Predicting the extent of a reaction

The numerical value of $K_C$ is itself a direct measure of how far a reaction has proceeded toward product formation at a given temperature. A large $K_C$ means the reaction reaches equilibrium with a high yield of product; a small $K_C$ means the reaction reaches equilibrium having formed very little product. As a rule of thumb: if $K_C$ is greater than about $10^3$, the reaction proceeds nearly to completion; if $K_C$ is less than about $10^{-3}$, the reaction barely proceeds at all; and if $K_C$ falls in between, $10^{-3}$ to $10^3$, significant amounts of both reactants and products coexist at equilibrium (Table 8.2). Real examples span this whole range dramatically -- the decomposition of water at 500 K has $K_C = 4.1\times10^{-48}$ (reaction essentially does not happen), while the formation of HCl at 300 K has $K_C = 4\times10^{31}$ (reaction runs essentially to completion). …