Intermediate Topic 506 of 1102 Kinetics & Equilibrium
Heterogeneous Equilibrium Vapor
Vapor pressure of a liquid in a closed container reflects dynamic phase equilibrium.
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In-depth explanation of Heterogeneous Equilibrium Vapor
When a liquid is placed in a closed container, evaporation and condensation eventually reach a dynamic equilibrium in which the rates of the two processes are equal. The pressure exerted by the vapour in equilibrium with the liquid is called the vapour pressure, and it depends only on the temperature and the nature of the liquid. It does not depend on the volume of the liquid or the size of the container, provided some liquid remains in contact with its vapour. Vapour pressure rises with temperature because a larger fraction of molecules have enough energy to escape the surface, and the boiling point is the temperature at which vapour pressure equals the external pressure.
Going deeper: Heterogeneous Equilibrium Vapor
Key ideas: Clausius-Clapeyron: ln(P2/P1) = -(delta Hvap/R)(1/T2 - 1/T1), with delta Hvap in J per mol. It gives vapour pressure at other temperatures or delta Hvap from a plot of ln P against 1/T. K for evaporation of a liquid is just P. Normal boiling point is where P = 1 atm, and at higher altitude water boils lower since external pressure is lower.
Common mistakes and connections: Students think vapour pressure depends on the amount of liquid or the container volume, or that a liquid stops evaporating at equilibrium. Another mistake is confusing the vapour pressure with atmospheric pressure or relative humidity. Adding solute lowers vapour pressure, linking to Raoult's law. The same ln versus 1/T form appears in the van't Hoff and Arrhenius equations.
Visualization of Heterogeneous Equilibrium Vapor
Vapour pressure of water rises steeply with temperature and equals 1 atm at 373 K, the boiling point; it is independent of the amount of liquid. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The vapour pressure of water at 25 degrees C is about 23.8 mmHg, so a closed flask of water at that temperature contains vapour at that pressure.
Frequently asked questions about Heterogeneous Equilibrium Vapor
What is Heterogeneous Equilibrium Vapor?
Vapor pressure of a liquid in a closed container reflects dynamic phase equilibrium.
Is Heterogeneous Equilibrium Vapor a beginner, intermediate or advanced topic?
Heterogeneous Equilibrium Vapor is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Heterogeneous Equilibrium Vapor?
Study these earlier topics in Kinetics & Equilibrium first: Reaction Coordinate Diagram, Equilibrium in Biology, Buffer Equilibria.
What comes after Heterogeneous Equilibrium Vapor?
The next topics in Kinetics & Equilibrium are Partition Coefficient and Distribution Law.
Can I practise Heterogeneous Equilibrium Vapor with quizzes?
Yes. The interactive lesson includes a self-check question on Heterogeneous Equilibrium Vapor, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Heterogeneous Equilibrium Vapor fits in Kinetics & Equilibrium
Heterogeneous Equilibrium Vapor is topic 87 of 91 in Kinetics & Equilibrium.