Reactions proceed until the free energy reaches a minimum
In this section we consider how to interpret the free energy function to predict how far a reaction will proceed. If the equilibrium constant is in the range from 0.1 to 10 then there will be significant concentrations of both reactants and products when equilibrium is reached. Only if the equilibrium is very large do we expect products to be formed in at yield that approaches 100% (as we would observe the yield). Likewise, if the equilibrium is very small then we expect the reaction not to proceed at all. However, when making such statements we must keep in mind that there is always some amount of product formed in a reacton, however small that amount may be. In chemical systems one must always be aware of the enormous range of possible concentrations. Somcetimes even small amounts of a substance can be very important for reactivity or for the properties of a mixture. Thus, we must also be aware of that aspect of the system.
The textbook example used for extent of reaction is the equilibrium between NO2 and its dimeric form N2O4. This equilibrium can be called the "smog reaction" since the brown color of N2O4 is one component of the smog that we can detect visually. The molecule NO2 is produced by the heat of tail pipes of cars that permits reaction of nitrogen and oxygen in the atmosphere. Since the eQuilibrium constant for this reaction is within the range indicated above the reaction has a significant amount of both components at equilibrium. This makes it a good example of the concept of extent of reaction.
There are two condittions that we must carefully distinguish in considering gas phase reactions. If the system is closed then the overall pressure is a constant and we must include that fact in the calculation of the mole fractions (and therefore the partial pressures). In an open system the pressure may chnage, which gives a different result for the calclation of the equilibrium pressures. We consider both of these cases in the following section.
