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Answer:
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Explanation:
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.
This could include the time taken for, say, 5 cm3 of gas to be produced. Or it could be the time taken for a small measurable amount of precipitate to be formed. Or you could measure the time taken for some dramatic colour change to occur. We will look at examples of all these below.
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.
This is repeated for a range of concentrations of the substance you are interested in. 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.
Obviously, you could then repeat the process by changing something else - the concentration of a different substance, or the temperature, for example.
Understanding the results
We will take a simple example of an initial rate experiment where you have a gas being produced. This could be a reaction between a metal and an acid, for example, or the catalytic decomposition of hydrogen peroxide.
If you plotted the volume of gas given off against time, you would probably get the first graph below.
A measure of the rate of the reaction at any point is found by measuring the slope of the graph. The steeper the slope, the faster the rate. 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. That is only a reasonable approximation if you are considering a very early stage in the reaction. The further into the reaction you go, the more the graph will start to curve.
Measuring the slope of a straight line is very easy. The slope in this case is simply V/t.
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