Rates of reaction - kinetics (A2)
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I am trying to find out what order the potassium iodide and the potassium peroxodisulphate are for the rate equation Rate = k[KI][K2S2O8] using colorimetry. I am a bit confused how the colorimetry technique works in terms of finding the orders. Please help. Thanks very much.
Igloo writes ...
Your colorimeter readings - via a calibration curve - will have given you concentrations of (coloured) I2 at different times. From knowledge of the equation:
K2S2O8 + 2KI ---> 2K2SO4 + I2
you can easily calculate the concentrations of KI or K2S2O8 remaining at any stage.
At this point, it is hard for me to be very specific in my help, without knowing more about what you've done. What follows is my best guess as to the procedure you've used.
Presumably you have followed the reaction using differing concentrations of KI, whilst keeping the concentration of K2S2O8 constant, perhaps by using an excess of K2SO4. If so, your colorimetry readings, as explained above, can give you measurements of concentration of KI at given times, and these results can then be plotted on a graph. If, for example, the graph has a constant half-life, this would tell you that the reaction was first order with respect to KI. If, on the other hand, the graph is a straight line, i.e. the rate is constant, then the reaction is zero order wrt KI.
If the reaction is also followed using different concentrations of K2S2O8, whilst keeping KI constant, in a similar fashion you will be able to find the order of reaction with respect to K2S2O8.
If you've used a different approach then you'll need to get back to us via React.
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I do not understand why you need to use the calibration curve. You have a known concentration of both the reactants (one of which you vary)and you are timing the reaction until a certain, set, colour has been reached. Surely thats all the information you need to get the order of reaction with respect to the reactants. Why would you need to read off the calibration curve and work out the concentration of iodine? I do not see why we need to know the concentration of iodine.
GREG 13 January 2008
updated: 02 March 2007
