2.3 Vapor Pressure, IMFs, and Boiling Point

Main Ideas

Vapor pressure is the pressure exerted by gas molecules above a liquid in a closed container. The gas forms when some liquid molecules escape from the surface.

At the same temperature, a liquid with more molecules in the gas phase above it has a higher vapor pressure. A liquid with fewer gas-phase molecules has a lower vapor pressure.

How many molecules escape depends on the strength of the intermolecular forces (IMFs):

• Weak IMFs: Molecules escape more easily, producing more vapor and a higher vapor pressure.
• Strong IMFs: Molecules are held together more tightly, so fewer escape and the vapor pressure is lower.

IMF strength is also related to boiling point:

• Liquids with weaker IMFs boil at a lower temperature because their molecules escape more easily.
• Liquids with stronger IMFs boil at a higher temperature because more heating is needed to overcome the attractions.

The video compares acetone (liquid A) and water (liquid B). Acetone is shown with more vapor above it, a higher vapor pressure, weaker intermolecular attractions, and a lower boiling point than water.

Suggested Approach for Related Questions

1. Compare the amount of vapor above each liquid. More vapor indicates higher vapor pressure.
2. Use vapor pressure to infer how readily molecules escape the liquid.
3. Relate ease of escape to IMF strength: easier escape indicates weaker attractions; harder escape indicates stronger attractions.
4. Connect IMF strength to boiling point: weaker attractions generally mean a lower boiling point, while stronger attractions mean a higher one.
5. Draw a diagram showing the liquid, the vapor above it, and the attractions between liquid molecules.

Speaker and Source

Ms. Long is the instructor and narrator.