Work
Energy transferred when a force moves through a distance, most often in chemistry as a gas expanding against external pressure.
Formula / equation
w = −p_ext ΔV
Unit
J
In depth
By the modern convention, work done on the system is positive and work done by the system is negative. An expanding gas does work on the surroundings and so loses energy. Free expansion into a vacuum does no work at all, since the external pressure is zero.
Examples in the real world
A gas expanding by 2.0 L against 1.0 atm does 203 J of work on the surroundings.