Rates of reaction - kinetics
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I am investigating the rate of reaction between manganate(VII) and oxalate ions. As far as my experiment goes, I have noticed that the rate of reaction with respect to the manganate(VII) ions is first order. However, I cannot find any material which exactly the states the rate equation for this reaction, so I am a bit unsure. Also please could you give me a prompt as to how I can investigate the catalytic effect of the manganese(II) ions?
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Ulex writes
It seems surprising but this much-studied reaction does not appear to have been thoroughly investigated and I don’t know where you might find a complete rate equation. Attention usually focuses on its autocatalytic behaviour, Mn2+ ions being a product of the reaction.
I have noticed, however, that when my own students have done the conventional experiment they get a first order curve for the catalysed reaction. I am sure you are right about this being first order with respect to manganate(VII).
As to investigating the effect of the catalyst. I take it that you have already seen the anomalous graph for the ‘uncatalysed’ reaction where the rate is very slow to begin with and then accelerates before decelerating again in what looks like a first order manner. You have to bear in mind the existence of this effect if you are to investigate the effect of changing the catalyst concentration because this concentration is not going to remain constant.
You could try repeating your experiment using varying initial concentrations of manganese(II) sulphate but you will have to compensate for the fact that this concentration increases during the reaction. A set of experiments where you measure the initial rate would be best because the increase of concentration will not be excessive over the first few seconds/minutes of the reaction. You could even calculate what the increase is because the increase of [Mn2+] would equal the decrease of [MnO4-].
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updated: 11 April 2008
