delta U and delta h


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Kayla Tchorz-Dis 1F
Posts: 30
Joined: Thu Jul 13, 2017 3:00 am

delta U and delta h

Postby Kayla Tchorz-Dis 1F » Sun Jan 21, 2018 5:08 pm

I'm confused how delta U is related to delta H, also if this will be on the upcoming test. Thanks!

mayasinha1B
Posts: 70
Joined: Fri Sep 29, 2017 7:04 am

Re: delta U and delta h

Postby mayasinha1B » Sun Jan 21, 2018 5:15 pm

H = U +PV
ΔH = Δ(U + PV)
ΔH = ΔU = Δ(PV)

At constant pressure, ΔH = ΔU + PΔV

So if you are at constant pressure and constant volume, then ΔH = ΔU + P(0) and therefore ΔH = ΔU

If we don't have ΔV but we have other info (still at constant P)
PV = nRT
V = nRT/P

If we are at constant P and T then only n will vary giving us:

ΔV = (Δn)(nRT/P)

Therefore:

ΔH = ΔU + PΔV
ΔH = ΔU + P(Δn)(RT/P)
ΔH = ΔU + (Δn)RT

We were given these equations in class. As far as the definitions, delta U refers to the change in internal energy of a system while delta H refers to the change in enthalpy of a system.


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