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Old Jan 4th 2019, 12:22 PM   #1
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Enthalpy formula

Hi guys,
I am relatively new in Thermodynamics as I am an electrical engineer. Right now, I am looking for an exact enthalpy calculation formula.

The Gas is N2 with Temperature 27C and Pressure 124.73 bar. From a paper the enthalphy is 288.28 kJ/kg. I also found the same answer from online calculator ( http://www.peacesoftware.de/einigewe...tickstoff.php7 ).

However, I don't know how to calculate it.
I can describe all attempts enthusiastically that I have done but I am afraid it might waste your time.

Could you please help me to find the formula for this enthalphy?

Thank's in advanced
Donny
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Old Jan 5th 2019, 11:57 AM   #2
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As far as I know absolute value of enthalpy can't be calculated. Only change in enthalpy of rxns can be calculated.
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Old Jan 6th 2019, 09:49 AM   #3
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Chapter suggestion

Donny,

I suggest you read Chapter 45 of The Feynman Lectures on Physics - Vol I
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Old Jan 8th 2019, 02:38 AM   #4
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Originally Posted by mscfd View Post
Donny,

I suggest you read Chapter 45 of The Feynman Lectures on Physics - Vol I

Hi mscfd,

Thank you for your suggestion. I read the book and I can see the point. However, I am lost in translation in equation (45.1). I don't know how to elaborate delta f become delta x and delta y.
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Old Jan 8th 2019, 04:01 AM   #5
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Originally Posted by Donny View Post
Hi mscfd,

Thank you for your suggestion. I read the book and I can see the point. However, I am lost in translation in equation (45.1). I don't know how to elaborate delta f become delta x and delta y.
Donny, eq. 45.1 is the definition of the total derivative. You can easily find a topic about this content in a Calculus book.

$\displaystyle f(x_1,x_2,x_2,....,x_n)$
$\displaystyle df=dx_1\frac{\partial f}{\partial x_1}+dx_2\frac{\partial f}{\partial x_2}+dx_3\frac{\partial f}{\partial x_3}+.....+dx_n\frac{\partial f}{\partial x_n}$

Anyway, if the internal energy can be defined as a function of the temperature and volume (this is not the only form): $\displaystyle U(T,V)$

$\displaystyle dU=dT\left(\frac{\partial U}{\partial T}\right)_V+dV\left(\frac{\partial U}{\partial V}\right)_T$

The letters under the parentheses mean that the variation of the derivative is for V or T constant. In pure mathematics this information is redundant, since it is a partial derivative, for a physicist or an engineer who is in the laboratory measuring, it is important to know which property should remain constant during the variation.
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Last edited by mscfd; Jan 8th 2019 at 06:09 AM.
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