 So, let's summarize reaction orders. Note that we can talk about reaction order in two contexts. We can talk about it with respect to a single reactant. That's like saying if we hold all other concentrations and conditions constant, how will the rate vary with this reactant? And we can talk about the overall reaction order, which is the sum of the orders with respect to all the reactants. Reactions with overall orders of zero or first order are simple. There's only one possible type of rate law for each. If we have a reaction with overall second order, however, there are two principle ways that this could happen. One is that the rate could depend on the square of one reactant concentration, as with the bromine atoms becoming bromine molecules in the previous slide. In this case, the reaction is second order with respect to the reactant and its second order overall. However, our ammonium and nitrite reaction is also second order overall. It's first order with respect to ammonium, and first order with respect to nitrite. But when you sum those two orders together, you have an overall second order reaction, a reaction where the rate depends on two concentrations.