# Enthalpy formula

#### Donny

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/einigewerte/calc_stickstoff.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.

Donny

#### mak29

As far as I know absolute value of enthalpy can't be calculated. Only change in enthalpy of rxns can be calculated.

1 person

#### mscfd

Chapter suggestion

Donny,

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

1 person

#### Donny

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.

#### mscfd

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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1 person

#### Donny

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 pressure 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.

Yes, It is defined as function of pressure... Right now, I am going to search how I can put those two parameters (temperature and pressure, no volume) become an enthalpy... Should it require other constant ( R, Cp, or Cv)?

Thank you