Rates of reaction - kinetics
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I'm having difficulty explaining the continuous rate method and the initial rate method.
Ulex says
This is quite a tricky bit of theory and students quite often get confused, so you are in good company!
Continuous method
Let us take a hypothetical example to illustrate this. Suppose you have a reaction involving a substance X reacting with an alkaline solution which, of course, contains OH- ions. The reaction has the equation:
X + OH- -> products
You set up one batch of reaction mixture, starting the clock when the mixture is made. You make sure that the concentration of X is large compared to teh concentration of hydroxide ions, so that the concentration of X does not change significantly.
At measured time intervals you withdraw a sample of the mixture and titrate it with an acid. The more acid you need, the greater the OH- ion concentration. As the reaction goes on, this titration value will decrease because the OH- ions are being used up. You now have a list of times and a list of titration values which are proportional to the concentrations of OH- ions. You plot titration value against time.
If your graph is a straight line, the reaction is zero order with respect to OH-.
If your graph is a curve with constant half life, the reaction is first order with respect to OH-
If your graph is a deeper curve with a half life which increases markedly, the reaction is probably second orders. Orders higher than this are rare and outside the scope of the half-life method.
Notice the characteristic feature: you only have one reaction mixture and you make continuous measurements on it.
Initial rate method
Again, lets take an example. You have a 1-cm long piece of magnesium and you drop it into an excess of hydrochloric acid with concentration 1 mol dm-3. You time how long it takes for the piece of magnesium to react completely. In this time, the acid concentration will not have changed much so you assume that it has not changed at all.
You repeat the experiment with another 1 cm of magnesium but this time you use acid of concentration 0.9 mol dm-3. It takes a bit longer this time.
You repeat again several times with a lower concentration each time.
You now have a list of concentrations and a corresponding list of times.
Here’s the tricky bit: for each of these times, t, the same amount of magnesium has reacted so the rate of the reaction (the initial rate) is given by Moles Mg / t
Since the amount of magnesium is the same for each separate experiment, the initial rate is proportional to 1/t.
You now compare with each rate equation in turn so see which fits the data best.
Notice the characteristic feature of the method: you do lots of experiments, each one giving you one piece of information.
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Risk assessment
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updated: 26 January 2006
