Internal Energy, Enthalpy, and Heat

[Hemichordate]

---

Members do not see this ad.

What is the scientific explanation for why deltaH = q when pressure is constant?

Also, why is deltaU (internal energy) = q when volume is constant?

When either pressure or volume is constant, shouldn't the PdeltaV variable be eliminated? In that case, why wouldn't deltaH = deltaU = q in both situations?

 

[Pulsar]

What is the scientific explanation for why deltaH = q when pressure is constant?

Also, why is deltaU (internal energy) = q when volume is constant?

When either pressure or volume is constant, shouldn't the PdeltaV variable be eliminated? In that case, why wouldn't deltaH = deltaU = q in both situations?

There are two ways to get energy into or out of a system: in the form of heat or PdV work. Therefore, dU = dQ + dW = dQ - PdV

If you define enthalpy as H = U + PV then dH = dU + PdV + VdP. At constant P, the VdP term drops out and you get dH = dU + PdV. Now substitute in the expression for dU from above (dU = dQ - PdV) and you end up with dH = (dQ - PdV) + PdV = dQ at constant pressure. Enthalpy is valuable at constant pressure. The constant-volume analog of enthalpy is the Helmholtz free energy. And the more familiar Gibbs free energy combines the two.

As for your second question, we already established that dU = dQ + dW = dQ - PdV. If volume is constant, that reduces to dU = dQ (i.e. there's two ways to change internal energy, heat and/or PdV work. We're imposing the restriction that there is no PdV work by saying volume doesn't change, so the only other way internal energy can change is through heat lost/gained).

And your final question: just because P is constant doesn't mean P is zero. So at constant pressure, for example, VdP must equal zero, but PdV does not necessarily.


Hope that answers your questions