Question

What intermolecular forces must the following substances overcome in the process of vaporization? (a) ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\), (b) acetonitrile \(\left(\mathrm{H}_{3} \mathrm{CCN}\right)\), (c) phosphorus pentachloride \(\left(\mathrm{PCl}_{5}\right)\), and (d) sodium chloride \((\mathrm{NaCl})\)

Short answer

Ethanol overcomes hydrogen bonding, dipole-dipole interactions, and London dispersion forces. Acetonitrile overcomes dipole-dipole interactions and London dispersion forces. Phosphorus pentachloride overcomes London dispersion forces, and sodium chloride needs to overcome ionic bonds during vaporization.

Step-by-step solution

Identify Intermolecular Forces for Ethanol (C2H5OH)

Ethanol has a hydroxyl group (OH) which is capable of forming hydrogen bonds due to the presence of hydrogen directly bound to an electronegative oxygen atom. Ethanol also exhibits dipole-dipole interactions because of its polar nature and London dispersion forces which are present in all molecules regardless of polarity.

Identify Intermolecular Forces for Acetonitrile (H3CCN)

Acetonitrile contains a triple bond between carbon and nitrogen which results in a significant difference in electronegativity, creating a polar molecule. Therefore, acetonitrile experiences dipole-dipole interactions and London dispersion forces. However, it cannot form hydrogen bonds because it lacks a hydrogen atom bound directly to a highly electronegative atom such as oxygen, nitrogen, or fluorine.

Identify Intermolecular Forces for Phosphorus Pentachloride (PCl5)

Phosphorus pentachloride is a nonpolar molecule as the symmetry of its trigonal bipyramidal structure cancels any dipole moments. Therefore, the primary intermolecular forces it must overcome are London dispersion forces.

Identify Intermolecular Forces for Sodium Chloride (NaCl)

Sodium chloride is an ionic compound composed of sodium cations (Na+) and chloride anions (Cl-). The main intermolecular force in NaCl is the ionic bond which is much stronger than other types of intermolecular forces. Upon vaporization, it requires overcoming these ionic bonds.

Key concepts

Vaporization of Substances

Vaporization is the process by which a substance transitions from a liquid to a gaseous state. This transformation requires the substance to overcome intermolecular forces that hold the particles together within the liquid. A common example is when water boils and turns into steam; the heat energy supplied breaks the hydrogen bonds and other interactions between water molecules.

Key factors affecting vaporization include the temperature, the strength of intermolecular forces within the substance, and the presence of atmospheric pressure. Substances with stronger intermolecular forces require more energy to vaporize. In the examples provided, each substance has a different set of intermolecular forces to overcome for vaporization to occur.

Hydrogen Bonding

Hydrogen bonding is an especially strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. These bonds are much stronger compared to other dipole-dipole interactions because of the significant polarity and close proximity of the involved atoms.

For instance, in ethanol (\( \text{C}_{2}\text{H}_{5}\text{OH} \)), the presence of a hydroxyl group allows the formation of hydrogen bonds. The hydrogen atom, partially positive, is attracted to the electronegative oxygen atom of another ethanol molecule, creating a network of strong intermolecular forces which are pivotal in determining the boiling point and the amount of energy needed for vaporization.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. The strength of these interactions depends on the polarity of the molecules—the greater the difference in electronegativity between the atoms, the stronger the dipole.

In the case of acetonitrile (\( \text{H}_{3}\text{CCN} \)), the significant difference in electronegativity between the carbon and the nitrogen creates a dipole moment, leading to the dipole-dipole interactions. These interactions are less strong than hydrogen bonds but are still important in determining a substance's physical properties like boiling point and vapor pressure.

London Dispersion Forces

London dispersion forces, also known as van der Waals forces, are weak intermolecular attractions that result from temporary fluctuations in electron distribution within molecules, leading to fleeting dipoles. These forces are universal and present in all atoms and molecules, whether they are polar or nonpolar, though they are more pronounced in larger and more malleable atoms or molecules.

The molecule phosphorus pentachloride (\( \text{PCl}_{5} \)) experiences London dispersion forces as the primary intermolecular force due to its nonpolar structure. As these forces are relatively weak, the energy required to overcome them during vaporization is less compared to substances with stronger intermolecular forces.

Ionic Bonds

Ionic bonds are formed between positively charged cations and negatively charged anions. These are not considered typical intermolecular forces; they are actual chemical bonds and are much stronger than any intermolecular forces. Ionic bonds result from the electrostatic attraction between oppositely charged ions and are the primary forces at play in ionic compounds.

In sodium chloride (\( \text{NaCl} \)), an example of an ionic compound, the ionic bonds must be overcome for vaporization to occur. This requires a significant amount of energy, as these bonds firmly hold the ions in a crystalline lattice structure. Therefore, ionic compounds typically have high boiling points, and achieving vaporization is a considerably energy-intensive process.