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.
If you added it to the flask using a spatula, and then quickly put the bung in, you might lose some gas before you got the bung in.
Alternatively, as you pushed the bung in, you might force some air into the measuring cylinder. To start the reaction, you just need to shake the flask so that the weighing bottle falls over, and then continue shaking to make sure the catalyst mixes evenly with the solution.
The maths goes like this: If you have a reaction involving A, with an order of n with respect to A, the rate equation says: If you plotted log(rate) agains log[A], this second equation would plot as a straight line with slope n.
If you measure the slope of this line, you get the order of the reaction. Note: Don't worry if you don't understand logs (logarithms), or how I got from the first equation to the second one!You can then plot 1/t as a measure of rate against the varying concentrations of the reactant you are investigating.If the reaction is first order with respect to that substance, then you would get a straight line.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.Then you find the time taken for the same event to take place with that new concentration.The maths of this might not be familiar to you, but you may find that you are asked to do this as a part of a practical exam or practical exercise.If it is an exam, you would probably be given help as to how to go about it.This page is an introduction to some of the experimental methods that can be used in school labs to find orders of reaction. How initial rate experiments work An outline of the experiments The simplest initial rate experiments involve measuring the time taken for some easily recognisable event to happen very early on in a reaction.There are two fundamentally different approaches to this - you can either investigate what happens to the initial rate of the reaction as you change concentrations, or you can follow a particular reaction all the way through, and process the results from that single reaction. This could include the time taken for, say, 5 cm of gas to be produced.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.