How is the change in enthalpy at constant pressure expressed?

The correct expression for the change in enthalpy at constant pressure is represented by the equation ∆H = ∆U + P∆V.

To understand this, we first need to define what enthalpy (H) is. Enthalpy is a thermodynamic potential that is particularly useful in processes occurring at constant pressure. It combines internal energy (U) with the product of pressure and volume (PV):

H = U + PV

When looking at the change in enthalpy (∆H), we can express it as:

∆H = ∆U + ∆(PV)

At constant pressure, as the system changes, the term P∆V accounts for the work done by the system as it expands or contracts against the ambient pressure:

∆(PV) = P∆V

Thus, substituting this into the change in enthalpy equation, we derive:

∆H = ∆U + P∆V

This relationship illustrates how changes in internal energy, which encompasses the heat and work interactions, combine with the work associated with volume changes under constant pressure conditions.

The other options do not provide accurate representations of this relationship. For example, the statement that ∆H equals