The path taken during the extraction of energy from combustion will affect how much work you can extract from the process. Paths are central to our understanding of thermodynamics because of considerations of reversibility, which is related to how much work can be extracted from an engine, but also because we live in a constant pressure world so the "easiest" path for us to consider in Chemistry is a constant pressure path. This statement is true at the earths surface, but geochemists study processes that occur deep beneath the surface where the volume cannot chnage due to the severe restrictions of the pressure and therefore a constant volume path makes the most sense. When considering atmospheric processes, air masses are not at constant volume or pressure, but the gas in the atmosphere is so sparse that the path an "insulated" or adiabatic path. Thus, there are reasons based in the nature of things that lead to consideration of certain paths. We also will show that using defined paths can help us clarify our thinking abou processes. Indeed, the path is crucial to an understanding of a process.

Reversibility and irreversibility

The distinction between reversible and irreversible paths is important as a starting point. The reversible process is a hypothetical process since to really be reversible the process (e.g. expansion or compression of a gas) would need to be infinitely slow and perfectly at equilibrium during the entire process. This is an ideal that is not realized in any real process, although we can approach the ideal by careful choice of conditions. The crucial feature of a reversible expansion or compression is that it is also a constant temperature process. For this reason it is a central aspect of our understanding. The point is that the internal energy state function depends only on temperature (at least for an ideal gas) and therefore the internal energy does not change along a reversible path. For this reason we can relate the heat and work along this path, w = -q. This relationship is used in a number of important derivations. There are many irreversible processes, but the most important one is the constant pressure expansion (or compression).

The constant pressure path and the enthalpy

The constant pressure path is important because it defines the enthalpy state function. The heat transferred at constant pressure is related to the internal energy by a work term. We can write this in general terms as H = E + PV. This equation tells that enthalpy includes the work term in a chemical process. The work that is meant here is the work of expansion against the atmosphere when moles of gas are produced in a chemical process (or work of compression if moles of gas are consumed). The enthalpy is a convenient state function since it includes this term. This means that we can use the tabulated enthalpy to calculate a heat of reaction with no further consideration of the work done. IF, on the other hand, we used the internal energy as the state function under normal conditions in the laboratory (i.e. constant pressure and temperature) we would need to calculate the work term as an additional step to obtain the correct heat of reaction. Clearly, we want to use the simplest state function. We use the enthalpy for normal conditions.