irreversible systems

[Funmi Baruwa]

In an irreversible system, how come the external pressure doesn't change even though the volume expands? Wouldn’t the volume expansion push against the external pressure and cause it to get smaller ?

[Chem_Mod]

if we are talking about expanding against the atmosphere, the change in volume is so negligible as the system expands that P can be considered constant. If we look at a piston, expansion of the system will compress the other side to a smaller volume, which would increase the external pressure (pressure of the other side of the piston which the system is expanding against), if temperature remains constant. However, this expansion occurs instantaneously for an irreversible expansion and expansion stops as soon as Pex= Psystem, so Pex can be thought of as constant.

[Funmi Baruwa]

Thank you that makes sense now!