Heat of Fusion and Vaporisation Calculator
Energy for a phase change, where temperature does not move.
Work out Heat of Fusion and Vaporisation. Energy for a phase change, where temperature does not move. Free, with no account and nothing to download.
Water: fusion 6.01, vaporisation 40.66. Ethanol: 4.9 and 38.56.
Energy for the phase change
33.361 kJ
5.5509 moles at 6.01 kJ/mol
There is no temperature term, because a phase change happens at constant temperature. All of the energy goes into overcoming intermolecular forces rather than raising kinetic energy, which is why a pan of boiling water stays at 100 °C no matter how hard you heat it, and why an ice bath holds 0 °C until the last ice melts. Vaporisation costs far more than fusion — for water, 40.66 against 6.01 kJ/mol, nearly seven times — because melting only loosens the lattice while boiling separates the molecules entirely. That large enthalpy of vaporisation is why sweating cools so effectively, and why steam burns are far worse than boiling water burns at the same temperature.
How the Heat of Fusion and Vaporisation Calculator works
Enter a mass, molar mass and the enthalpy of fusion or vaporisation to get the energy required. There is no temperature term, because a phase change happens at constant temperature — that is exactly why the calculation is separate from specific heat.
Also known as: latent heat calculator · heat of vaporization calculator · energy to melt ice calculator · phase change energy calculator
Frequently asked questions
How do I calculate the energy to melt something?
Convert mass to moles, then multiply by the enthalpy of fusion. Melting 100 g of ice needs 5.55 mol × 6.01 kJ/mol, or about 33.4 kJ.
Why is there no temperature in the formula?
Because the temperature does not change. All the energy goes into breaking intermolecular forces rather than raising kinetic energy, which is why boiling water stays at 100 °C however hard you heat it.
Why does boiling take so much more energy than melting?
Melting only loosens the lattice enough for molecules to slide past each other. Boiling separates them entirely. For water that is 40.66 against 6.01 kJ/mol — nearly seven times as much.
Why do steam burns hurt more than boiling water burns?
Because steam condensing on skin releases its full enthalpy of vaporisation, about 2,260 J per gram, before the water even starts to cool. Boiling water at the same temperature delivers only the cooling.
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