Question

Discuss the types of intermolecular forces acting in the liquid state of each of the following substances. a. \(\mathrm{N}_{2}\) b. \(\mathrm{NH}_{3}\) c. He d. \(\mathrm{CO}_{2}\) (linear, nonpolar)

Short answer

In the liquid state, the types of intermolecular forces present in each substance are: a. \(\mathrm{N}_2\): London dispersion forces only. b. \(\mathrm{NH}_3\): Dipole-dipole interactions and hydrogen bonding. c. He: London dispersion forces only. d. \(\mathrm{CO}_2\): London dispersion forces only.

Step-by-step solution

Identify intermolecular forces

Intermolecular forces are forces that exist between molecules and play a crucial role in determining the physical properties of substances. There are three main types of intermolecular forces: London dispersion forces, dipole-dipole forces, and hydrogen bonding.

(a) Interactions in \(\mathrm{N}_{2}\)

Nitrogen gas, \(\mathrm{N}_{2}\), is a nonpolar molecule because it is made up of two identical nitrogen atoms sharing electrons equally. Thus, the only intermolecular forces acting in the liquid state of \(\mathrm{N}_{2}\) are London dispersion forces. London dispersion forces are temporary, weak attractive forces between nonpolar molecules caused by the constant movement of electrons.

(b) Interactions in \({\mathrm{NH}}_{3}\)

Ammonia (\(\mathrm{NH}_3\)) is a polar molecule due to the presence of a lone pair of electrons on the nitrogen atom and the difference in electronegativities between nitrogen and hydrogen atoms. As a result, there are dipole-dipole interactions between the molecules, as the positive end of one molecule is attracted to the negative end of another. Additionally, since there is hydrogen bonding with nitrogen, which is one of the few elements that hydrogen can bond with to create hydrogen bonding, there are hydrogen bonding forces acting in the liquid state of \(\mathrm{NH}_{3}\).

(c) Interactions in He

The helium atom, He, is a monoatomic substance and is a noble gas, which means it is nonpolar with no bonds. The only intermolecular forces acting in the liquid state of helium are London dispersion forces. These forces are typically weaker than in other substances due to the small size and low polarizability of helium atoms.

(d) Interactions in \(\mathrm{CO}_{2}\)

Carbon dioxide, \(\mathrm{CO}_{2}\) is a linear, nonpolar molecule due to the equal sharing of electrons between the carbon and oxygen atoms. The polarity of the molecule cancels out, making it linear and nonpolar. Consequently, the only intermolecular forces acting in the liquid state of \(\mathrm{CO}_{2}\) are London dispersion forces. These forces are present as a result of temporary dipoles forming as electrons move in the molecule.

Key concepts

London Dispersion Forces

Imagine tiny temporary magnets created by the movement of electrons in nonpolar molecules—these are London dispersion forces. Because electrons are constantly on the move, they can create temporary regions of negative charge that attract nearby positive regions in other molecules. This fleeting attraction, named after German-American physicist Fritz London, is weak compared to other intermolecular forces, but it's the primary force at play in nonpolar substances like nitrogen gas (2) and helium (He).

These forces get stronger with more electrons (larger molecules) or with increased molecular surface area, which allows more close contact between molecules. For substances with similar molecular weights, the shape can influence the strength of London dispersion forces. For example, a long, skinny molecule can experience stronger dispersion forces than a compact one with the same molecular weight.

Dipole-Dipole Interactions

Attraction Between Polar Players

When molecules have areas of permanent partial positive and negative charge – a separation known as a dipole – they enter into a tug-of-war called dipole-dipole interactions. Think of magnets with north and south poles; the positive end of one molecule naturally aligns with the negative end of another. Substances like ammonia (H3) possess a lopsided electron distribution creating such dipoles.

These interactions are notably stronger than London dispersion forces and are crucial in determining the high boiling points of polar substances. In a cooling liquid, molecules slow down, and these dipole attractions help stabilize the liquid state. However, when heating up, adding enough energy can overcome these dipole-dipole interactions, allowing the substance to vaporize.

Hydrogen Bonding

The Special Case of H-Bonds

Hydrogen bonding is the ace of attractions in the intermolecular forces deck. It's a strong, specific type of dipole-dipole interaction that happens when hydrogen is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. The bond produces a highly polar molecule, with hydrogen carrying a partial positive charge. This charge is attracted to the lone pair of electrons on the electronegative atom in another molecule, creating a hydrogen bond.

In substances like ammonia (H3), these hydrogen bonds are why we observe unusually high boiling points and unique properties like surface tension and viscosity. Hydrogen bonds are the behind-the-scenes heroes in the liquid state of water and biological molecules like DNA, lending to the extraordinary properties of life itself.