Final answer:
The question centers on chemical equilibrium and when it is appropriate to consider changes in concentration as negligible in the calculation of equilibrium concentrations. The use of approximations in chemistry simplifies the computation of equilibrium states, particularly when the changes in reactant or product concentrations are small compared to their initial values.
Step-by-step explanation:
The discussion focuses on the concept of chemical equilibrium and how changes in concentration can be considered negligible in the context of reaching equilibrium. A common assumption made in these problems is that the change in concentration of reactants or products during the reaction is so small that it can be neglected when calculating the equilibrium concentrations. This makes calculations more straightforward, as illustrated in the examples involving CO2, COCl2, and N2O4 where changes in concentration during the reaction are deemed negligible.
In chemical equilibrium problems, such as those involving the equilibrium constant (K), it is often assumed that the concentrations of pure liquids and solids remain constant and thus do not appear in the reaction quotient Q. For gas-phase reactions and solutions, however, changes in concentrations must be considered unless demonstrated to be negligible relative to the concentrations of other reaction species.