What is equilibrium in regards to chemical reactions?

A + B -----> AB In the chemical reaction above, reactant A reacts with reactant B to form product AB. However, it is a common misconception that this reaction only goes in the forward direction (which is the one shown above). As more and more AB gets made, the reaction starts going in the opposite direction, with AB breaking down into A and B again: AB -----> A + B. In every reaction, if it goes on for long enough, there is a certain point where the rates of the forward reaction(A + B ------> AB) and reverse reactions (AB ------> A + B) are equal. At this point, the system is said to be in equilibrium. As a general statement, all chemical reactions try to achieve equillibrium. At equilibrium, the concentrations of A, B and AB are equal, and changing the concentration of any of the three will cause the system to shift in favor of the products or reactants to re-achieve equilibrium. For example, if more reactant A is added, the system will shift in favor of the products to use up the extra A that has been added, and once this has been done, the system will return to equilibrium.