If the interaction were a simple collision of two molecules (we will discuss the actual mechanism in the next chapter), then doubling the concentration of either or would double the rate of reaction, and doubling both at the same time would make the collisions, and hence the reaction, occur four times as fast. The rate of forward reaction then would be given by

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This expression says that the rate of forward reaction is proportional to each of the reactant concentrations independently, with a proportionality constant, . If the rate is measured in moles per second, and concentrations are in moles per liter, then has units of liter . These rather strange units for mean that as one type of molecule sweeps through the available volume, colliding with other molecules, its rate of sweep can be expressed in liters per second, per unit of concentration (moles per liter) for that type of molecule.