Nuffield Advanced Chemistry - I'm doing an investigation on the rate of reaction between butanone and iodine, expanding on the iodine and propanone. I have just completed a colorimetry experiment - measuring the change in colour of the iodine. As I made up stock solutions of hydrochloric acid, distilled water and iodine, to save time, I did repeats of the investigation. As a consequence, my results do not agree, and although the plots have roughly the same gradient - for example the first time I did it the initial colour absorbance was 1.08, the second 1.20, the third 1.29. Yes, I did them on different days, which I think may have had an affect. I am not sure how to analyse the results, as they are all so different, and taking an average would be unreliable?! I am also not sure how the stock solutions would have affected the results, or whether there was any other explanation?

Rates of reaction - kinetics (A2)

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I'm doing an investigation on the rate of reaction between butanone and iodine, expanding on the iodine and propanone. I have just completed a colorimetry experiment - measuring the change in colour of the iodine.

As I made up stock solutions of hydrochloric acid, distilled water and iodine, to save time, I did repeats of the investigation. As a consequence, my results do not agree, and although the plots have roughly the same gradient - for example the first time I did it the initial colour absorbance was 1.08, the second 1.20, the third 1.29. Yes, I did them on different days, which I think may have had an affect. I am not sure how to analyse the results, as they are all so different, and taking an average would be unreliable?! I am also not sure how the stock solutions would have affected the results, or whether there was any other explanation?

Ulex writes

I think the most likely source of the discrepancy is that on different days the temperature was different. As you know, temperature change makes a big difference to reaction rates. As confirmation of this, the gradients of the two sets of result are the same, suggesting that each set of results is valid and that one set does, in fact, serve as a check on the other.

I think your teacher is quite correct to suggest that making stock solutions should not be a problem; in fact it is surely the more satisfactory thing to do because it eliminates one source of inconsistency.

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updated: 18 December 2007

Rates of reaction - kinetics (A2)

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