حرارة الإنصهار
حرارة الانصهار أو طاقة الذوبان هي كمية الطاقة الحرارية اللازمة لتحويل مول واحد من مادة ما من الحالة الصلبة إلى الحالة السائلة أو بالعكس، وتدعى درجة الحرارة التي تحدث عندها العملية بنقطة الانصهار.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
القيم المرجعية للمواد الشائعة
Substance | Heat of fusion (cal/g) | Heat of fusion (J/g) |
---|---|---|
water | 79.72 | 333.55 |
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 |
هذه القيم مأخوذة من CRC Handbook of Chemistry and Physics, 62nd edition. The conversion between cal/g and kJ/kg in the above table uses the thermochemical calorie (calth) = 4.184 joules rather than the International Steam Table calorie (calINT) = 4.1868 joules..
الاستخدامات
To heat one kilogram (about 1 litre) of water 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
توقع قابلية الذوبان
The heat of fusion can also be used to predict solubility for solids in liquids. Provided an ideal solution is obtained the mole fraction of solute at saturation is a function of the heat of fusion, the melting point of the solid and the temperature (T) of the solution:
Here, R is the gas constant. For example the solubility of paracetamol in water at 298 K is predicted to be:
This equals to a solubility in grams per liter of:
which is a deviation from the real solubility (240 g/L) of 11%. This error can be reduced when an additional heat capacity parameter is taken into account [1]
البرهان
عند الاتزان، يتساوى الكمون الكيميائي للمذيب النقي وذلك للصلب النقي:
أو
with the gas constant and the temperature.
إعادة تنظيم الحدود يعطينا:
and since
the heat of fusion being the difference in chemical potential between the pure liquid and the pure solid, it follows that
Application of the Gibbs-Helmholtz equation:
ultimately gives:
or:
and with integration:
the end result is obtained:
طالع أيضاً
- حرارة التبخر
- السعة الحرارية
- السعة الحرارية النوعية
- Thermodynamic databases for pure substances
- Joback method (Estimation of the heat of fusion from molecular structure)
المصادر
- ^ Measurement and Prediction of Solubility of Paracetamol in Water-Isopropanol Solution. Part 2. Prediction H. Hojjati and S. Rohani Org. Process Res. Dev.; 2006; 10(6) pp 1110 - 1118; (Article) doi:10.1021/op060074g