolar molecules like neon gas condense to liquids at low temperatures or why geckos can walk on walls — their setae exploit these forces.Marvelously, Van der Waals forces are universal, emanating from all atoms and molecules, which makes them fundamentally important in the cohesion of materials.

Dipole-Dipole Interactions

When we start discussing polar molecules, a new character enters the stage - the dipole-dipole interaction. These attractions occur in substances where molecules exhibit permanent dipoles, which means there's a side that's always slightly negative and a side that's slightly positive. It's as if each molecule has a built-in magnet with a plus and minus pole.

Think of water (H₂O) molecules, with oxygen having a higher electronegativity than hydrogen. This difference pulls the molecule's electron cloud towards the oxygen, leaving the hydrogens slightly positive and the oxygen slightly negative. When many water molecules come together, the positive hydrogen of one attracts the negative oxygen of another. It's a perpetual dance of attraction, much like a ballroom filled with pairs of magnets gracefully aligning positive to negative.

The result is a higher boiling point for polar substances compared to nonpolar ones, because more heat is needed to break these interactions apart. This explains why water has a relatively high boiling point for a molecule of its size. Dipole-dipole interactions are also why certain liquids mix well together. For example, alcohol mixes with water because both are polar, allowing their dipoles to interact harmoniously.

Hydrogen Bonding

Among the types of intermolecular forces, hydrogen bonding is the VIP, famously known for its role in the vitality of life itself, such as the double helix structure of DNA. A hydrogen bond is a more intense version of a dipole-dipole interaction, but it only occurs when hydrogen is bonded to very electronegative elements--namely oxygen, nitrogen, or fluorine.

In a molecule of water, for instance, the difference in electronegativity between hydrogen and oxygen creates a strong dipole. The hydrogen atoms carry a δ+ (delta positive) charge while the oxygen bears a δ− (delta negative) charge. When water molecules come close, the δ+ hydrogen gets attracted to the δ− oxygen of another molecule, creating a hydrogen bond. It's akin to a firm handshake between molecules – stronger and more specific than a casual wave (Van der Waals force) or a polite bow (generic dipole-dipole interaction).

Hydrogen bonds may be individual and delicate, but collectively, they're mighty. They give water its unique properties, like its high surface tension, high specific heat capacity, and the fact that ice floats on liquid water. By holding biological molecules in specific shapes, hydrogen bonds are crucial in biochemistry, affecting enzyme function, protein folding, and the properties of nucleic acids. This special intermolecular force significantly determines the behavior of molecules where it's present, making it a focal point in the study of chemistry and biology.