Many reactions do not go to completion, where all of the reactants turn into products and the reaction is over. Instead, most reactions reach a state called equilibrium. At equilibrium, reactants turns into products at the same rate as products turn into reactants. Therefore, both forward and reverse reactions keep on going.
Equilibrium constant, Keq, gives a ratio of the concentration of products to reactants (Keq= [products]/[reactants]).
Thus, equilibrium constant tells us at equilibrium whether the formation of products or reactants is favored. A high equilibrium constant tells us that at equilibrium, formation of products (or the forward reaction) is favored because in order for a ratio to be big, numerator has to be greater than denominator. A very low equilibrium constant, tells us that at equilibrium, we have a lot more reactants (reverse reaction is favored).
There are many different equilibrium constants. Ka is the equilibrium constant for acid, and when it is low it signifies a weak acid. Kb is the equilibrium constant for base, and when low, signifies that the base is weak. Ksp is the equilibrium constant for solubility product (solids that don't dissolve well). Kp is the equilibrium constant for reactions that involve gases (p stands for pressure).
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