Constant Pressure and Delta U

[Ava Ward 1K]

Why is it that when there is constant pressure, the work is not included in the delta U equation?

[Nicholas Flores 2F]

At Constant pressure, the significance of work depends on the reaction that occurs. For reactions that involve changes in the number of moles of gas, the work equation P(Delta V) is significant as there is a change in volume. If a reaction creates more moles of gas or decreases the number of moles of gas, the volume must change in order to maintain constant pressure. However, at constant pressure for reactions that involve solids and liquids, the change in volume is insignificant because solids and liquids do not have significant changes in volume. For solids and liquids Delta U= qp because W= P(Delta V) and the change in volume is 0. At constant volume, Delta U always equals qp as there is no work (P(delta V), no change in volume means that W=0). Hope this helps!