A mixture of gaseous reactants is put into a cylinder, where a chemical reaction turns them into gaseous products. The cylinder has a piston that moves in or out, as necessary, to keep a constant pressure on the mixture of 1 atm. The
cylinder is also submerged in a large Insulated water bath. (See sketch at right.)
From previous experiments, this chemical reaction is known to absorb 174. kJ of energy.
The position of the piston is monitored, and it is determined from this data that the system does 389. kJ of work on the
piston during the reaction.