Question

Give an example for each type of intermolecular force: (a) dipole-dipole interaction, (b) ion-dipole interaction, (c) dispersion forces, (d) van der Waals forces.

Short answer

(a) HCl shows dipole-dipole interaction. (b) Na⁺ with H₂O demonstrates ion-dipole interaction. (c) Argon shows dispersion forces. (d) Methane illustrates van der Waals (dispersion) forces.

Step-by-step solution

Understanding Dipole-Dipole Interaction

Dipole-dipole interactions occur between polar molecules, where the positive end of one dipole is attracted to the negative end of another. An example of a dipole-dipole interaction is found in hydrogen chloride (HCl) molecules, where the partially positive hydrogen atom of one molecule is attracted to the partially negative chlorine atom of a nearby molecule.

Understanding Ion-Dipole Interaction

Ion-dipole interactions occur between an ion and a polar molecule. The strength of this interaction depends on the charge of the ion and the magnitude of the dipole. An example of ion-dipole interaction is the interaction between sodium ions (Na⁺) and water (H₂O) molecules, which is common when salt dissolves in water.

Understanding Dispersion Forces

Dispersion forces, also known as London dispersion forces, occur between nonpolar molecules or atoms due to temporary fluctuations in electron distribution, leading to a temporary dipole. An example of dispersion forces can be seen in noble gases like argon (Ar), where these forces allow them to exist in a liquid state at low temperatures.

Understanding Van der Waals Forces

Van der Waals forces is a broad term, including all types of intermolecular forces such as dipole-dipole, ion-dipole, and dispersion forces. The term is often used synonymously with dispersion forces when addressing nonpolar molecules, such as methane (CH₄), where it describes the weak attraction between molecules.

Key concepts

Understanding Dipole-Dipole Interaction

Dipole-dipole interactions are forces that occur between polar molecules. These molecules have an uneven distribution of electrons, resulting in partial positive and negative charges at opposite ends. This alignment creates a dipole, essentially a molecule with a north and a south pole, much like a tiny magnet. In dipole-dipole interactions, the positive end of one polar molecule is attracted to the negative end of a neighboring molecule.
An example is hydrogen chloride (HCl). In this molecule, the hydrogen atom carries a slight positive charge, while the chlorine atom carries a slight negative charge. This charge difference causes the hydrogens in one HCl molecule to attract to the chlorines in neighboring HCl molecules, forming a network of attractions. In essence, dipole-dipole interactions are like a crowd of people holding hand, where each hand-to-hand link represents the attraction between positive and negative charges.

Understanding Ion-Dipole Interaction

Ion-dipole interactions take place between an ion and a polar molecule. Ions are charged atoms or molecules, with the charge originating because they have gained or lost electrons. When these charged particles come near polar molecules with dipoles, a strong attraction develops.
Consider sodium chloride (common table salt, NaCl) dissolving in water. The water molecules are polar, with a partial negative charge on the oxygen and a partial positive charge on the hydrogens. When Na⁺ ions are introduced, the negative oxygen faces the ion, drawing it in with electrostatic force. This connection helps dissolve the salt, illustrating the importance of ion-dipole forces in processes like dissolving and hydration.

Understanding Dispersion Forces

Dispersion forces, or London dispersion forces, are present in all molecules, regardless of their polarity. These forces are particularly important in nonpolar molecules and arise from temporary shifts in electron density, creating a temporary dipole. As electrons move, they can momentarily cause an imbalance, creating a temporary positive and negative side.
Imagine the noble gas argon (Ar) at a low temperature. Each argon atom exhibits fleeting moments where electron distribution is uneven. During these instances, a weak dipole is created, prompting neighboring atoms to develop complementary dipoles. This chain reaction of rapid, weak attractions forms the basis of dispersion forces, enabling atoms like argon to condense into a liquid under certain conditions.

Understanding Van der Waals Forces

Van der Waals forces encapsulate all types of intermolecular forces, including dipole-dipole, ion-dipole, and dispersion forces. However, the term is often used synonymously with dispersion forces, especially when discussing nonpolar molecules.
An example can be seen in methane (CH₄). Methane is a nonpolar molecule, meaning it lacks permanent dipoles. Despite their nonpolar nature, methane molecules interact weakly through van der Waals forces. These interactions occur due to transient fluctuations in electron negating repulsion in nearby molecules and allowing them to come closer. While individually weak, these forces play a crucial role in the behavior of nonpolar molecules, especially gases, in condensed phases.