Non-flow processes
In a non-flow process, gas does not flow across the boundary of the system, so the system is closed. The gas is treated as ideal and obeys
For an ideal gas, intermolecular forces are neglected. There is therefore no intermolecular potential-energy contribution to the internal energy, so changes in internal energy are associated with changes in molecular kinetic energy and temperature.
Constant pressure change
A constant-pressure process is also called an isobaric process. The pressure remains unchanged while the volume and temperature can change.
Heating a gas at constant pressure causes it to expand. Its volume and temperature increase, and the gas does positive work on its surroundings.
Cooling at constant pressure causes the volume and temperature to decrease, so work is done on the gas.
On a – diagram, a constant-pressure change is represented by a horizontal line.
Work and the first law
For a volume change at constant pressure , the work done by the system is
Using gives
| Change | Temperature | ||
|---|---|---|---|
| Heating and expansion | Positive | Positive | Increases |
| Cooling and compression | Negative | Negative | Decreases |
Exam Tip: At constant pressure, calculate as final volume minus initial volume. This automatically gives the correct sign for .