[Music] hi and welcome back to three sighs lessons kool dot uk' by the end of this video you should be able to draw and interpret graphs of the quantity of reactants or products in a chemical reaction you should then be able to calculate the mean rate of a chemical reaction now this sounds a bit tricky but the actual ideas are relatively straightforward I'm showing you a chemical reaction here as you can see the reactant is turning into the product if we plot the quantity of product formed against time then we get this graph now the slope of the line gives us an idea of the rate of reaction the steeper the slope the faster the reaction so as you can see initially the reaction is very fast we can tell that as we're making a lot of product in a short amount of time that's because we've got a large number of reactant molecules so lots of the more reactant aren't forming the product gradually the slope of the line becomes less steep this tells us that the reaction is slowing down in other words the rate of the reaction is decreasing now that's because a lot of the reactant molecules have already reacted an turned into product this means there are fewer reactant molecules available to react at the end we can see that the slope of the line is now zero in other words the line is flat this tells us that at this point the reaction has stopped and that's because all of the reactant molecules have already reacted now you'll notice that on this reaction we're measuring the quantity of product formed in grams but if a product a gas then we'd measure the quantity in centimeters cubed so as we've seen me in much of the quantity of product formed in a chemical reaction another way of looking at the weight is to measure the quantity of reactant used in this case the graph looks like this again we can see that initially the rate is fast then the reaction slows down and find the reaction stops now in the exam you could be asked to calculate the mean rate of a reaction so we're going to look at that now here's a sample question in a chemical reaction sixty grams of product was formed in fifteen seconds partly the mean rate of reaction now to do this we use this calculation the mean rate of reaction equals the quantity of product formed divided by the time taken the quantity of product formed is 60 grams and the time taken is 15 seconds 60 divided by 15 gives us a mean rate of reaction of 4 grams per second here's a question for you to try in a chemical reaction 20 grams of product was formed in 40 seconds calculate the mean rate of reaction pause the video and try this yourselves so the mean rate of reaction is a quantity of product formed divided by the time taken we've got 20 grams of product formed in 40 seconds 20 divided by 40 gives the mean rate of reaction of Nohr point 5 grams per second now we can also calculate the mean rate of reaction from the quantity of reactant used here's a sample question 30 grams of reactant was used in 10 seconds calculate the mean rate of reaction so to do that we use this calculation the mean rate of reaction is the quantity of reactant used divided by the time taken so the quantity of reactant used as 30 grams on the time taken as 10 seconds dividing 30 by 10 gives us a mean rate of reaction of the 3 grams per second she has a question for you to try 150 grams of reactant was used in 75 seconds calculate the mean rate of reaction pause the video and try this yourself okay so the quantity of reactant used is 150 grams and the time taken is 75 seconds putting these into the equation gives us a mean rate of reaction of two grams per second remember you'll find plenty of questions and rates of reaction in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to draw an interpret graphs of the quantity of reactants or products and a chemical reaction you should then be able to calculate the mean rate of the chemical reaction [Music]