Constant Pressure

[ng1D]

When a reaction has a constant pressure, why is PdeltaV insignificant when the reaction involves solids and liquids?

[madawy]

When there's a reaction at constant pressure that involves solids and liquids, a change in volume will be insignificant because V of the reactants will equal (or almost equal) V of the products. ΔV=0, so PΔV=0. Also, there is no expansion work. Thus, with constant pressure, for a reaction that involves solids and liquids, ΔU=ΔH through the First Law of Thermodynamics.
Under constant pressure with no expansion work for reactions involving solids and liquids (w=0):
ΔU = q+w = q = ΔH

[Fatemah Yacoub 1F]

This is because for reactions with solids or liquids there are no volume changes as compared to a gas that is able to expand and change its volume relatively easily.