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Some sample reactions The catalytic decomposition of hydrogen peroxide This is a simple example of measuring the initial rate of a reaction producing a gas.A simple set-up to do this might be: The reason for the weighing bottle containing the catalyst is to prevent introducing errors at the beginning of the experiment.Or you could measure the time taken for some dramatic colour change to occur. You then change the concentration of one of the components of the reaction, keeping everything else constant - the concentrations of other reactants, the total volume of the solution and the temperature and so on.
A measure of the rate of the reaction at any point is found by measuring the slope of the graph. Since we are interested in the initial rate, we would need the slope at the very beginning.
If you then look at the second graph, enlarging the very beginning of the first curve, you will see that it is approximately a straight line at that point.
Measuring the slope of a straight line is very easy. Now suppose you did the experiment again with a different (lower) concentration of the reagent.
Again, we will measure the time taken for the same volume of gas to be given off, and so we are still just looking at the very beginning of the reaction: The initial rates (in terms of volume of gas produced per second) are: of gas, you just collected the gas up to a mark which you had made on the side of a test tube. If you are simply wanting to compare initial rates, then it doesn't matter.
So you would convert all the values you had for rate into log(rate). I suspect that in the unlikely event of you needing it in an exam at this level, it would be given to you. You probably have to enter 2 and then press the log button, but on some calculators it might be the other way around.
Convert all the values for [A] into log[A], and then plot the graph. All you need to do is find the log button on your calculator and use it to convert your numbers. If you do it the wrong way around, you will just get an error message.
Since this is the part of the reaction you are most interested in, introducing errors here would be stupid!
You have to find a way of adding the catalyst to the hydrogen peroxide solution without changing the volume of gas collected.
That's because in a first order reaction, the rate is proportional to the concentration. It might be second order - but it could equally well have some sort of fractional order like 1.5 or 1.78.
The best way around this is to plot what is known as a "log graph".