In a certain experiment, a piston chamber is used as part of a primitive engine. The apparatus consists of a pipe closed at one end with a piston at the other end. A valve in the cylinder allows fuel gases to be introduced or waste gases to be expelled.
In the operation of this engine, hydrogen and oxygen are introduced in a 2:1 ratio (in order to ensure complete combustion) at ambient temperature Tamb and atmospheric pressure Patm. The following reaction is ignited
with a heat of reaction
ΔHrea = –4.8 x 105 J/mol
The pressure rises to Pburn.
Next the piston slowly moves back a distance l, from which the engine derives useful work. The distance l is short enough that the pressure and temperature inside the chamber remain roughly constant.
The waste gases are then expelled, and the piston is restored to its original position.
The radius of the cylinder is r, and the cross-sectional area is A. The length of the cylinder before the piston moves back is L, which is much larger than l. The number of moles of oxygen introduced is n.
If the reaction shown were performed in a closed chamber isothermally at 500˚C , what would happen to the pressure?