What Enthalpy of Fusion Means
Enthalpy of fusion, also called latent heat of fusion or heat of melting, is the energy needed to change a solid into a liquid at its melting point. During this phase change, the temperature stays constant while energy breaks or rearranges the solid structure.
In this formula, q is heat energy, m is mass, and Lf is the specific latent heat of fusion.
Important Unit Detail
A common mistake is mixing kilograms and J/g. If Lf is in J/g and mass is in kg, the mass must be converted to grams first.
q = mkg × LkJ/kg
For example, melting 1 kg of ice at 0°C requires about 334 kJ, not 0.334 kJ.
When Preheating Matters
The basic enthalpy of fusion equation assumes the material is already at its melting point. If the solid starts below its melting point, you first need sensible heat to warm it up.
qtotal = qpreheat + mLf
The calculator only includes this preheating part if you enter both the initial temperature and the solid specific heat capacity.
Preset Values Are Approximate
The preset values are educational reference values. Real values can vary slightly by source, purity, pressure, material phase, and measurement conditions. For professional engineering or laboratory work, use the data source specified by your course, lab, supplier, or standard.
| Substance | Approx. Lf | Approx. melting point |
|---|---|---|
| Ice / water | 334 kJ/kg | 0°C |
| Ethanol | 108 kJ/kg | −114.1°C |
| Benzene | 127.4 kJ/kg | 5.5°C |
| Aluminum | 399 kJ/kg | 660.3°C |
| Copper | 207 kJ/kg | 1084.6°C |
| Lead | 22.4 kJ/kg | 327.5°C |
| Mercury | 11.6 kJ/kg | −38.83°C |
| Gallium | 80.1 kJ/kg | 29.76°C |
| Sodium chloride | 520 kJ/kg | 801°C |
Example: Melting Ice
Suppose you want to melt 2 kg of ice that is already at 0°C.
m = 2 kg Lf = 334 kJ/kg q = mLf q = 2 × 334 q = 668 kJ
Example with Preheating
If 2 kg of ice starts at −10°C, you must first warm the ice to 0°C before melting it. Using cp ≈ 2.1 kJ/(kg·K):
qpreheat = mcΔT qpreheat = 2 × 2.1 × 10 qpreheat = 42 kJ qfusion = 2 × 334 qfusion = 668 kJ qtotal = 710 kJ
Common Mistakes
- Multiplying kg directly by J/g without converting units.
- Using molar enthalpy in kJ/mol as if it were J/g.
- Forgetting that enthalpy of fusion assumes the solid is already at its melting point.
- Including liquid heating after melting when the question only asks for fusion energy.
- Using approximate values without checking the required data source for a lab or assignment.
Frequently Asked Questions
What does enthalpy of fusion mean?
It is the energy required to convert a solid to a liquid at the melting point without changing temperature.
What is the formula for heat of fusion?
The basic formula is q = mLf, where m is mass and Lf is the specific latent heat of fusion.
Is J/g the same as kJ/kg?
Yes, numerically. For example, 334 J/g equals 334 kJ/kg.
Does this calculator heat the material to the melting point?
Only if you enter an initial temperature and a solid specific heat capacity. Otherwise, it assumes the material is already at its melting point.
Can I use kJ/mol?
Yes. Select kJ/mol and enter the molar mass in g/mol so the calculator can convert it to kJ/kg.