ΔH vs ΔU?

[Anna O 2C]

Can someone explain why heat transfer is ΔU at constant volume and interpreted as ΔH at constant pressure?

[Becky Belisle 1A]

The formula for internal energy is: ΔU= q + w, so when volume is constant, w=0. Thus, when no work is done, ΔU = q. ΔH = q at constant pressure because if the pressure changed that would affect the enthalpy, but not the heat transfer.

[haleyervin7]

Can deltaU ever equal deltaH?

[Cole Elsner 2J]

Yes at constant pressure. If there is no net change in moles, there will be no work done. So there is the potential for deltaU=deltaH.

[Soumya Ravichandran 4H]

Yes, this can occur when there is no volume change (free expansion), but generally speaking, delta H will only equal delta U when there is no work done and pressure is held constant

[Anjali_Kumar1F]

If enthalpy is given do we automatically assume the reaction is at constant pressure.

[Destiny Diaz 4D]

When enthalpy is given I would not say it is safe to assume that pressure is constant, unless other wise stated.

[Pritish Patil 1K]

When volume is constant, work = 0 J. This means that deltaU = q.

[CHEM 14B Lover]

Can't there be expansion with constant pressure? Wouldn't that still be work done?