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Chapter 11: Problem 95

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

Short Answer

Expert verified
At -35 degrees Celsius, HI has a higher vapor pressure than HF because it only experiences dipole-dipole and London dispersion forces, which are weaker than the hydrogen bonding in HF. Therefore, it requires less energy for HI to vaporize than HF, and more HI molecules escape into the vapor phase, creating a higher vapor pressure.

Step by step solution

01

Recognize the Types of Forces in Each Molecule

We first need to determine the types of forces acting in each molecule. For hydrogen fluoride HF, hydrogen bonding is present (a special type of dipole-dipole interactions), whereas for hydrogen iodide HI, we have simpler dipole-dipole forces and London dispersion forces.
02

Understand the Impact of Intermolecular Forces on Vapor Pressure

A molecule with higher intermolecular forces will require more energy to convert from a liquid phase to a gaseous phase (vaporize). Therefore, HF with hydrogen bonding would require more energy compared to HI and therefore have a lower vapor pressure.
03

Relate the Forces to the Given Temperature

At -35 degrees Celsius, it's easier for the HI molecules to overcome these weaker forces (dipole-dipole and London dispersion forces) and escape into the vapor phase than it is for the HF molecules to overcome their stronger (hydrogen bonding) forces. Hence, HI has a higher vapor pressure than HF at this temperature.

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