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  • Home / Ask Me Stuff
  • Lessons
    • Thermodynamics >
      • Thermochemistry Worksheet + Answers
      • First Law of Thermodynamics
      • Pressure-Volume Work
      • Enthalpy
      • Hess' Law
      • Enthalpy of Formation
      • Heat Capacity
      • Calorimetry
      • Entropy
      • Third Law of Thermodynamics
      • Spontaneity: Gibbs Free Energy
      • Second Law of Thermodynamics
      • Spontaneity at Different Temperatures
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      • Redox Reactions
      • Introduction to Half-Reactions
      • Calculating Oxidation Number
      • Has a Redox Reaction Occurred?
      • How to Balance Redox Reactions (Acidic Solution)
      • How to Balance Redox Reactions (Basic Solution)
      • Galvanic Cells
      • Standard Reduction Potentials
      • Electrolytic Cells
      • Nernst Equation
    • Kinetics >
      • Introduction
      • Relative Rates of Reaction
      • Rate Laws
      • Zero-Order Reactions
      • First-Order Reactions
      • Second-Order Reactions
      • Half-Life Expressions
      • Arrhenius Equation
      • How Long will it Take to Decay?
      • What Order is this Reaction?
      • Find the Rate Constant
      • Find the Activation Energy
    • Gases >
      • Pressure, Volume, Temperature
      • Ideal Gas Law
      • Density of Gases
      • Ideal Gas Law and Changes in P, V, T
      • Kinetic Molecular Theory
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      • Partial Pressures
      • Kinetic Energy and Temperature
    • Equilibrium >
      • Writing Equilibrium Expressions
      • Le Chatelier's Principle
    • Acids and Bases >
      • Acids and Bases Worksheet + Answers
      • Arrhenius vs Bronsted-Lowry vs Lewis Acids
      • Solve Titration Questions
    • Intermolecular Forces >
      • Phase Diagrams
      • Phase Changes
      • Intermolecular Forces
      • Effects of Intermolecular Forces
      • Ranking by Boiling/Melting Point
      • Clausius-Clapeyron Equation
      • Heating Curves
    • Solids >
      • Ionic/Metallic/Covalent
      • Symmetry of Solids
      • Simple Cubic, fcc and bcc
      • How to Find Edge Length
    • Organic Reactions >
      • Br2 + Alkene (Adding across a double bond)
      • HCl + Alkene (Adding across a double bond)
      • Reaction of OH with Alkyl Halide
      • What is Regioselectivity?
    • Moles and Mass >
      • Average Atomic Mass
      • Solve for Isotopic Abundance
      • Limiting Reagents
      • Percent Yield
      • Actual Yield and Percentage Yield
      • Percent Composition
    • Atomic Structure >
      • What's in an Atom?
      • Quantum Numbers
      • Pauli, Aufbau, Hund
      • Light: E h ν λ
      • Energy Levels of Hydrogen
      • Energy Levels of Non-Hydrogen Atoms
    • Organic Naming >
      • Naming Organic Molecules
      • How to Name Amines
      • How to Name Amides
  • Privacy Policy
  • Lewis Structures

Clausius-Clapeyron Equation


This is the equation that tells you what vapour pressure a liquid has at a certain temperature.  It involves the enthalpy of vaporization, ΔH(vap) and a random constant that you'll probably never use (we call it "A"):
Picture
The reason you'll "probably never use it" is that most questions give you the ΔH(vap), and the vapour pressure at a certain temperature.  Then, they'll ask you to predict the vapour pressure at a second temperature.  To do this, you need to have a formula that relates two pressure and two temperatures.  Here it is:
Picture
Given the enthalpy of vaporization is 41 kJ/mol and the Vapour Pressure at 373 K is 101.3 kPa, what is the vapour pressure at 298?  Find out in the video below.

Like math?

Maybe you'll care how the second equation comes from the first.

1. Divide one Clausius-Clapeyron Equation by a second one, as shown in the first step below.
2. Combine the exponents (remember exponent laws?)
3. Take the "ln" of both sides to "undo" the "e"
4. Factor out ΔH(vap) / R
5. Re-order the last two terms to clean up the expression.
Picture
Created by Nathan Oldridge (aka ChemistNATE).  All lessons here are Public Domain.