 Okay, now let's look at some actual data from a rates experiment. The reaction equation is here, we're going to react butyl chloride with water to give butanol and hydrochloric acid. An experimentalist has performed this reaction and she measures the concentration of butyl chloride at various times. What can she do with this information? Well, what can you ever do with a table of data? You can graph it. Graphs are lovely and also useful. So we'll use a scatter graph and this is what we get. You can see the concentration of butyl chloride starts high and then decreases as the reaction proceeds and it's used up. So what feature of this graph gives us information about how fast the reaction is proceeding? Well, it's a bit like a speed graph. Here's a random distance time graph that I just made up. Say it's tracking a runner. Distance is on the y-axis, that's how far the runner's gone. And time is on the x-axis, that's how long they're taking. And you can see from the slope of the graph it's gradient how fast the runner was moving. You can see that for the first four seconds the runner was going at a constant speed because the line is straight. But then the line gets steeper. This indicates the runner got faster and then later at about seven seconds they slowed down again. The reason you know they're getting faster is that the slope or the gradient of the line changed. Gradient is calculated as rise over run. For this graph that means the distance divided by the time. And that gives you the speed. It's the same for our chemical rate graph. The gradient of the line tells you the rate of the reaction. And here again the gradient is found as rise over run, which is the change in concentration of butyl chloride divided by the change in time. Now you can see that this line isn't straight, it's curving. It starts off quite steep but gets less and less steep as the reaction goes on. This means that the rate of reaction is decreasing, it's slowing down as time goes on. Alright, so how do we calculate the rate? Well it's still rise over run, but because this graph is a curve the gradient is always changing. So we're going to need to choose a time interval and then calculate the average rate for that time interval. So say we choose the time interval from 200 to 500 seconds. And for that change in time, that's 300 seconds, we work out how much the concentration change and then we divide one by the other. And that gives us the average rate for that time period. You can see that if you chose a different time, for instance the first 100 seconds, then you would get a different rate.