In this video we will be looking at calculating relative rate. It is assumed at higher that you can already calculate average rate using the equation from National 5, so change in quantity divided by change in time. We use relative rate at higher when we're looking at reactions where you may not have a quantity that you can measure over the course of your reaction, and instead you might have something like a really obvious colour change.
You can find... the equation here, this one, on page 4 of your data book. But you may need to rearrange it to be able to calculate time as well.
So just be aware that you may want to learn how to do it in this form as well. The rates that we look at are usually in time, in seconds. So that means that your rate is per second. And if you're doing it the other way around, it's so you can find out the full length of the time for the reaction. We're going to have a look at doing calculations using both of these and then I'll show you an instance of where this would actually be used.
So here are two calculations for you to try. So if you want to pause the video and give it a try and then I'll show you a worked example of how to calculate them. Calculating relative rate is really straightforward.
We're just going to do rate equals one divided by the time. then the question, which is 58 seconds, which will give us a relative rate of 0.017 per second. If for some reason you had the relative rate and you want to know how long the reaction takes, say you're trying to plan your experiment, then you're just going to rearrange the equation.
So this time you have time equals 1 divided by the rate, which is 0.2 per second. This is going to give you your answer in seconds. So the whole reaction took...
5 seconds to complete with that as your relative weight. A reaction where calculating average rate is not possible and therefore we need to calculate relative weight is that of acidified potassium permanganate and oxalic acid. In this reaction you get a really sharp colour change at the end point where the potassium permanganate decolourises from deep purple to colourless.
Below 40 degrees. the colour change isn't too sharp so we don't usually do it below 40 and above 70 it's really fast you're not able to necessarily get the timing right so we usually do it between these temperatures. If you were to carry out this reaction these are the sorts of results that you might get so you can pause the video again see if you can calculate the rates and then I'll just write in the rates for you at the end.
So if you were to be calculating the rates for these times Just doing 1 divided by time for the whole column, you would end up having 0.011, 0.025, 0.059 and 0.120. So it's clear already from the table that there is a relationship between your temperature and the rate. So for every 10 degree rise you get roughly a doubling of the rate and we can see this if we plot it as a graph.
So when plotted as a graph you don't get a linear relationship, you get this curve here. Now it could be that you could be given a rate and temperature or maybe a rate and concentration graph and asked to work out how long a reaction would take at a specific variable. So in this question here we're being asked How long does the reaction take at 55 degrees? So 55 was not one of the temperatures that we were looking at but we can extrapolate from the graph.
So if we just draw on to our graph we trace up a line to the at the 55 point and you can see that it hits here at this line. So our rate is equal to 0.04 per second. Squeeze that in there. But we want to calculate time, so it's just rearranging your rate equation. So you're going to have 1 divided by rate.
So that will give you 1 divided by 0.04. And your final answer would be that at 55 degrees, you would expect the reaction to take 25 seconds. I hope that you've enjoyed this video on the first topic of higher chemistry, controlling the rate.
Do come back for more videos as the course goes on. And remember to subscribe and follow me on Twitter at Miss Adams Chem. Thank you.