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Iodine clock reaction9/2/2023 ![]() If you get a curve, then it isn't first order. That's because in a first order reaction, the rate is proportional to the concentration. If the reaction is first order with respect to that substance, then you would get a straight line. You can then plot 1/t as a measure of rate against the varying concentrations of the reactant you are investigating. In experiments of this sort, you often just use 1/t as a measure of the initial rate without any further calculations. If you look at the expressions in the table above, you should recognise that the initial rate is inversely proportional to the time taken. ![]() If you are simply wanting to compare initial rates, then it doesn't matter. Suppose, for example, that instead of measuring the time taken to collect 5 cm 3 of gas, you just collected the gas up to a mark which you had made on the side of a test tube. Now suppose you didn't actually know what the volume V was. The initial rates (in terms of volume of gas produced per second) are: experiment 1 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: Now suppose you did the experiment again with a different (lower) concentration of the reagent. Measuring the slope of a straight line is very easy. The further into the reaction you go, the more the graph will start to curve. That is only a reasonable approximation if you are considering a very early stage in the reaction. 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. Since we are interested in the initial rate, we would need the slope at the very beginning. The steeper the slope, the faster the rate. If you plotted the volume of gas given off against time, you would probably get the first graph below.Ī measure of the rate of the reaction at any point is found by measuring the slope of the graph. This could be a reaction between a metal and an acid, for example, or the catalytic decomposition of hydrogen peroxide. We will take a simple example of an initial rate experiment where you have a gas being produced. ![]() Obviously, you could then repeat the process by changing something else - the concentration of a different substance, or the temperature, for example. You would need to cover a reasonably wide range of concentrations, taking perhaps 5 or so different concentrations varying from the original one down to half of it or less. This is repeated for a range of concentrations of the substance you are interested in. Then you find the time taken for the same event to take place with that new concentration. 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. We will look at examples of all these below. ![]() Or you could measure the time taken for some dramatic colour change to occur. Or it could be the time taken for a small measurable amount of precipitate to be formed. This could include the time taken for, say, 5 cm 3 of gas to be produced. ![]() The simplest initial rate experiments involve measuring the time taken for some easily recognisable event to happen very early on in a reaction. I wouldn't really recommend that you try to read it all in one go. We will look at these two approaches separately. 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 page is an introduction to some of the experimental methods that can be used in school labs to find orders of reaction. FINDING ORDERS OF REACTION EXPERIMENTALLY ![]()
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