Question

In each of the following groups of substances, pick the one that has the given property. Justify your answer. a. highest boiling point: \(\mathrm{HBr}, \mathrm{Kr}\), or \(\mathrm{Cl}_{2}\) b. highest freezing point: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{NaCl}\), or \(\mathrm{HF}\) c. lowest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{Cl}_{2}, \mathrm{Br}_{2}\), or \(\mathrm{I}_{2}\) d. lowest freezing point: \(\mathrm{N}_{2}, \mathrm{CO}\), or \(\mathrm{CO}_{2}\) e. lowest boiling point: \(\mathrm{CH}_{4}, \mathrm{CH}_{3} \mathrm{CH}_{3}\), or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) f. highest boiling point: \(\mathrm{HF}, \mathrm{HCl}\), or \(\mathrm{HBr}\) g. lowest vapor pressure at \(25^{\circ} \mathrm{C}: \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}, \mathrm{CH}_{3} \mathrm{CCH}_{3}\), or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Short answer

a. HBr has the highest boiling point because it has dipole-dipole forces, which are stronger than the London dispersion forces present in Kr and Cl2. b. NaCl has the highest freezing point, due to strong ionic bonding between its ions. c. I2 has the lowest vapor pressure at 25°C because its stronger London dispersion forces require more energy to be exerted before it vaporizes. d. N2 and CO have the lowest freezing points due to their lower molar masses and therefore weaker London dispersion forces. e. CH4 has the lowest boiling point due to its lower molar mass and therefore weaker London dispersion forces. f. HBr has the highest boiling point due to its greater molar mass and thus stronger hydrogen bonding. g. CH3CH2CH2OH has the lowest vapor pressure at 25°C due to the stronger hydrogen bonding compared to the London dispersion forces in other molecules.

Step-by-step solution

Consider Intermolecular Forces

HBr has dipole-dipole forces due to its polarity while Kr and Cl2 are nonpolar with London dispersion forces. Dipole-dipole forces are stronger than London dispersion forces.

Compare Molecular Weights

HBr has a molar mass of 80 g/mol, Kr has a molar mass of 83.8 g/mol, and Cl2 has a molar mass of 70.9 g/mol. In general, higher molar mass correlates with stronger London dispersion forces.

Solution

HBr has the highest boiling point because it has dipole-dipole forces, which are stronger than the London dispersion forces present in Kr and Cl2. b. highest freezing point: H2O, NaCl, or HF

Consider Intermolecular Forces

H2O has hydrogen bonding, ionic bonding is present in NaCl, and HF has hydrogen bonding. Hydrogen bonding is a special type of dipole-dipole interaction, and ionic forces are even stronger than hydrogen bonds.

Solution

NaCl has the highest freezing point, due to strong ionic bonding between its ions. c. lowest vapor pressure at 25°C: Cl2, Br2, or I2

Consider Intermolecular Forces

All three molecules are non-polar, so they have London dispersion forces.

Compare Molecular Weights

Cl2 has a molar mass of 70.9 g/mol, Br2 has a molar mass of 159.8 g/mol, and I2 has a molar mass of 253.8 g/mol. Higher molar mass correlates with stronger London dispersion forces.

Solution

I2 has the lowest vapor pressure at 25°C because its stronger London dispersion forces require more energy to be exerted before it vaporizes. d. lowest freezing point: N2, CO, or CO2

Consider Intermolecular Forces

All of these molecules are nonpolar and have the London Dispersion forces.

Compare Molecular Weights

N2 has a molar mass of 28 g/mol, CO has a molar mass of 28 g/mol, and CO2 has a molar mass of 44 g/mol.

Solution

N2 and CO have the lowest freezing points due to their lower molar masses and therefore weaker London dispersion forces. e. lowest boiling point: CH4, CH3CH3, or CH3CH2CH3

Consider Intermolecular Forces

All of these molecules are nonpolar and have the London Dispersion forces.

Compare Molecular Weights

CH4 has a molar mass of 16 g/mol, CH3CH3 has a molar mass of 30 g/mol, and CH3CH2CH3 has a molar mass of 44 g/mol.

