Standard enthalpy change of fusion
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When you withdraw thermal energy from a liquid or solid, the temperature falls. When you add heat energy the temperature rises. However, at the transition point between solid and liquid (the melting point), extra energy is required (the heat of fusion). To go from liquid to solid, the molecules of a substance must become more ordered. For them to maintain the order of a solid, extra heat must be withdrawn. In the other direction, to create the disorder from the solid crystal to liquid, extra heat must be added.
The heat of fusion can be observed if you measure the temperature of water as it freezes. If you plunge a closed container of room temperature water into a very cold environment (say −20 °C), you will see the temperature fall steadily until it drops just below the freezing point (0 °C). The temperature then rebounds and holds steady while the water crystalises. Once completely frozen, the temperature will fall steadily again.
The temperature stops falling at (or just below) the freezing point due to the heat of fusion. The energy of the heat of fusion must be withdrawn (the liquid must turn to solid) before the temperature can continue to fall.
The units of heat of fusion are usually expressed as:
- joules per mole (the SI units)
- calories per gram (old metric units now little used, except for a different, larger calorie used in nutritional contexts)
- British thermal units per pound or Btu per pound-mole
- Note: These are not the calories found in food. The calories found in food are more properly known as kilocalories—equal to 1000 calories. 1000 calories = 1 kilocalorie = 1 food calorie. Food calories are sometimes abbreviated as kcal as if small calories were being used, while calories are abbreviated as cal. Another distinguishing method, though often confusing, uses capitalisation. A Calorie is a food calorie, or 1000 calories. So 1 Cal = 1000 cal.
Reference Values
The heat of fusion of water is
- 79.72 calories per gram
- or 334.5 kilojoules per kilogram
| Substance | Heat of fusion (cal/g) | Heat of fusion (kJ/kg) |
|---|---|---|
| methane: | 13.96 | 58.41 |
| ethane: | 22.73 | 95.10 |
| propane: | 19.11 | 79.96 |
| methanol: | 23.70 | 99.16 |
| ethanol: | 26.05 | 108.99 |
| glycerol: | 47.95 | 200.62 |
| formic acid: | 66.05 | 276.35 |
| acetic acid: | 45.91 | 192.09 |
| acetone: | 23.42 | 97.99 |
| benzene: | 30.45 | 127.40 |
| myristic acid: | 47.49 | 198.70 |
| palmitic acid: | 39.18 | 163.93 |
| stearic acid: | 47.54 | 198.91 |
Application
To heat one kilogram (about 1 liter) of cool water 20 °C from 10 °C to 30 °C requires 20 kcal.
However, to melt ice and raise the resulting water temperature 20 °C requires extra energy. To heat ice from 0 °C to water at 20 °C requires:
- (1) 80 cal/g (heat of fusion of ice) = 80 kcal for 1 kg
- PLUS
- (2) 1 cal/(g·°C) = 20 kcal for 1 kg to go up 20 °C
- = 100 kcal
See also
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