Rates of reaction - kinetics (A2)
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When carrying out an experiment to find the effect of temperature on the rate of reaction between sodium thiosulphate and hydrochloric acid - why is 1/time plotted on the graph?
Corrie writes ....
As you probably know, the rate constant, k, varies with temperature as described by the Arrhenius equation. Written in terms of log (or ln) k, it looks like this:
Log (k) = -Ea/RT + a constant
Plotting log (or ln) k vs 1/T (T in Kelvin) should produce a straight line, the gradient of which is = -Ea/R. From this you can find a value for Ea.
Let's say the rate equation for this reaction is
Rate = k[HCl]x.[Thiosulphate]y
If the concentrations are kept the same in each experiment, as they in your experiment, then it follows that k is proportional to the rate of the reaction. Assuming the reaction proceeds at a more or less contant rate initially, then 1/time (where the time is that taken to reach a certain point) is a measure of the rate - if the reaction takes twice the time, then the rate is halved, and so on.
Thus 1/time is proportional to the rate, which is in turn proportional to the rate constant here. So instead of log (or ln) k for the Arrhenius plot, log (or ln) 1/time can be used. The slope of the resulting line will be the same and can be used to find a value for the activation energy.
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updated: 28 November 2007
