This change is the negative of the change in the system, so we can conclude that
ΔStot = 0, which is what we should expect for a reversible process. If, on the other hand,
the isothermal expansion occurs freely (w = 0), then q = 0 (because ΔU = 0).
Consequently, ΔSsur = 0, and the total entropy change is given by eqn 3.17 itself: