this video is a walkthrough of an exam question about the third required practical of AQA a-level chemistry before I walk you through how to answer this question pause the video and have a go at answering it for yourself this chemical reaction in particular between the sodium thyssulfite and the dilute hydrochloric acid is probably already really familiar to you because it's one of the few that we can do in the school or sip form laboratory that involves a change in turbidity so you would probably have used this reaction at GCSE as part of the concentration required practical and then again it's really likely that you did this particular reaction as part of your practical endorsement so it's a nice example of a reaction where you can use one over time as a surrogate for rate rather than monitoring the rate of reaction using a continuous method but even if you encounter a question like this with a reaction that you haven't actually done for yourself you can still go through the process of describing how to set it up and you're going to cover the exact same points in this investigation we're looking at the impact of temperature on the rate of reaction so our independent variable is the temperature of the reagents and we need to talk about how we're going to change that the dependent variable well we're basically using one over the time taken for the reaction to finish as a surrogate for rate and then in terms of control variables we need to talk about the fact that we're always using the same volumes of solutions and the same concentrations of solutions so to begin with I want to make it clear that I am going to change the independent variable so I'm going to start by talking about how I would use water baths to heat up separate samples of my two reactants to a bunch of different temperatures you don't have to specify the temperatures but if you do you should be specifying more than two of them and actually for this reaction in particular we don't really want the temperature to go above 55 degrees because it's producing a load of sulfur dioxide gas and the hotter it is the faster it's going to produce that and sulfur dioxide of course is not great if you're breathing it in so that's a bit of a safety concern I'm also going to specify that I'm going to determine some volumes for these I can of course just say later do it again using the same volumes but sometimes it's just a bit easier to include a volume within your method and again because of the production of sulfur dioxide gas it's fairly typical that we would do this experiment on a bit of a micro scale to try to minimize the amount of that gas that's being produced now when you did this practical at GCSE you almost certainly Drew across on a piece of paper using a pencil and put your conical flask on top of that or your McCartney bottle on top of that um and waited until you couldn't see the cross anymore so that's still a perfectly valid method but at a level you might also have been fortunate enough to have access to a light sensor in which case you can pre-determine a level of light transmission and once the transmission Falls below that um then we say that the reaction has finished because the same amount of sulfur has been produced then of course we need to actually add our reactants together so we're going to add our first sample of hydrochloric acid to the sodium thiosulfate and of course as we do that it's important that we say that we will start the timer so then we're going to wait until either the cross has disappeared if we're going for the low-tech method or if we're using a Colorimeter then you're waiting for a predetermined point where you've set the absorbance to a certain amount or the transmission to a certain amount and whichever one of those you're doing you're going to record how long it took for that reaction to be completed and then we need to talk about changing the independent variable so we're going to repeat the experiment using the water bath of 35 degrees but keeping the temperatures um sorry changing the temperature but keeping the volumes and the concentrations of the solutions the same then for each one of these um we're going to calculate a value of one over time for the temperature and we're doing that as a surrogate measure of the rate of reaction and then once we've got that information we can plot a graph of um the temperature on the x-axis against one over time on the y-axis thank you very much for watching and I hope that you're now feeling a little bit more confident about answering questions about this third required practical of AQA a level chemistry if you are finding the videos useful don't forget to like And subscribe for more a level chemistry content coming soon