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Chapter 12: Problem 90

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

Short Answer

Expert verified
Liquid hydrogen iodide (HI) has a higher vapor pressure than liquid hydrogen fluoride (HF) at -35\(掳C\) because the intermolecular forces in HI (London dispersion forces) are weaker than those in HF (hydrogen bonds), making it easier for HI molecules to escape to the gas phase.

Step by step solution

01

Understanding Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. It is a measure of the tendency of molecules and atoms to escape from a liquid or solid.
02

Intermolecular Forces and Vapor Pressure

The strength of intermolecular forces in a substance influences its vapor pressure. Weaker intermolecular forces typically lead to a higher vapor pressure, since the molecules have less attraction to each other and more can escape into the gas phase.
03

Characteristics of HI and HF

In the case of HI, it forms London dispersion forces which are weaker than the hydrogen bonds formed by HF. Therefore, it is easier for HI molecules to break free from each other and become gas, hence leading to a higher vapor pressure.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Intermolecular Forces
Intermolecular forces are the forces that act between molecules, influencing their physical properties such as boiling point, melting point, and vapor pressure. These forces are generally weaker than the forces that hold atoms together within a molecule, known as covalent bonds.
While there are different types of intermolecular forces, they all play a role in determining how easily molecules can escape into the vapor phase.
  • Types of Intermolecular Forces: The main types are London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
  • Weaker Forces and Vapor Pressure: Generally, weaker intermolecular forces lead to higher vapor pressures. This occurs because molecules with weak attractions can more easily escape into the gas phase.
The interplay between these forces can affect the physical behavior of liquids and solids, such as how HI and HF behave differently under similar conditions.
London Dispersion Forces
London dispersion forces are the weakest type of intermolecular force, but they are present in all molecules, whether polar or nonpolar. These forces arise from temporary fluctuations in electron density, which create temporary dipoles in the molecules.
Despite their temporary nature, they can have significant effects, especially in molecules with large electron clouds.
  • Molecular Size and Electron Clouds: Molecules with larger electron clouds exhibit stronger London dispersion forces because they have more electrons that can move around and create temporary dipoles.
  • Impact on Molecules Like HI: For molecules like HI, which are relatively larger and more polarizable because of their heavy iodine atoms, London dispersion forces become the predominant intermolecular force.
Because they are weak, substances like HI, with only London dispersion forces, tend to have higher vapor pressures compared to molecules involved in stronger interactions, such as hydrogen bonding.
Hydrogen Bonding
Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is directly bonded to highly electronegative atoms like fluorine, oxygen, or nitrogen. This results in a significant attraction between molecules, which profoundly affects their properties.
In substances like HF, hydrogen bonds play a crucial role in defining their physical characteristics.
  • Formation of Hydrogen Bonds: Hydrogen forms a bond with electronegative atoms, which makes the hydrogen atom partially positive, allowing it to attract nearby electronegative atoms.
  • Effect on Physical Properties: These bonds make it more difficult for molecules to escape into the vapor phase, thereby lowering vapor pressure compared to substances with only London dispersion forces.
Because of the strong nature of hydrogen bonds, molecules like HF experience much stronger attractions, leading to lower vapor pressure compared to HI at the same temperature.

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Most popular questions from this chapter

The molar heats of fusion and sublimation of molecular iodine are \(15.27 \mathrm{~kJ} / \mathrm{mol}\) and \(62.30 \mathrm{~kJ} / \mathrm{mol}\), respectively. Estimate the molar heat of vaporization of liquid iodine.

Define these terms: crystalline solid, lattice point, unit cell, coordination number.

Explain in terms of intermolecular forces why (a) \(\mathrm{NH}_{3}\) has a higher boiling point than \(\mathrm{CH}_{4}\) and \((\mathrm{b}) \mathrm{KCl}\) has a higher melting point than \(\mathrm{I}_{2}\).

Define these terms: (a) molar heat of vaporization, (b) molar heat of fusion, (c) molar heat of sublimation. What are their units?

If you lived in Alaska, state which of these natural gases you would keep in an outdoor storage tank in winter and explain why: methane \(\left(\mathrm{CH}_{4}\right),\) propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right),\) or butane \(\left(\mathrm{C}_{4} \mathrm{H}_{10}\right)\)
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