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precede3 · Jul 17, 2011, 11:15 PM

what is the differnce of molar enthalpy and enthalpy of formation?

because I can find molar enthalpy value but I can't find formaion enthalpy value.

Unknown008 · Jul 18, 2011, 12:33 AM

Molar enthalpy is just a unit, this is not a reaction or some energy. It is often used as kJ/mol or J/mol.

Enthalpy of formation is the energy released when a substance is formed from its constituent elements and has its units in kJ/mol or J/mol most of the time, that is it has the units of molar enthalpy.

Just like enthalpy of formation, enthalpy of reaction, enthalpy of vaporisation, enthalpy of fusion and so on all are expressed in molar enthalpy.

precede3 · Jul 18, 2011, 04:21 PM

First thank you for your answering. And also I'm wondering about molar enthalpy function.
Between the molar enthalpy function and molar enthalpy formation is pretty different.

Unknown008 · Jul 18, 2011, 11:40 PM

Ok, to be honest, I never heard of that function and there isn't any useful information online either on that.

But what I found is:

$\text{Molar enthalpy function} = \frac{H_m(T)-H_m(0)}{T} \ J/(Kmol)$

Hm(T) is the melting enthalpy of bulk crystals as a function of temperature T

And $H_m(T) = \frac{H_mT^2}{T_m\ ^2}$

where Hm is the melting enthalpy of bulk crystals.
Tm is the melting point of the crystals.

You can already see the difference.

precede3 · Jul 19, 2011, 04:21 PM

Thank you very much. That's what I want to know!

Unknown008 · Jul 19, 2011, 10:26 PM

You're welcome :)