Ahh.. I have this problem writing the evaluation for my piece of coursework. I've looked everywhere but can't find a better solution. Here is my problem: I had to mix HCl and sodium thiosulphate in a beaker and simultaneously start timing how long it takes for the reaction to reach the end-point. Now, in the above, a procedural error has formed as it is difficult to start the stopwatch exactly when the two solutions are mixed and this will slighlt vary for each repeat. I can't find any way to remedy this. I could say that I'd use a friend to start the timing while I combine the two reagents but I can't think of anything else. Any chemists out there who can think of a different method that would improve accuracy? This is for my A2 OCR: Determination of the rate equation Assessment.
To be honest I think the error will be that small that it's barely worth thinking about other than to make a qualitative mention of it somewhere in your report.
Yep, I mentioned it but the mark scheme also says that I have to suggest a suitable modification to improve the accuracy. I fail to see how it can be improved :\.
You're kind of limited with the kit you can use at A2 level. The way of doing it properly would be to monitor the concentration decay of thiosulfate spectrometrically (UV-vis) and then make a plot showing how [thiosulfate] varied with time: 3Na2S2O3 + 2HCl -> 2Na2SO4 + 2NaCl + 4S + H2O
Yeah, that's one way of doing it. Increase the range of data and you decrease the standard error. Another consideration would be to stir your thiosulfate as you added your acid. Under your experimental conditions there will always be a time lag between mixing the reagents and starting the clock.
Yeah, we still used the KS3/ GCSE technique where you count the time it takes for the black cross placed underneath the beaker to disappear . We don't have that kind of apparatus at our college. I'm already repeating the same experiment three times until I get results within 1 second of each other so it should be pretty accurate. Thanks for the advice though, I think I will say exactly that under these conditions there will always be a time lag as you can't really fix it even if there were two persons. It's also the fact that it depends when you start the clock - if you do it on initial contact between the two solutions (when not all the solution is porued in) then the reaction will be slower because of the low initial volume (in the first 1-2 seconds). I'm not good at explaining this sort of stuff -
Given the experiment you've described I'd be less concerned about the time lag at the beginning, more worried about when the actual end point (cross disappearing) is. I don't suppose it's easy to tell the moment the cross has just disappeared. It's a pants experiment.
It is, yes and that was another point I mentioned as an error in measurement. It's especially harder when the concentration is lower (as I'm varying the concentrations of the two). I'm currently researching to see if there's a way to improve the end-point determination.
Your sad Nah i'm kinda into chemistry a bit myself, well I was in school and that, then I left and didn't really continue it, the same with maths actually...I was real good with that.
This is real deja vu. I am doing this experiment for my 'SAT's practical assessment for chemistry' We use the black crosses.
I remember doing chemistry coursework, though it was quite a while ago. I did the other option though. I'd say use a friend because the time difference is going to be less than a second or so and if your smallest unit of time is seconds, it will hardly make any difference.