Example 34.1 Calculating the entropy change for the isothermal expansion of a perfect gas Calculate the entropy change of a sample of perfect gas when it expands isothermally from a volume V, to a volume V Method "The definition of entropy instructs us to find the energy supplied as heat for a reversible path between the stated initial and final states regardless of the actual manner in which the pro- cess takes place. A simplification is that the expansion is isother- mal, so the temperature is a constant and may be taken outside the integral in eqn 3A.2. The energy absorbed as heat during a reversible isothermal expansion of a perfect gas can be calcu- lated from AU=q+w and AU=0, which implies that q=-w in general and therefore that grev=-Wrey for a reversible change. The work of reversible isothermal expansion is calculated in Topic 2A. The change in molar entropy is calculated from AS.=AS