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U and H

Posted: Wed Jan 17, 2018 10:56 pm
Can someone please clarify the difference between deltaU and deltaH and how they relate to each other in the equation deltaU = deltaH-pdeltaV

Re: U and H

Posted: Wed Jan 17, 2018 11:17 pm
deltaU is the change in internal energy. deltaH is the change in enthalpy. Enthalpy is the amount of heat released at constant pressure. So the change in internal energy equals the change in enthalpy minus the product of pressure times change in volume.

Re: U and H

Posted: Wed Jan 17, 2018 11:20 pm
Delta U is the change in the internal energy of the system. In the piston model, when there is expansion energy, the system is doing work on the surroundings which causes a positive increase in volume, thus yielding w= -PΔV since the system lost energy when expanding. As a result, delta U = q-PΔV.

However, when the pressure is constant, the equation becomes delta U = q(p)-PΔV. Since q(p) is also equal to a change in enthalpy, then delta U = deltaH-PΔV.

Hope that helps.

Re: U and H

Posted: Wed Jan 17, 2018 11:27 pm
deltaU is the internal energy of just the system. It takes into account heat (q) and work (w) of the system. This means that it is affected by changes in those values. More q transferred into system or more w done on system (or more of both) means there is more energy being put into the system and therefore internal energy increases. q transferred out of the system or w done by the system (or less of both) means there is energy leaving the system and therefore internal energy decreases.

deltaH is the energy required or released to cause a phase change/energy absorbed or released by a reaction. It is equal to q at constant pressure.

Since deltaU (the internal energy) is affected by changes in q and w and deltaH is a change in q at constant pressure, whenever q changes, it will affect both of these values.
deltaU = deltaH-pdeltaV is just showing how they're related/the same when p is constant.
deltaU = [q(at constant p) + w] - pdeltav
deltaU = q(at constant p)+w-w
deltaU = q(at constant p)
deltaU = deltaH