Intermediate Topic 264 of 1102 States of Matter & Solutions
Enthalpy of Vaporization
Heat required to convert one mole of liquid to vapor at constant temperature.
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In-depth explanation of Enthalpy of Vaporization
The enthalpy of vaporization is the heat energy required to convert one mole of liquid into vapour at its boiling point, at constant temperature and pressure. This energy is used to overcome the intermolecular forces holding the liquid together rather than to raise its temperature. Liquids with strong hydrogen bonding have large enthalpies of vaporization. The value is always positive for vaporization and is much greater than the enthalpy of fusion for the same substance. It is used in calculating the heat needed for boiling and the cooling produced by evaporation.
Going deeper: Enthalpy of Vaporization
Key ideas: Heat required q = n ΔHvap, with ΔHvap positive for vaporization and equal and opposite for condensation. Water: 40.7 kJ/mol or about 2260 J/g, which is many times the energy needed to warm water from 0 to 100 C at 4.18 J/(g K) times 100 K = 418 J/g. Decreases with temperature and reaches zero at the critical point.
Common mistakes and connections: A typical error is applying ΔHvap using the mass in grams without dividing by the molar mass. Another is forgetting the sign when condensing, or the units: kJ per mol vs J per g. Steam burns are worse than boiling water burns because condensation releases this energy. Related to heating curves, Clausius-Clapeyron and Trouton's rule.
Visualization of Enthalpy of Vaporization
Hvap is heat absorbed to vaporize one mole at the boiling point; water's 40.7 kJ/mol reflects strong hydrogen bonding. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Water has an enthalpy of vaporization of 40.7 kJ/mol at 100°C, whereas methane has only about 8.2 kJ/mol at its boiling point.
Frequently asked questions about Enthalpy of Vaporization
What is Enthalpy of Vaporization?
Heat required to convert one mole of liquid to vapor at constant temperature.
Is Enthalpy of Vaporization a beginner, intermediate or advanced topic?
Enthalpy of Vaporization is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Enthalpy of Vaporization?
Study these earlier topics in States of Matter & Solutions first: Capillary Action, Cohesion and Adhesion, Meniscus.
What comes after Enthalpy of Vaporization?
The next topics in States of Matter & Solutions are Enthalpy of Fusion and Heating Curve.
Can I practise Enthalpy of Vaporization with quizzes?
Yes. The interactive lesson includes a self-check question on Enthalpy of Vaporization, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Enthalpy of Vaporization fits in States of Matter & Solutions
Enthalpy of Vaporization is topic 45 of 109 in States of Matter & Solutions.