General chemistryLesson 3 of 6 in the course Liquids and Solids

Vapor Pressure, Phase-Change Energies, and Heating Curves

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

This lesson introduces the exchange of molecules between a liquid and its vapor, and explains how this exchange creates vapor pressure.

Vaporization is the transition from liquid to gas. Molecules in a liquid have a range of kinetic energies; some at the surface have enough energy to escape intermolecular attractions and enter the gas phase.

Condensation is the reverse transition, from gas to liquid. Gas molecules move randomly and may collide with the liquid surface. Some are captured by the liquid and rejoin it.

All flashcards

What is dynamic equilibrium in a liquid–vapor system?
Vaporization and condensation continue at equal rates, so the overall amounts of liquid and vapor stay constant.
What is the normal boiling point?
The temperature at which a liquid's vapor pressure equals 1 atm (101.3 kPa).
What does the Clausius–Clapeyron relationship relate?
A substance's vapor pressure and absolute temperature, allowing vapor-pressure changes to be estimated.
What is enthalpy of fusion?
The energy required to convert one mole of a solid into liquid.
What does a flat region on a heating curve represent?
A phase transition during which heat changes particle arrangement rather than temperature.

Quiz questions

  1. At what condition does a liquid boil?Answer: When its equilibrium vapor pressure equals the external pressure.
  2. What temperature scale must be used in the two-point Clausius–Clapeyron equation?Answer: Kelvin, because the equation uses absolute temperature.
  3. Which equation calculates heat for a phase change when the amount and molar enthalpy are known?Answer: q=nΔHq=n\Delta H
  4. What does a temperature plateau indicate on a heating curve?Answer: A phase change is occurring while the temperature remains constant.
  5. How does condensation enthalpy compare with vaporization enthalpy for the reverse process?Answer: It has the same magnitude and the opposite sign.

Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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