Advanced Topic 258 of 1102 States of Matter & Solutions
Clausius-Clapeyron Equation
Relates vapor pressure to temperature through the enthalpy of vaporization.
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In-depth explanation of Clausius-Clapeyron Equation
The Clausius-Clapeyron equation relates the vapour pressure of a liquid to temperature through the enthalpy of vaporization. In its two-point form it is ln(P2/P1) = (ΔHvap/R)(1/T1 - 1/T2), with temperatures in kelvin. A plot of ln P against 1/T gives a straight line with slope -ΔHvap/R, so the enthalpy of vaporization can be found from experimental data. The equation can also predict the vapour pressure at another temperature or the boiling point at another pressure. It is derived from thermodynamic principles and is accurate when the vapour behaves as an ideal gas.
Going deeper: Clausius-Clapeyron Equation
Key ideas: For calculations, convert temperatures to kelvin, use R = 8.314 J/(mol K) with ΔHvap in J, not kJ. Steps: ln(P2/P1) = -(ΔHvap/R)(1/T2 - 1/T1). It assumes constant ΔHvap and ideal vapour behaviour over the range. It can be applied to sublimation using ΔHsub.
Common mistakes and connections: The most common errors are leaving ΔH in kJ with R in J, using Celsius, and swapping the sign. A straight line of ln P versus 1/T has a negative slope. Accuracy declines over wide temperature ranges or near the critical point. Related to Antoine equation, vapour pressure and boiling at altitude, and phase diagram slopes.
Visualization of Clausius-Clapeyron Equation
Clausius-Clapeyron: ln P against 1/T is a straight line with slope -Hvap/R; for water the slope gives about 40.7 kJ/mol. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Using the boiling point of water at 1 atm and ΔHvap = 40.7 kJ/mol, the equation gives a vapour pressure of about 0.13 atm at 50°C, close to the measured value of 12.3 kPa.
Frequently asked questions about Clausius-Clapeyron Equation
What is Clausius-Clapeyron Equation?
Relates vapor pressure to temperature through the enthalpy of vaporization.
Is Clausius-Clapeyron Equation a beginner, intermediate or advanced topic?
Clausius-Clapeyron Equation is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Clausius-Clapeyron Equation?
Study these earlier topics in States of Matter & Solutions first: Evaporation, Boiling Point, Normal Boiling Point.
What comes after Clausius-Clapeyron Equation?
The next topics in States of Matter & Solutions are Viscosity and Surface Tension.
Can I practise Clausius-Clapeyron Equation with quizzes?
Yes. The interactive lesson includes a self-check question on Clausius-Clapeyron Equation, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Clausius-Clapeyron Equation fits in States of Matter & Solutions
Clausius-Clapeyron Equation is topic 39 of 109 in States of Matter & Solutions.