Enthalpy & Constant Pressure and Volume


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Jonathan Pai 2I
Posts: 121
Joined: Fri Sep 28, 2018 12:27 am

Enthalpy & Constant Pressure and Volume

Postby Jonathan Pai 2I » Sat Feb 02, 2019 7:51 pm

Why does deltaH=q at constant pressure?

Why does deltaH=q at constant volume?

Thanks CC

David Effio 1H
Posts: 38
Joined: Wed Feb 14, 2018 3:01 am

Re: Enthalpy & Constant Pressure and Volume

Postby David Effio 1H » Sat Feb 02, 2019 8:40 pm

Mathematically, constant pressure deltaH would be represented as:

deltaU = qP + w
w = -P * deltaV, under constant pressure.

Since deltaH = deltaU + (P * deltaV)
deltaH = (qP - P * deltaV) + (P * deltaV)
deltaH = qP

Similarly, the heat content at constant volume, qV, will be the same as deltaH, because:

deltaU = qP + w
w = -P * deltaV

Since deltaV = 0, because pressure remained the same, we get

deltaU = qP - (P*0) = qP

Since deltaU = deltaH at constant volume, deltaH = qP

Carissa Young 1K
Posts: 65
Joined: Fri Sep 28, 2018 12:17 am

Re: Enthalpy & Constant Pressure and Volume

Postby Carissa Young 1K » Sat Feb 02, 2019 9:06 pm

Thanks for explaining this! I was also kind of confused about deltaH.


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