Pressure-volume work is the work associated with a change in volume against an external pressure. This is one type of work that can be performed by a reaction. An internal combustion engine uses the combustion of fuel to convert the chemical potential energy in the fuel to mechanical energy. The combustion reaction causes gases in the cylinder to expand, pushing the piston outward against an external pressure. We can calculate pressure-volume work against a constant external pressure using the equation: work equals negative pressure times change in volume. The negative sign is needed because if the change in volume is positive, the system has expanded its volume against the external pressure, thus doing work. When the system does work on the surroundings, it is a negative change in energy for the system. Let's practice using this equation. Our problem reads, "If you inflate a balloon from 0.05 liters to 3.05 liters against an external pressure of 1 atmosphere, how much work is done in joules?" We'll start with our equation: pressure volume work for a constant external pressure equals negative pressure times change in volume. Our change in volume is the final volume minus the initial volume. Substituting in our numbers and completing the calculation, we get -3 liter atmospheres. Our last step is to convert this to joules using dimensional analysis. We're given the conversion factor to convert from liter atmospheres to joules in the problem: 101.3 joules is one liter atmosphere. Completing the calculation, we get -304 joules.