we have an evacuated thermally isolated chamber with a balloon inside. the balloon is filled with a volume V1 of an ideal gas at pressure P1 and temperature T1. the balloon is pricked so that the gas fills the chamber. we have to find the entropy change of the system. I'm not sure if we can treat the ideal gas as having constant internal energy (is the work done by the balloon in forcing the gas out negligible?) If so, then it's just a standard joule expansion, which is easy to calculate. thanks for your help.