What counts as work?

[RJ Mathews 1K]

On Wednesday Dr.Lavelle mentioned that we need to control for a reaction's changes in pressure and any work done. He also mentioned that a reaction pushing air out of the way counted as work, so should be considered. In that case, how can we account for or control the work that a reaction does with something as small as pushing away air? I also remember that doing stuff in an enclosed contained could result in a pressure difference, so that isn't an option.

[Allison Peng 1D]

usually for gases that can do work by expansion, you can attach it to a container that has a piston, so when it does work by pushing away air, you can actually see it by the new position of the piston (therefore, the volume changes when it does expansion work at constant pressure). I think for most of our problems, we assume that the only work that can be done is expansion work.

[Jake 1F]

Yeah you can either use a piston to measure the work that's being done by expansion or you can use a sealed container to ensure that any energy involved is contributing to changes in heat, not work. Since this only applies for gases, they have two different specific heat values, Cv which represents the specific heat when bvolume is constant (sealed container) and Cp which is when pressure is constant (unsealed/piston). He also said Cp is larger than Cv because of the energy that gois into expansion.