PHYSICAL CHEMISTRY IN BRIEF

CHAP. 4: APPLICATION OF THERMODYNAMICS [CONTENTS] 110

Shaft work is applied for machines with a steady flow of the substance [see4.3.3].

Note: Do not let yourself be mistaken by the name of the quantityWsh. In contrast to

volume, surface or electrical work, shaft work is not a kind of workW embraced by the

first law of thermodynamics.

Example

If a system exchanges only volume work with its surroundings during an adiabatic process, shaft

work is equal to the enthalpy change. Prove this assertion.

Proof

It follows from the specification and from the first law of thermodynamics (3.3) that

∆U=Wvol, [Q= 0].

From the definition of enthalpy (3.9) and relation (4.12) we then obtain

∆H= ∆U+ ∆(pV) =Wvol+p 2 V 2 −p 1 V 1 =Wsh.