Final answer:
The equilibrium constant, Kc, indicates a reaction's extent at equilibrium. A very large Kc means the reaction goes virtually to completion, predominantly forming products. A very small Kc indicates that the reaction hardly proceeds, favoring reactants significantly.
Step-by-step explanation:
The value of the equilibrium constant, Kc, is indicative of the extent of a chemical reaction at equilibrium. When Kc is very large (often > 10³), it implies that the reaction has gone virtually to completion, meaning that the concentrations of reactants are converted almost entirely to products. The concept of the reaction reaching completion can be understood as most, or all, of the reactants being used up to form products, leaving a mixture that is predominantly products with negligible amounts of reactants.
Conversely, when Kc is very small, the reaction has hardly taken place at all, and the equilibrium lies far to the left, favoring the reactants. In this case, the concentration of the reactants remains much greater than that of the products, and very few products are produced. This signifies a reaction that does not occur to a significant extent under the given conditions.
When calculations involving Kc are performed for such reactions where the equilibrium constant is very large, it is often simplified by the assumption that the concentration changes for reactants are negligible, since they are nearly all consumed. This simplification is useful for solving problems without the need for complex ICE (Initial, Change, Equilibrium) tables, which would otherwise show the minute remaining concentrations of reactants.