When we talk about reaction kinetics, we're simply talking about the rates of reaction. How quickly do products form in reaction? Or how quickly are reactants consumed? Rates of reaction are measured by finding the change in concentration over time. When talking about the rates of reactions for forming a product, you'll get a positive rate because the change in concentration is increasing. When talking about the rate of reaction of a reactant, we'll get a negative rate because the change in concentration is decreasing. But you're always measuring the change in concentration over time. Generally when we're measuring concentration, we're measuring it in molarity. So our rate would be molarity per second, or moles per liter's time second. Most in time, the rates of reactions are not constant. They'll change over time. As reactants consume, it's harder and harder for them to react with each other, and so the rate of reaction will decrease. We'll talk a little bit more about that later this chapter. In this assignment tonight, we're going to focus mostly on the initial rates of reaction, how fast the reaction start off. As you can see from this graphic of the text, here's our reaction of the dinitrogen pentoxide decomposing, and you can see the concentration of the dinitrogen pentoxide decreasing as it gets consumed. But the rate at which it's decreasing changes. We can talk about the rate as being the concentration over time, or the slope of this curve. For my calculus piece out there, you would say the first derivative of this function. And you can see that the slope is negative because we are consuming reactant, so we have a negative rate of reaction. But the slope is getting less and less negative as the reaction proceeds. So again, when doing these problems for now, we'll focus on the initial rate of reaction, how fast is the reaction going right at the beginning? One more thing before we get into the math. As we learned when we did Hess's law, a lot of these reactions occur in multiple steps. So when talking about the rate of reaction, you don't necessarily have to worry about all the steps. You just have to focus on the rate determining step. The rate determining step is the slowest step in the process. Whenever you have a reaction occurring, there's going to be some kind of bottleneck that slows up the whole process. It might be the first step, it could be the second step, it could be the third step. It doesn't necessarily matter where it happens in the order. But whatever your slow step is, that will be your rate determining step. And if you measure the rate of the rate determining step, that will very closely approximate the rate of your overall reaction.