Rates of reaction - kinetics (A2)
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When determining the order of reaction of iodine with propanone, iodine is allowed to react with excess propanone. Why is the iodine in excess and not the propanone? Why do we use sodium hydrogencarbonate to stop the reaction but cannot use sodium hydroxide?
Jenkin writes …
When you are trying to discover the order of reaction with respect to a certain reactant, in this case iodine, you use a large excess of all other reactants so that their concentrations stay virtually constant throughout the reaction. You are therefore ensuring that only one variable is changing. You could then carry out another experiment in which you used a large excess of iodine, so as to find the order with respect to propanone.
Sodium hydrogencarbonate will stop the reaction by neutralising the acid present (and acid concentration is a third possible term in the rate concentration). You cannot use sodium hydroxide because it will react with the remaining iodine, aand you are probably following the reaction by titrating this iodine with standard sodium thiosulphate solution.
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updated: 20 August 2003
