The book that I use (Concepts in Thermal Physics by S. and K. Blundell) states the second law in two ways. The way they state the Kelvin version is "no process is possible whose sole result is the complete conversion of heat into work." How does that fit in with the isothermal expansion of an ideal monatomic gas? From the first law we have, Delta(U)=0=-W(by gas)+Q(added to gas) giving W(by gas)=Q(added to gas) Assuming there is no friction from the piston, this should be correct. However, this seems to suggest that isothermal expansion violates the second law if it is not part of a cycle. How do we reconcile it with the second law?