Calculating the Equilibrium ConstantFor the following chemical reaction:
aA(g) + bB(g) ↔ cC(g) + dD(g)
The equilibrium constant Kc is:
Kc = [C]c[D]d / [A]a[B]b
[A], [B], [C], [D] etc. are the molar concentrations of A, B, C, D
a, b, c, d etc. are the coefficients in the balanced chemical equation
Significance of the Equilibrium ConstantFor any given temperature, there is only one value for the equilibrium constant. Kc only changes if the temperature at which the reaction occurs changes.
If the value for Kc is very large, then the equilibrium favors the reaction to the right and there are more products than reactants. The reaction may be said to be "complete" or "quantitative."
If the value for the equilibrium constant is small, then the equilibrium favors the reaction to the left and there are more reactants than products. If the value of Kc approaches zero the reaction may be considered not to occur.
If the values for the equilibrium constant for the forward and reverse reaction are nearly the same then the reaction is about as likely to proceed in one direction and the other and the amounts of reactants and products will be nearly equal. This type of reaction is considered to be reversible.