And suppose this reaction achieves equilibrium before one of the reactants is used up. What that means is that the forward reaction rate is equal to the reverse reaction rate. So the rate at which our reactants react to produce our product is the same as the rate at which the product associates into our reactants. In other words, the concentration of these guys and this guy remains the same even though the reactions are occurring. And that's because they're occurring at the same rates. Now, suppose we have the following combustion reaction in which methane combusts in the presence of oxygen to produce carbon dioxide and water.