Question

Consider the molecules HF and HCl. The electronegativities of the atoms involved are \(\mathrm{H}, 2.1 ; \mathrm{Cl}, 3.0 ; \mathrm{F}, 4.0 .\) (a) Which of the two molecules is more polar? Explain your answer. (b) For which substance are the dipole-dipole attractions between the molecules stronger? Explain your answer. (c) Which gas would, upon cooling, liquefy first? Explain your answer. Also, indicate which compound would have the higher boiling point based on your answer. (d) Do both molecules also experience London forces of attraction? If yes, which would have the greater London forces?

Short answer

(a) HF is more polar than HCl due to a larger electronegativity difference (1.9 for HF versus 0.9 for HCl). (b) HF has stronger dipole-dipole attractions than HCl due to its higher polarity. (c) HF will liquefy first upon cooling and has a higher boiling point due to stronger dipole-dipole attractions. (d) Both HF and HCl experience London forces, but HCl has greater London forces due to its larger electron cloud.

Step-by-step solution

Determine the polarity of HF and HCl molecules

To determine the polarity of these molecules, we can find the electronegativity difference between the atoms involved. A larger difference indicates a more polar molecule. For HF: \(\Delta \text{EN} = 4.0 - 2.1 = 1.9\) For HCl: \(\Delta \text{EN} = 3.0 - 2.1 = 0.9\) (a) Since the electronegativity difference is larger in the HF molecules (1.9) than in the HCl molecules (0.9), the HF molecule is more polar.

Determine which substance has stronger dipole-dipole attractions

(b) Dipole-dipole attractions are stronger between more polar molecules. Therefore, HF has stronger dipole-dipole attractions between the molecules than HCl, due to its higher polarity.

Determine which gas would liquefy first upon cooling

(c) A gas will liquefy when the intermolecular forces, such as dipole-dipole attractions, become strong enough to overcome the kinetic energy of the molecules. Since HF has stronger dipole-dipole attractions than HCl, HF will liquefy first upon cooling.

Determine which compound has a higher boiling point

A higher boiling point indicates stronger intermolecular forces between the molecules. Since HF has stronger dipole-dipole attractions, it will have a higher boiling point than HCl.

Determine if both molecules experience London forces

(d) London forces, or dispersion forces, are experienced by all molecules, regardless of their polarity. Thus, both HF and HCl experience London forces of attraction.

Determine which molecule has greater London forces

Although both molecules experience London forces, the molecular size and the electron cloud play a role in the strength of these forces. Larger molecules with more electrons have greater London forces. In this case, the Cl atom is larger than the F atom, so the HCl molecule has a larger electron cloud and possesses greater London forces compared to the HF molecule.

Key concepts

Electronegativity Difference

Electronegativity refers to the ability of an atom to attract bonding electrons towards itself. When two atoms with different electronegativities form a bond, the electronegativity difference can lead to the development of a partial charge on each atom. This is because the more electronegative atom will pull the electron cloud closer to itself, creating a slight negative charge, while the less electronegative atom becomes slightly positive.

The electronegativity difference between hydrogen (H) and fluorine (F) is 1.9, which is significantly higher than the difference between hydrogen (H) and chlorine (Cl), 0.9. This greater difference in the case of HF leads to a stronger partial charge development, hence a more polar molecule compared to HCl.

Dipole-Dipole Attractions

Dipole-dipole attractions are intermolecular forces that 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 degree of polarity; more polar molecules have stronger dipole-dipole attractions.

As HF is more polar than HCl due to a larger electronegativity difference, the dipole-dipole attractions between HF molecules are consequently stronger. This increased attraction leads to a higher resistance to changes in physical state, such as melting or boiling.

Intermolecular Forces

Intermolecular forces include various types of attractions between molecules, such as dipole-dipole attractions, hydrogen bonding, and London dispersion forces. These forces are responsible for the physical properties of substances, like boiling point and melting point.

HF's stronger dipole-dipole attractions, which are a type of intermolecular force, mean that HF will generally have stronger overall intermolecular forces than HCl. As a result, HF will liquefy at a higher temperature than HCl, demonstrating that substances with stronger intermolecular forces require more energy, in the form of heat, to overcome these forces and transition into a gaseous state.

London Dispersion Forces

London dispersion forces, also known as Van der Waals forces, are a type of weak intermolecular force that occurs in all molecules, regardless of polarity. They arise due to temporary fluctuations in the electron distribution within molecules, which create instantaneous dipoles that induce corresponding dipoles in neighboring molecules.

The strength of London forces is influenced by the size of the electron cloud, which is generally larger in molecules with more electrons or larger molar mass. Despite HF being more polar, HCl experiences stronger London dispersion forces due to the larger size and greater electron cloud of the chlorine atom compared to the fluorine atom.

Boiling Point

The boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric pressure, causing the liquid to turn into vapor. A higher boiling point suggests stronger intermolecular forces within a substance.

Given that HF has stronger dipole-dipole interactions due to a greater electronegativity difference, it has a higher boiling point than HCl. In essence, more energy is required to break the bonds between HF molecules compared to those between HCl molecules when transitioning from the liquid phase to the gas phase.