Law of mass action

In 1864, the Norwegian chemists Cato Maximilian Guldberg and Peter Waage formulated the law of mass action, generalising the experimental behaviour of a great many reversible reactions they had studied. The law states:

"At any instant, the rate of a chemical reaction at a given temperature is directly proportional to the product of the active masses of the reactants at that instant."

For a reactant present with stoichiometric coefficient x in the balanced equation, this is written

where the square-bracket notation $[\ ]$ denotes the reactant's active mass -- its molar concentration: …