Calculating Energy of an inlet gas stream; 2 conflicting methods

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Homework Statement



I want to calculate the energy of an inlet gas stream (I will add heat later but need to know the rate at which energy is coming into the system [J/s]).

The fluid is air and I can decompose it into molar flow rates of the constituent gases. I now need to use lookups to find the energy of each constituent gas.

I believe what I'm after here is a state variable because its based on the ambient condition and not any process that happened beforehand.

Homework Equations



I can think of 2 equations here:
Q = m_dot * Cp * T
Q = m_dot * H
(where H = enthalpy)

The Attempt at a Solution



I have found some lookup data for values of H and Cp of oxygen and nitrogen. However the 2 equations give different plots; when I plot T vs H and T vs (Cp * T) the plots are similar but significantly different.

I'm not sure why they are different or which one I should use. Also, is what I'm trying to calculate defined as the internal energy of the gas?

Thanks for your help.
 

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