[BLANK_AUDIO]. Chemical kinetics is a study of how the rate of a reaction depends upon a number of different things. So, for example, if we change the concentration in the reaction chamber. Or, for instance we change the temperature. What happens to the rate? If there are different species present in the reaction, what happens? And finally, if we add something to the reaction such as a catalyst which normally speeds up the reaction, what happens to the rate? If you can understand all this, ultimately this should lead to an elucidation of the reaction mechanism. So how do we define the rate of reaction? Let's just start by looking at the simplest process we could possibly imagine. Where we start, there's some reactants R, going to products P. At this, at this point in time we're not even going to consider that this process might be in equilibrium. Then for this reaction, let's consider the rate of reaction, in terms of only the reactants. So, towards the beginning of the reaction at some time t equals t1. The concentration of the reactants R is going to be equal to R1. And at some later time t equals t2, concentration of R will have changed to R2. Now as the reaction proceeds, the concentration of R is dropping. As a consequence, concentration R2 must be less than the concentration R1. So using this information we should be able to determine the rate of reaction, which is defined in its simplest terms as how much has reacted divided by the time taken. So, in our particular case that's going to be equal to the change in concentration of the reactants R2 minus R1, divided by the time taken, which is t2 minus t1. More normally, we would write this as the change which is capital delta in the concentration R divided by the change in the time. The problem with describing the rate in this manner is that it's actually going to be a negative quantity because the concentration R2 is less than the concentration R1. So in order to take care of this and describe the rate as a positive term, we take minus delta R by delta t, and now we have a positive quantity to describe the rate of the reaction. So all that was done in terms of the reactants. But actually we can use anything in the, that is present in the reaction. Any species at all and it's perfectly valid, for example, to use the products as well. In that case we do exactly the same and describe the rate as the change in concentration of the products, which is P2 minus P1 divided by the change in time t2 minus t1. Describing it the same as we did before, therefore as delta P upon delta t. Now we have something which is already positive, because concentration P2 is greater than concentration P1, so this is positive and we have a perfectly valid way to describe the rate of the reaction. So here we now have two ways, we can either use the products or we can use the reactants and the only difference in these two ways are described in the rate, is this minus sign. If we use something which is disappearing as a function of time. Now, these rates have been measured over relatively large period of time, delta t as t2 is substantially different to t1. But what we really need is the instantaneous rate as the time delta t approaches 0. So in order to do this, we change the capital delta into a d. So we have dR by dt. Where the d now means an infinitesimally small change. So we have an infinitesimally small change in R divided by an infinitesimally small change in t minus dR by dt, or we can have dP by dt. So there's the expression for the rate. And from that expression we can immediately get the units of the rate, because we have a concentration divided by a time. So, a typical unit would be a unit of concentration, such as moles per decimeter cubed, or moles per liter cubed if you like, and then we've got divided by time. So we're going to have per second, moles per decimeter cubed, per second. [BLANK_AUDIO]