Rates of reaction - kinetics (A2)
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I am carrying out a investigation into the rate of reaction between sodium thiosulphate and hydrochloric acid. I am investigating how concentration of each of the reactants affects reaction rate and hence find the overall rate equation for the reaction. I know I will measure the time for the cross on the paper underneath the reaction vessel to become obscured by solid sulphur for each concentration and I will the be able to get the rate using rate=1/time for cross to obscure. However, I do not know how to then calculate the order of the reaction with respect to each reactant and the overall rate equation
Ulex says
You are almost there! Think in terms of rate equations. The general form of a rate equation is
Rate = k[A]n[B]m
You have, I hope, kept one concentration constant while you vary the other so you have two sets of results. When the thiosulphate concentration varies, the rate equation becomes
Rate = constant x [thiosulphate]n
You now have two ways to proceed, a clumsy way and an elegant way!
The clumsy way is to plot a graph of rate against [thiosulphate]. If n=1 the graph will be a straight line. If it isn’t, try plotting rate against [thiosulphate]2. A straight line resulting from this indicates an order of 2. The weakness of this method is that experimental error may result in the line not being quite straight in either case. The elegant method gets round this.
Taking logarithms of both sides of the rate equation gives:
Lg(rate) = lg(constant) + n Lg(thiosulphate)
If you plot lg(rate) against lg[thiosulphate] you should get a straight line whatever the order of the reaction. Draw the best straight line you can and find its gradient, which is n, the order of the reaction.
Repeat for the other set of results.
By the way, Experiment 11.4 in the Nuffield Chemistry Students’ Book has a rather neater alternative to using a cross on a piece of paper under the reaction vessel, which you might adapt if you have access to a copy. The piece of paper tends to get wet and soggy. Experiment 11.4 also uses rather smaller volumes of solution which might appeal to your lab technician!
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updated: 26 January 2006
