افتح القائمة الرئيسية

حرارة الإنصهار

(تم التحويل من حرارة إنصهار)
Molar heat content of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points. The enthalpy of melting (ΔH°m) of zinc is 7323 J/mol, and the enthalpy of vaporization (ΔH°v) is 115 330 J/mol.

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.


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فهرست

القيم المرجعية للمواد الشائعة

 
Standard enthalpy change of fusion of period three.
 
Standard enthalpy change of fusion of period two of the periodic table of elements.
SubstanceHeat of fusion
(cal/g)
Heat of fusion
(J/g)
water 79.72333.55
methane 13.9658.41
ethane 22.7395.10
propane 19.1179.96
methanol 23.7099.16
ethanol 26.05108.99
glycerol 47.95200.62
formic acid 66.05276.35
acetic acid 45.91192.09
acetone 23.4297.99
benzene 30.45127.40
myristic acid 47.49198.70
palmitic acid 39.18163.93
stearic acid 47.54198.91

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

المصادر

  1. ^ 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