Question

Define these terms and give an example for each category: (a) dipole-dipole interaction, (b) dipoleinduced dipole interaction, (c) ion-dipole interaction, (d) dispersion forces, (e) van der Waals forces.

Short answer

Dipole-dipole interactions occur between molecules that have permanent dipoles, like in HCl. Dipole-induced dipole interactions occur between polar and nonpolar molecules, like in H2O and Ar. Ion-dipole interaction is seen between an ion and the partial charge on a dipole, found in a solution of NaCl in water. Dispersion forces are temporary attractive forces that occur when electrons in two closely placed atoms move to create temporary dipoles, seen in molecules like He and CH4. Van der Waals forces refer to a set of weak attractions between molecules, seen in structures like graphite.

Step-by-step solution

Define Dipole-Dipole Interaction

Dipole-dipole interaction occurs between the molecules that have permanent dipoles. This means molecules with polar bonds. They align themselves so that the positive region of one molecule is close to the negative region of another molecule. For example, a molecule of hydrogen chloride (HCl) shows dipole-dipole interaction, as the hydrogen end is slightly positive, and the chlorine end is slightly negative.

Define Dipole-Induced Dipole Interaction

Dipole-induced dipole interaction occurs between a polar molecule and a nonpolar molecule. The polar molecule induces a temporary polarity in the nonpolar molecule, leading to attractions. An example can be seen between a water (H2O) molecule and an argon (Ar) atom.

Define Ion-Dipole Interaction

Ion-dipole interaction occurs between an ion and the partial charge on a dipole. It is stronger than the dipole-dipole interaction. In solutions where ions are present, such as in a solution of sodium chloride (NaCl) in water, this interaction is particularly important.

Define Dispersion Forces

Dispersion forces, also known as London dispersion forces, are temporary attractive forces that occur when electrons in two close atoms move in such a way that they create temporary dipoles. All atoms and molecules experience these forces. Non-polar molecules such as helium (He) or methane (CH4) rely solely on these forces for interactions.

Define Van der Waals Forces

Van der Waals forces refer to a set of weak intermolecular attractions. They include attractions and interactions between molecules, molecular dipoles, quadrupoles, and other multipoles, as well as between moieties within individual molecules. An example of Van der Waals forces is the attraction between the layers in a graphite structure.

Key concepts

Dipole-Dipole Interaction

When molecules display areas of positive and negative charge, known as dipoles, they can attract each other through dipole-dipole interactions. This force plays a central role in the behavior of polar substances.

Take, for instance, water molecules (H₂O), each with a slightly positive end (near the hydrogen atoms) and a slightly negative end (near the oxygen atom). These molecules align themselves so that the opposite charges are next to each other, resulting in a stable structure. In liquid water, this interaction is what gives the substance its unique properties, such as high surface tension and boiling point.

Dipole-Induced Dipole Interaction

Sometimes, a polar molecule can influence a neighboring nonpolar molecule and induce a temporary dipole in it. This is the essence of dipole-induced dipole interaction.

For clarity, consider the interaction between a molecule of chloroform (CHCl₃), which is polar, and oxygen (O₂), which is typically nonpolar. The chloroform can cause a temporary imbalance in the electron cloud of the oxygen molecule, leading to a fleeting attraction between them. These interactions are vital in explaining solubility phenomena and the physical properties of mixtures involving both polar and nonpolar substances.

Ion-Dipole Interaction

Even stronger than dipole-dipole, ion-dipole interactions occur between ions, which have a full charge, and polar molecules, which have a partial charge. This type of force dictates how salts dissolve in water.

Let's take table salt (NaCl) as an example. When salt is mixed into water, the sodium (Na⁺) and chloride (Cl^-) ions interact with the partially charged ends of the water molecules. The negative end of water aligns with the sodium ions, while the positive end aligns with the chloride ions, effectively separating the ions and allowing the salt to dissolve.

Dispersion Forces

Dispersion forces, also known as London forces, are the weakest of all intermolecular forces and occur due to temporary dipoles that arise from momentary uneven electron distributions in atoms or molecules. These forces are universal and affect all substances, whether they are polar or nonpolar.

An example of dispersion forces at work is seen in nitrogen gas (N₂). Despite being a nonpolar molecule, nitrogen exhibits these fleeting attractions that allow the gas molecules to liquefy at very low temperatures. In larger molecules, dispersion forces can become quite significant, playing a role in phenomena such as boiling points and viscosity.

Van der Waals Forces

Van der Waals forces encompass a variety of weak attractions between particles, including the aforementioned dipole-dipole, dipole-induced dipole, and dispersion forces. With these forces, molecules can exert subtle attractive and repulsive powers that impact how they arrange themselves and interact.

A classic instance is found in gecko feet, which allow these reptiles to climb walls and ceilings with ease. The secret lies in the thousands of tiny hairs on their toes that exploit Van der Waals forces to create a strong cumulative attraction to surfaces.