Rates

You know you can figure out the average and instantaneous rates for a car traveling on a highway (mph - the speed of a car; average over the whole time of the trip, and instantaneous at a specific instant). Similarly, you can determine the rates of a reaction (or any  change, for that matter). Reaction rates is this rate for a chemical reaction.

Here's how the textbook defines it: A reaction rate is the decrease in reactant concentration or increase in product concentration per unit time of time as a reaction process. That's nice and clear, right?

Put simpler, the reaction rate of a chemical reaction is just the measurement of the change in concentration for whatever unit of time being used. Think of the car on the highway: its rate is the number of miles it travels per hour (mph). For a chemical reaction, just use the change in molarity per unit of time (so, M/min, M/sec, etc.).

It's really pretty easy. Let's try it.

Let's say a reaction occurs in which the concentration of a substance changes from 4.0 M to 2.0 M in 40 minutes (if this looks familiar, its an example from your book...and of course you've been reading your book). First of all, the substance is a reactant, since the concentration is decreasing (and a product would increase in concentration ). What's the average rate of the reaction? Well, average means its for the whole duration of the reaction, not at a specific instant. So, the change in concentration is ( final - initial) 2.0-4.0=-2.0. Divide that by the time span you want; in this case, its asking for the rate over 40 minutes. So, the average rate of this reaction is -2.0/40=-0.05 M/min. Now, because it was the substance that disappeared (reactant), you multiply the result by -1 to get a positive rate for the reaction See, that was fun, wasn't it?

Remember, reaction rates are usually stated as concentration/time, but you can determine the  average rate of a reaction by using one of the measurable properties of the reaction and dividing the change in that quantity by the total time of the reaction. Just like we did above. Usually, concentration is the quantity used, but I guess you could use mass or volume if you really wanted to.

Let's take a minute to discuss the constancy of the rate during the reaction. Few reactions occur at a constant rate for the whole process. Often, reactions are fast at the beginning (when all the reaction concentrations are high), and as the reactants are consumed, the reaction slows down. For some reactions, as the reaction nears equilibrium, the rate becomes constant, while in other reactions, the rate will go all the way to zero as it gets closer to completion.