Question

In each of the following groups of substances, pick the one that has the given property. Justify your answer. a. highest boiling point: HBr, Kr, or \(\mathrm{Cl}_{2}\) b. highest freezing point: \(\mathrm{H}_{2} \mathrm{O}, \mathrm{NaCl}\) , or 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 b. NaCl c. I2 d. N2 e. CH4 f. HF g. CH3CH2CH2OH

Step-by-step solution

a. Highest Boiling Point

To determine the highest boiling point, we consider the strength of the intermolecular forces. HBr has dipole-dipole forces, Kr has London Dispersion forces, and Cl2 also have London Dispersion Forces. However, HBr is a polar molecule with stronger dipole-dipole forces so it will have the highest boiling point among the three substances. Answer: HBr

b. Highest Freezing Point

When looking at the highest freezing point, we need to consider the type of bonding and the intermolecular forces. H2O has hydrogen bonding, NaCl has ionic bonding, and HF has hydrogen bonding. NaCl has the strongest ionic bond which leads to a much higher freezing point compared to H2O and HF. Answer: NaCl

c. Lowest Vapor Pressure at 25°C

The substance with the lowest vapor pressure at 25°C is expected to have stronger intermolecular forces. Cl2, Br2, and I2 are all nonpolar molecules having London Dispersion Forces. As molecular weight increases, the London Dispersion Forces also increase. Hence, I2 has the highest molecular weight and thus the strongest London Dispersion forces, leading to the lowest vapor pressure. Answer: I2

d. Lowest Freezing Point

To determine the lowest freezing point, we'll consider intermolecular forces. N2, CO, and CO2 are all nonpolar molecules with London Dispersion forces, but CO and CO2 also have dipole-dipole forces. As molecular weight increases, London Dispersion forces increase. N2 has the lowest molecular weight, so it will have the weakest intermolecular forces and the lowest freezing point. Answer: N2

e. Lowest Boiling Point

For the lowest boiling point, we'll again consider intermolecular forces. CH4, CH3CH3, and CH3CH2CH3 are all nonpolar molecules with London Dispersion forces. The molecule with the smallest molecular weight will have the weakest intermolecular forces and the lowest boiling point. CH4 has the smallest molecular weight among the three. Answer: CH4

f. Highest Boiling Point

HF, HCl, and HBr are polar molecules with dipole-dipole forces, and since HF also forms hydrogen bonds, it should have a higher boiling point than the other two. Answer: HF

g. Lowest Vapor Pressure at 25°C

CH3CH2CH3, CH3CCH3, and CH3CH2CH2OH can be compared considering their intermolecular forces. CH3CH2CH3 and CH3CCH3 have London Dispersion forces, while CH3CH2CH2OH has hydrogen bonding. Since hydrogen bonding is stronger than London Dispersion forces, CH3CH2CH2OH will have the lowest vapor pressure at 25°C. Answer: CH3CH2CH2OH

Key concepts

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. This process involves breaking the intermolecular forces holding the molecules together. Substances with stronger intermolecular forces will have higher boiling points because more energy is needed to break these forces. For example, polar molecules, such as hydrogen bromide (HBr), exhibit dipole-dipole forces, which are relatively strong. This makes the boiling point of HBr higher than that of krypton (Kr) or chlorine gas (\(\mathrm{Cl}_2\)) which both have weaker London Dispersion forces. Furthermore, when comparing hydrogen compounds like hydrogen fluoride (HF), hydrochloric acid (HCl), and hydrogen bromide (HBr), HF exhibits hydrogen bonding—a type of dipole-dipole interaction that is particularly strong. As a result, HF has a much higher boiling point compared to HCl and HBr.- Polar molecules with strong dipole-dipole forces = higher boiling points- Hydrogen bonding even further increases boiling pointsUnderstanding the connection between molecular structure and boiling point helps predict how substances behave under heat.

Freezing Point

The freezing point is the temperature at which a liquid becomes a solid. It is directly affected by the strength of intermolecular forces, similar to boiling points. When analyzing the freezing point, stronger intermolecular forces typically lead to higher freezing points. Ionic compounds like sodium chloride (NaCl) have strong ionic bonds which contribute to a high freezing point, much higher than hydrogen-bonded compounds like water (\(\mathrm{H}_2\mathrm{O}\)) and hydrogen fluoride (HF).Conversely, when compounds are mostly nonpolar, the presence of weak intermolecular forces such as London Dispersion forces results in lower freezing points. Nitrogen gas (\(\mathrm{N}_2\)) has low intermolecular forces, hence it will freeze at much lower temperatures compared to carbon monoxide (CO) or carbon dioxide (CO2), which have additional dipole interactions.- Strong intermolecular forces in ionic compounds = higher freezing points- Weak forces like in nonpolar molecules = lower freezing pointsThis illustrates the importance of considering both the type and strength of bonds when predicting freezing points.

Vapor Pressure

Vapor pressure refers to the pressure exerted by a vapor in equilibrium with its liquid or solid form in a closed container. Substances with strong intermolecular forces tend to have low vapor pressures because fewer molecules can escape from the liquid to the vapor phase. For example, iodine (I2), a substance with high molecular weight, has stronger London Dispersion forces than chlorine (Cl2) or bromine (Br2), resulting in the lowest vapor pressure among them.Additionally, the presence of hydrogen bonding in molecules like \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\) significantly lowers vapor pressure compared to compounds of similar molar mass that lack hydrogen bonding, such as propane (\(\text{CH}_3\text{CH}_2\text{CH}_3\)).- Stronger intermolecular forces = lower vapor pressure- Hydrogen bonding reduces vapor pressureThis understanding is essential for predicting how rapidly a substance might evaporate or condense in various conditions.

Dipole-Dipole Forces

Dipole-dipole forces arise in polar molecules where there is a significant difference in electronegativity between atoms. These forces occur as a result of the attraction between the positive end of one polar molecule and the negative end of another. Dipole-dipole interactions are generally stronger than London Dispersion forces due to their permanent nature. This means that polar molecules with these forces usually have higher melting and boiling points.For instance, in substances like HBr, the presence of dipole-dipole attractions leads to higher boiling points compared to nonpolar molecules like Kr or \(\mathrm{Cl}_2\). Moreover, these forces are an essential factor when determining phase changes and other properties of molecules.- Occur between polar molecules- Stronger than London Dispersion forces- Increase boiling and melting pointsKnowing the role of dipole-dipole forces helps in predicting and rationalizing the physical properties of molecules.

Hydrogen Bonding

Hydrogen bonding is a specific, often stronger type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to a small, highly electronegative atom like nitrogen, oxygen, or fluorine. This bond is one of the strongest types of intermolecular interactions, significantly affecting the physical properties of substances.In molecules like water (\(\mathrm{H}_2\mathrm{O}\)) and HF, hydrogen bonding is responsible for abnormally high boiling and freezing points. Similarly, the alcohol \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\) has a lower vapor pressure than other hydrocarbons due to these forces.- Requires hydrogen bonding to N, O, or F- Very strong, affecting boiling, melting points- Reduces vapor pressure significantlyHydrogen bonding's strength and directionality are key to understanding the behavior of many substances, particularly solvents and biologically relevant molecules.