CHEMISTRY COMMUNITY

Homework problem 9.7 that wants us to find the entropy change of 1 mol of ideal gas with a temperature change under constant pressure and constant volume. For the final, would we need to know that constant pressure is (5/2)R and that constant volume is (3/2)R? Also, for that same problem, how can something expand reversibly under constant pressure? I thought that constant pressure implied that an expansion was irreversible?

Those equations are on the equation sheet, but you will probably have to know which one is for constant pressure or constant temperature. Also, a reversible heat transfer simply implies a change in volume under constant temperature; pressure can decrease or stay the same, but as you can see, the equation for work during a reversible heat transfer is
w=-nRT; there is no mention of pressure, although it could change during a reversible heat transfer. For an irreversible process, pressure must be constant. The equation for work during an irreversible heat transfer is
w=-P.

What equation would we use if the volume or pressure was not constant to find entropy?

If the volume is changing you would use: n*R*ln(V2 / V1)

If the pressure is changing you would use: n*R*ln(P1/P2)

where n = moles, V = volume, and P= pressure.