Practical investigations
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For my A2 coursework I am investigating the rate of reaction between peroxydisulphate(VI) ions and iodide ions. I have found that both reactants are first order. Please could you help me to link the collision theory to what it has to do with order?
Ulex writes ...
This is a very interesting question for two reasons. Firstly it illustrates a fundamental point in the philosophy of scientific progress and secondly because, as far as I am aware, the ‘answer’ is not fully understood. Between us we may be about to make a very small contribution to chemical knowledge!
In order to react, particles must collide with each other. The ionic equation for this reaction involves one ion of peroxodisulphate reacting with two iodide ions. If this equation were to represent the actual mechanism, two things would follow:
1 Three particles would have to collide simultaneously – what is called a three-body collision. Statistically, this is a very rare event; the reaction may be slow but it isn’t as slow as that.
2 The reaction would be first order with respect to peroxodisulphate (true) but second order with respect to iodide (not true). I have looked up various accounts of this reaction and all those that mention order agree with you. Are you happy with this connection between order and collision? If two iodide ions were involved in the collision, doubling the concentration would result in four times the number of collisions, hence second order.
OK, so the overall ionic equation does not represent the mechanism. So, what is the mechanism?
If the reaction does not occur in one step, it must happen in a series of steps. The whole reaction cannot go faster than the slowest of these steps. (Think of two cars going along a narrow lane, a slow one in front and a potentially fast one behind it – the second car may be capable of going quickly but it can’t do so because the first one is controlling the overall speed).
There must be at least two steps. Let’s guess it’s actually two, a slow one (rate-determining) and a faster one to follow. One peroxodisulphate ion and one iodide ion must collide in the slow step. This would explain the first-order kinetics with respect to each of the two reactants. The other iodide ion presumably takes part in the later, fast step.
That’s as far as we can go by deduction from the existing evidence and even to get this far we have had to make a guess ( philosophers of science call this a ‘hypothesis’ which is a polite word for a guess!).
Any description of a possible mechanism involves further guesses. People would then have to do more experiments to try to find out if any proposed mechanism is possible.
I have discussed this with several other chemists. One suggests a clever mechanism involving two steps, the slow, first step producing an ion I+. This is not as unlikely as it seems at first. I+ is not a stable ion but there is some evidence that it exists as a short-lived species. It would also react very quickly with I- to give I2.
Perhaps you would like to try to write out the mechanism proposed.
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updated: 26 March 2007
