 Graphing rate data is useful because you get a quick visual idea of how the rate varies through the reaction. You can also plot datasets from different experiments and visually compare which ones are faster or slower. However, it's also possible to calculate the rate from a dataset without having to graph it. So we'll stay with the same dataset as on the previous slide. We know that the average rate is calculated as the change in concentration over the change in time. So we can therefore calculate an average rate for each time interval that we've measured a concentration for. Note that the triangle here is the Greek capital letter delta and it means change in or the final value minus the initial value. So for the first two data points, the change in concentration of butyl chloride is 0.0905 minus 0.1 and the time interval is 50 seconds. So this gives an average rate of 0.00019 moles per litre per second. It's worth paying attention to the sign here for a moment. The rate of a reaction is defined to be positive. However, if you study the rate by measuring the change in concentration of a reactant, it's naturally going to be negative since the reactant is being used up. Hence, for this rate calculation, we put a minus sign at the front to account for this. If the data were for the increase in product concentration, then we wouldn't need the minus sign at the front to give a final positive value. Looking at the rate as the reaction proceeds, you can see that the average rate decreases as we go along. This is what the graph showed us as well. We looked at the factors that affect reaction rate in the last video. One of them was concentration. As the reaction proceeds, the concentration of the reactant decreases because it's being used up. That means that the rate of the overall reaction is decreasing as well.