Question

At \(-35^{\circ} \mathrm{C}\), liquid HI has a higher vapor pressure than liquid HF. Explain.

Short answer

HI has weaker intermolecular forces, leading to a higher vapor pressure compared to HF, which has strong hydrogen bonds.

Step-by-step solution

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. It depends on the intermolecular forces present in the liquid.

Examining the Molecules

HI and HF are both hydrogen halides, but they have different intermolecular forces. HI is a larger molecule compared to HF, meaning its dispersion forces are larger. HF has strong hydrogen bonds due to the hydrogen-fluoride connection.

Relating Intermolecular Forces to Vapor Pressure

Stronger intermolecular forces lead to lower vapor pressure because the molecules require more energy to escape into the vapor phase. HF has strong hydrogen bonds, resulting in a lower vapor pressure as these bonds restrain the molecules.

Comparing HI and HF

HI's intermolecular forces are primarily van der Waals (dispersion forces), which are weaker than HF's hydrogen bonds. Thus, HI molecules can escape into the vapor phase more easily, leading to a higher vapor pressure.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between molecules. They are crucial in determining many properties of substances, including boiling points, melting points, and vapor pressures. Intermolecular forces can vary in strength:

• Strong forces: These include ionic interactions and hydrogen bonding, leading to molecules being tightly held together.
• Weak forces: These include dispersion forces and dipole-dipole interactions, leading to looser molecular connections.

Vapor pressure, the pressure exerted by a vapor in equilibrium with its liquid at a given temperature, is heavily influenced by these forces. The stronger the intermolecular forces, the more energy is required for the molecules to escape the liquid phase and enter the vapor phase, resulting in a lower vapor pressure. Conversely, weaker intermolecular forces allow molecules to escape more readily, leading to a higher vapor pressure.
Understanding these forces helps explain why different substances have varying vapor pressures under the same conditions.

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that is exceptionally strong compared to other dipole-dipole interactions. It occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. In the case of HF (hydrogen fluoride), hydrogen bonds are formed because of the significant electronegativity difference between hydrogen and fluorine. These bonds are responsible for HF exhibiting properties like higher boiling points and lower vapor pressures compared to similar molecules lacking hydrogen bonds.

• Strength: These bonds are stronger than typical dipole-dipole interactions but weaker than covalent or ionic bonds.
• Effect on properties: Substances with hydrogen bonding often have higher surface tensions, viscosity, and boiling points than those without.

In HF, the strong hydrogen bonds keep the molecules strongly connected, meaning more energy is needed for them to enter the vapor phase. This results in a lower vapor pressure for HF when compared with substances that have weaker intermolecular forces.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest of the intermolecular forces. They arise due to temporary shifts in electron density within atoms or molecules, which induce instantaneous and temporary dipoles. These forces are present in all molecules, regardless of their polar or nonpolar nature, but they are the only intermolecular force acting between nonpolar molecules. Dispersion forces increase with:

• Increasing molecular size: Larger molecules have more electron clouds, leading to stronger temporary polarizations.
• Greater surface area: Molecules with more surface contact can have stronger interactions through dispersion forces.

For HI (hydrogen iodide), dispersion forces are more prominent due to its larger size compared to HF. These forces are relatively weaker than hydrogen bonds but sufficient to influence the properties of molecules like HI. Because of these weaker forces, HI molecules can enter the vapor phase more readily than HF, resulting in a higher vapor pressure under the same conditions. Understanding dispersion forces helps explain why substances like HI can have different properties compared to others like HF, which have different types of intermolecular interactions.