Rates of reaction - kinetics (A2)
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I have done a practical on rates of reaction of Na2S2O3 with HCl, Na2S2O3 was 1st order...simple. Using initial rates and plotting rate/conc the graph for HCl was a curve, but instead of getting steeper as the book says a second order curve should, it gets less steep. All I can think is that as you increase conc of HCl, increase in rate becomes less? But this makes no sense. Please could you explain what might have happened and what should have happened!
Ulex writes
To start with the last bit, I’m not at all sure what should have happened, i.e. I don’t know what the order of the reaction is with respect to HCl nor do I recall seeing anywhere the results of an experiment to determine it! We are therefore in unknown territory – genuine research in fact. You seem to be reversing the normal procedure and varying the concentration of HCl. Did you therefore use a relatively high concentration of thiosulphate (so that its concentration remains constant) and a succession of relatively low ones for the acid? If you did, then the next thing is the application of the initial rate method. You are effectively using the general rate equation
Rate = k [HCl]n where n is the order of reaction.
If you plot rate against [HCl] (assuming n=1) and get a curve, then the assumption is wrong and the reaction is not first order. The next thing to do is to try plotting rate against [HCl]2. If this is a straight line then the reaction is second order. If your mathematics is up to it, try plotting lg[rate] against lg[HCl]; you should get a straight line whatever the order of the reaction and the gradient of this line is the value of n.
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updated: 17 December 2003
