The other possible case would be that the second step is slow and rate-determining, meaning that it is slower than the first step in the reverse direction: "r"2 ≪ "r"−1. In this hypothesis, "r"1 − r−1 ≈ 0, so that the first step is (almost) at equilibrium. The overall rate is determined by the second step: "r" = "r"2 ≪ "r"1, as very few molecules that react at the first step continue to the second step, which is much slower. Such a situation in which an intermediate (here ) forms an equilibrium with reactants "prior" to the rate-determining step is described as a "pre-equilibrium" For the reaction of and CO, this hypothesis can be rejected, since it implies a rate equation that disagrees with experiment.