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# problem calculating rate constant (k) Watch

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1. im doing the iodine clock reaction for my A2 coursework (salters chemistry).

i have some results but when i use the equation i end up with a rate constant that changes between results:

KI = 1mol dm-3
potassium peroxodisulfate = 0.04 mol dm-3
sodium thiosulfate = 0.01 mol dm-3

Mixture.......[Iodide]........time/s..............rate
1................1.............. ...94.98 s............10.5x10-8
2................0.8............ ..151.01 s............6.62x10-8
3................0.6............ ...213.75 s...........4.6x10-8
4................0.4............ ...314.18..............3.18x10-8
5.................0.2........... ...853.36............1.17x10-8

the amount of iodine converted back to iodide ions is 1x10^-5, working from the calculations in the TL4.2 handout.

it said if you divide that by the time you get the rate. so i did it.

(i did the peroxodisulfate ions in a similar experiment and they are 1st order)

but when i divide the rate by the two reactants for the rearranged equation k keeps changing. am i missing something obvious?

thanks

p.s) in my calculations im only changing the [i-] do i need to change the [2so82-] as well. it doesnt say in the handout and im confused.
2. here are my calculations.

mix 1:
k = 10.5x10-8 / (1 x 0.04) = 2.625x10-6

mix 2:
k = 6.62x10-8 / (0.8 x 0.04) = 2.06 x10-6

mix 3:
k = 4.6x10-8 / (0.6 x 0.04) = 1.91x10-6

mix 4:
k = 3.18 x10-8 /(0.4x0.04) = 1.98x10-6

is the .7 change in value acceptable or should i redo the experiment? the first experiment had a much larger variation of k (about 30 times) and its worrying me.
3. (Original post by Spikeymikey22)
here are my calculations.

mix 1:
k = 10.5x10-8 / (1 x 0.04) = 2.625x10-6

mix 2:
k = 6.62x10-8 / (0.8 x 0.04) = 2.06 x10-6

mix 3:
k = 4.6x10-8 / (0.6 x 0.04) = 1.91x10-6

mix 4:
k = 3.18 x10-8 /(0.4x0.04) = 1.98x10-6

is the .7 change in value acceptable or should i redo the experiment? the first experiment had a much larger variation of k (about 30 times) and its worrying me.
The values are not outside the limits of possibility taking into account experimental inaccuracy.
If you repeat experiments you get more reliable results and improve precision (not necessarily more accurate)
4. thanks

i'll write a load of information about temperature/human error/equipment uncertainty and refer to collision theory. thanks for the help.

p.s) would an average value for k be acceptable between the 5 results or would that be frowned upon by an examiner?
5. (Original post by Spikeymikey22)
thanks

i'll write a load of information about temperature/human error/equipment uncertainty and refer to collision theory. thanks for the help.

p.s) would an average value for k be acceptable between the 5 results or would that be frowned upon by an examiner?
I can see nothing wrong with averaging your values. It doesn't remove systematic error, but it gives a ballpark figure for your experiment.
6. thanks for the help, you've made my christmas less stressful since we only have a week before school (and lab access) ends

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Updated: December 8, 2010
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