Daedalus Posted April 13, 2010 Report Share Posted April 13, 2010 Hi, this is a little bit last minute so if you know the answer, please reply asap!My experimental data for this experiment was off so I'm not exactly sure which direction to go.The reaction between sodium thiosulphate and hydrochloric acid isNa2S2O3 + 2HCl → 2NaCl + S + SO2 + H2OI think, but it also has something to do with this:"To demonstrate the concept of reaction rate in chemistry classes. The thiosulfate ion can decompose into the sulfite ion and a colloidal suspension of sulfur, which is opaque. The equation for this acid-catalysed reaction is as follows:S2O32−(aq) → SO32−(aq) + S(s)" [from wikipedia]What I'm looking for is the rate equation, in the form:rate = k[Na2S2O3]m[HCl]nDoes anybody know what m and n are??Thanks! Reply Link to post Share on other sites More sharing options...
Sandwich Posted April 13, 2010 Report Share Posted April 13, 2010 If I recall correctly, m and n are the number of times each of those things is involved before the rate determining step. So you have to hypothesise the mechanism the reaction followed, work out which is the slowest step and then see how many times those two species are involved prior to the slow step. That'll be what they're to the power of -- i.e. m and n. Reply Link to post Share on other sites More sharing options...
Daedalus Posted April 13, 2010 Author Report Share Posted April 13, 2010 If I recall correctly, m and n are the number of times each of those things is involved before the rate determining step. So you have to hypothesise the mechanism the reaction followed, work out which is the slowest step and then see how many times those two species are involved prior to the slow step. That'll be what they're to the power of -- i.e. m and n.yeah... thing is, my data is so abysmal that m and n are almost impossible to derive. i mean, it's tempting to change the data to suit the theory..but i think in the end i'll use my original data, and talk about the problems in the conclusion/evaluation. i guess sometimes an experiment <i> has </i> to not work out... or at least i hope they won't penalize me too much! Reply Link to post Share on other sites More sharing options...
Sandwich Posted April 13, 2010 Report Share Posted April 13, 2010 If I recall correctly, m and n are the number of times each of those things is involved before the rate determining step. So you have to hypothesise the mechanism the reaction followed, work out which is the slowest step and then see how many times those two species are involved prior to the slow step. That'll be what they're to the power of -- i.e. m and n.yeah... thing is, my data is so abysmal that m and n are almost impossible to derive. i mean, it's tempting to change the data to suit the theory..but i think in the end i'll use my original data, and talk about the problems in the conclusion/evaluation. i guess sometimes an experiment <i> has </i> to not work out... or at least i hope they won't penalize me too much!Have you checked there's not something you've overlooked which would help your data fit? I know I did several labs and was convinced my results were ridiculously rubbish, only to discover my chemistry teacher had explained it wrong and when I found the actual explanation online, suddenly what I'd written a whole paper dissing turned out to be pretty good xP Otherwise it's acceptable to use crap data and just complain about it in your write-up, but you do have to have a good reason which isn't...A) my equipment suckedB) my method suckedC) I sucked at doing it...as for why it went wrong. Preferably some out of control scientific thing like perhaps madly fluctuating room temperature which you couldn't avoid, or anything else which might comprehensibly have thrown your results off. Whatever you do don't pick out fundamental flaws in your method etc.!Good luck! 1 Reply Link to post Share on other sites More sharing options...
Daedalus Posted April 14, 2010 Author Report Share Posted April 14, 2010 hey I finished the lab (rather badly) but I did find something that might be useful to whoever is doing this in the future (I've read tons of people doing it at IGCSE and AP I think, not sure about IB...)The reaction is acid catalyzed and is technically *not*Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2Owhich was what my teacher actually wrote down (!) but simplyS2O32−(aq) → SO32−(aq) + S(s)which means that the rate in turn is, I think, simply rate = k[Na2S2O3]But don't take my word on this. The more I do chemistry the less I understand it! Reply Link to post Share on other sites More sharing options...
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