Question

What kinds of intermolecular forces are present in each substance? (a) \(\mathrm{Kr}\) (b) \(\mathrm{N}_{2}\) (c) \(\mathrm{CO}\) (d) \(\mathrm{HF}\)

Short answer

Kr and N2 experience London dispersion forces; CO has London dispersion forces and dipole-dipole interactions; HF has hydrogen bonding in addition to London dispersion forces.

Step-by-step solution

Identifying the Type of Particles in the Substance

Determine if the substance is atomic, diatomic or polyatomic to understand the types of bonds or interactions it may have.

Determining the Type of Intermolecular Forces for Kr

Since Kr is a noble gas with atoms that are nonpolar, the only intermolecular force present is London dispersion forces.

Determining the Type of Intermolecular Forces for N2

N2 is a diatomic molecule made up of two nitrogen atoms sharing a triple bond, making it nonpolar. Thus, the primary intermolecular force is London dispersion forces.

Determining the Type of Intermolecular Forces for CO

CO is a polar molecule due to the difference in electronegativity between carbon and oxygen, so it experiences dipole-dipole interactions, along with London dispersion forces.

Determining the Type of Intermolecular Forces for HF

HF is a polar molecule with a large difference in electronegativity between hydrogen and fluorine, and thus experiences hydrogen bonding, which is a strong type of dipole-dipole interaction, in addition to London dispersion forces.

Key concepts

London Dispersion Forces

Within the vast world of chemistry, intermolecular forces play a fundamental role in dictating the physical properties of substances. One of the weakest but most pervasive types are London dispersion forces. These forces are temporary attractive forces that result from the movement of electrons creating instantaneous dipoles in atoms or molecules. Interestingly, these forces occur in all molecular substances, whether polar or nonpolar.

For example, noble gases like Kr (krypton) are nonpolar atoms that exhibit these forces as the dominant form of intermolecular attraction. Molecules such as N₂ also rely on London dispersion forces since the molecule is nonpolar due to its diatomic nature with equal electronegativity in both nitrogen atoms.

Dipole-Dipole Interactions

Dipole-dipole interactions step into the spotlight when polar molecules are the subject of discourse. These interactions are stronger than London dispersion forces and occur between molecules that have permanent dipoles, meaning that there is an uneven distribution of charge within the molecule. This creates a scenario where the positive end of one molecule is attracted to the negative end of another molecule.

A classic example is carbon monoxide (CO), where the oxygen atom has a higher electronegativity than carbon, leading to a separation of charges within the molecule and thus, the dipole-dipole interactions. These forces iron out the differences in charge by aligning opposite charges of adjacent molecules in a sort of molecular dance.

Hydrogen Bonding

Among the various types of intermolecular forces is a uniquely strong interaction called hydrogen bonding. Don't let the name fool you; it's not a true bond but a potent form of dipole-dipole interaction that occurs when hydrogen is directly connected to highly electronegative elements like fluorine, oxygen, or nitrogen.

Hydrogen fluoride (HF) provides an excellent backdrop for understanding this concept, as the massive electronegativity difference between hydrogen and fluorine leads to a highly polar molecule. The hydrogen atom, bearing a partial positive charge, is extremely attracted to the negatively charged fluorine of another HF molecule, creating a strong intermolecular pull that greatly influences the boiling and melting points of the substance.

Polarity of Molecules

The polarity of molecules is a seminal topic that hinges on the distribution of electrical charge over the atoms joined by a chemical bond. Essentially, it tells us how equally or unequally electrons are shared between atoms in a molecule. Polarity essentially decides whether a molecule will engage in dipole-dipole interactions or boast a lack thereof.

When atoms in a diatomic molecule like N₂ share electrons equally due to identical electronegativity, the result is a nonpolar molecule, hence dispensing with any permanent charge separation. Conversely, in molecules like CO, differing electronegativities lead to a polar molecule with regions of partial positive and negative charge, setting the stage for dipole-related intermolecular forces.

Electronegativity

Electronegativity is the character trait of an atom that describes its tendency to attract shared electrons in a chemical bond. This property is pivotal in determining the polarity of a molecule.

The greater the difference in electronegativity between the bonded atoms, the more polar the bond becomes. Taking the molecule HF as a case study, the fluorine atom has a high electronegativity compared to hydrogen, which pulls the bonded electron pair closer to itself. This electronegativity discrepancy between the atoms in a bond is the backbone of molecular polarity and, by extension, the type of intermolecular forces that a substance will exhibit.