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How to derive ΔH = ΔU + nRΔT

Posted: Fri Jan 31, 2020 9:02 pm
by Hannah Lee 2F
I'm confused about this equation: ΔH = ΔU + nRΔT, which is mentioned in the textbook.

I understand how to derive/use ΔH = ΔU + ΔnRT, but how could ΔU = ΔH - nRΔT be possible? I thought that in order to use any variation of ΔU = ΔH - PΔV, pressure had to be constant. However, in the book, it says you can use ΔU = ΔH - nRΔT to calculate the enthalpy of an ideal gas heated at a constant volume (not pressure). How would you derive that equation from the ideal gas law if volume is constant?

Re: How to derive ΔH = ΔU + nRΔT

Posted: Fri Jan 31, 2020 10:23 pm
by Jared Khoo 1G
The standard formula for the first law of thermodynamics is . Here, W is equivalent to P , which by the ideal gas law is the same as nR , which is where this equation came from.