Question

What kinds of intermolecular forces are present in each substance? (a) \(\mathrm{HCl}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{Br}_{2}\) (d) \(\mathrm{He}\)

Short answer

HCl has dipole-dipole interactions and hydrogen bonding; H2O has strong hydrogen bonding; Br2 has London dispersion forces; He has very weak London dispersion forces.

Step-by-step solution

Identify Intermolecular Forces in HCl

HCl is a polar molecule with a significant difference in electronegativity between the hydrogen (H) and chlorine (Cl) atoms. This leads to a dipole-dipole attraction between molecules. Additionally, since it also has a hydrogen attached to a highly electronegative atom (chlorine), hydrogen bonds can also occur, although technically, these are usually considered a special type of dipole-dipole interaction and not actual bonds like covalent or ionic bonds.

Identify Intermolecular Forces in H2O

Water (H2O) is a polar molecule with two hydrogen atoms bonded to an oxygen atom. The electronegativity difference between the oxygen and hydrogen atoms leads to a strong dipole-dipole interaction. Moreover, water has a very strong hydrogen bonding due to the presence of hydrogen atoms bonded to a highly electronegative oxygen atom.

Identify Intermolecular Forces in Br2

Bromine (Br2) is a diatomic molecule with two bromine atoms sharing electrons equally. This molecule is nonpolar, so the primary intermolecular force is London dispersion forces, which are caused by momentary dipoles that occur due to the motion of electrons.

Identify Intermolecular Forces in He

Helium (He) is a noble gas with atoms that do not have a permanent dipole moment. The only intermolecular forces present are London dispersion forces, which are very weak due to helium's small size and the low polarizability of its electron cloud.

Key concepts

Dipole-Dipole Interactions

Imagine two magnets placed close to each other: just like the opposite poles attract, similar is the case with certain molecules. Dipole-dipole interactions occur in polar molecules where there's an uneven distribution of electrons. This happens when atoms with different electronegativities - like nitrogen, oxygen, or fluorine - are part of the molecule, causing one end to be slightly negative and the other end slightly positive.

To understand this better, think about hydrochloric acid (HCl). The electron-hungry chlorine pulls electrons towards itself, creating a partial negative charge, while hydrogen gets a partial positive charge. When HCl molecules get close, the positive end of one attracts the negative end of another, just like mini magnets!

Hydrogen Bonding

In the world of intermolecular forces, hydrogen bonding is like a superpower. It's a strong type of dipole-dipole interaction, but it doesn't happen just between any molecules - it's specific to molecules where hydrogen is bonded to very electronegative atoms like nitrogen, oxygen, or fluorine.

Take water (H2O) for example. Water's hydrogen atoms are attracted to the oxygen atoms of other water molecules, forming a unique bond that's sort of a bridge between full-blown ionic or covalent bonds and weaker dipole interactions. This special bond gives water its high boiling point and surface tension, making life as we know it possible.

London Dispersion Forces

Even when molecules don't have permanent dipoles, they can still have fleeting moments of polarity due to the constant motion of electrons. These transient moments create temporary dipoles which, in turn, induce dipoles in adjacent molecules. This phenomenon is known as London dispersion forces. These are the only type of intermolecular forces present in nonpolar molecules like bromine (Br2) and gases like helium (He), where electrons randomly create momentary attractions.

Despite being the weakest of all intermolecular forces, they play a significant role in the properties of noble gases and nonpolar molecules. For instance, dispersion forces are the reason why Br2 is a liquid at room temperature and why noble gases can be liquified under certain conditions.

Polarity of Molecules

Polarity is the Meryl Streep of molecular characteristics - versatile and central to performance. A molecule is considered polar if it has areas of positive and negative charge due to the uneven distribution of electrons across different atoms.

The electronegativity of an atom plays a key role here. Atoms like oxygen, fluorine, and nitrogen tend to hog electrons, creating an imbalance, or a dipole, in a molecule. Water (H2O), for example, is polar because of the significant differences in electronegativity between hydrogen and oxygen. Polarity affects many properties of substances, including their boiling and melting points, solubility, and how they interact with other molecules.