Solution

CH4 has the lowest boiling point due to its lower molar mass and therefore weaker London dispersion forces. f. highest boiling point: HF, HCl, or HBr

Consider Intermolecular Forces

All three molecules are polar and form hydrogen bonds.

Compare Molecular Weights

HF has a molar mass of 20 g/mol, HCl has a molar mass of 36.5 g/mol, and HBr has a molar mass of 80 g/mol.

Solution

HBr has the highest boiling point due to its greater molar mass and thus stronger hydrogen bonding. g. lowest vapor pressure at 25°C: CH3CH2CH3, CH3CCH3, or CH3CH2CH2OH

Consider Intermolecular Forces

CH3CH2CH3 and CH3CCH3 have London dispersion forces, while CH3CH2CH2OH has hydrogen bonding due to the -OH group.

Solution

CH3CH2CH2OH has the lowest vapor pressure at 25°C due to the stronger hydrogen bonding compared to the London dispersion forces in other molecules.

Key concepts

Boiling Point

Boiling point is a fundamental property that indicates the temperature at which a liquid turns into vapor. This transformation depends significantly on the strength of intermolecular forces. Strong intermolecular forces lead to higher boiling points because more energy is required to overcome these forces and change the liquid into a gas.
What's fascinating is the impact of different types of intermolecular forces:

• Dipole-Dipole Forces: Present in polar molecules like HBr, these forces are generally stronger than London dispersion forces because they result from the attraction between oppositely charged ends of molecules.
• Hydrogen Bonds: A special type of dipole-dipole interaction, hydrogen bonds are stronger than regular dipole-dipole forces. They occur in molecules where hydrogen is bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen.
• London Dispersion Forces: The weakest of the bunch, these forces are present in all molecules (polar and nonpolar) but dominate in nonpolar molecules like Kr and Cl₂.

The combined effects of these forces and molecular weight determine the boiling point. For example, HBr has a higher boiling point than Kr and Cl₂ because the dipole-dipole interactions in HBr are stronger than the London dispersion forces in Kr and Cl₂.

Freezing Point

Freezing point refers to the temperature at which a liquid becomes a solid. Just like boiling point, freezing point is influenced by intermolecular forces. Substances with strong intermolecular forces usually exhibit higher freezing points because they require more energy removal to solidify.
Here's a closer look:

• Hydrogen Bonds: Strong bonds present in substances like water (H₂O), they cause water to freeze at 0°C, a relatively high freezing point.
• Ionic Bonds: As seen in NaCl, ionic bonds are even stronger than hydrogen bonds, resulting in a much higher freezing point.
• Dipole-Dipole Forces: Seen in HF, these forces are weaker than ionic bonds, resulting in a lower freezing point compared to NaCl.

In the original exercise, NaCl was identified with the highest freezing point due to strong ionic bonds.

Vapor Pressure

Vapor pressure is an important concept that describes how readily a liquid evaporates. It is influenced by intermolecular forces; weaker forces result in higher vapor pressures because molecules escape more easily into a gas phase. Conversely, strong forces result in low vapor pressure.
Consider these factors:

• London Dispersion Forces: Found in nonpolar molecules such as Cl₂, Br₂, and I₂, where increased molecular weight and surface area enhance these forces, thereby lowering vapor pressure.
• Hydrogen Bonds: Molecules like CH₃CH₂CH₂OH, which have hydrogen bonds, will have much lower vapor pressure compared to similar molecules without hydrogen bonds.

In particular, I₂ has the lower vapor pressure among Cl₂, Br₂, and I₂ because its large structure creates more significant London dispersion forces.

Molecular Weight

Molecular weight, the sum of the atomic weights in a molecule, plays a significant role in dictating physical properties like boiling and freezing points. Generally, the heavier the molecule, the stronger the London dispersion forces it can exert. This leads to variations in physical behaviour.
See how this works:

• Heavy Molecules: They tend to have stronger dispersion forces. This is illustrated by bromine (Br₂) and iodine (I₂), where higher molecular weights contribute to lower vapor pressure and higher boiling and freezing points.
• Light Molecules: Such as methane (CH₄), with weaker dispersion forces, tend to evaporate more easily and possess lower boiling points.

In exercises involving molecular weight, it regularly collaborates with the type of intermolecular force to determine the property in question, whether it be boiling, freezing, or vapor pressure.