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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: 144730 on January 13, 2017, 11:36:02 AM

CH₃COCH₃ (aq) + I₂ (aq) > CH₃COCH₂I (aq) + H⁺ (aq) + I⁻ (aq)
Samples at 30, 60, 180, 270 and 360 seconds were neutralized and titrated with Na2S2O3.
The concentrations of I2 used were 0.01, 0.02, 0.03, 0.04 and 0.05 mol dm3.
According to literature the reaction is zero order with respect to iodine.
Concentration of iodine solution (mol dm3) Initial rate of reaction (ml s1)
0.010  0.000289
0.020  0.001000
0.030  0.001839
0.040  0.002984
0.050  0.004785
This produced an graph which was clearly first order for I2.
What could the reasons for this be?

It's clearly not first order. Have you done a loglog plot?
Did you keep everything else constant?
What is your "initial rate" measurement? Loss of iodine? Are your units mol/L/s? If so, except in the first case, you would have used up all your iodine in less than 30 seconds. Something is obviously wrong with your data.

I plotted the initial rate of reaction against the concentration of the iodine solution.
I have not done a loglog plot, I am not sure how to.
Everything else was kept constant.
The initial rate measurement is the tangent to the line of bestfit for the volume (cm3) of Na2S2O3 needed to titrate the samples at 0.0001 seconds.

It should be clear from your plot that the relationship is not linear.
For a loglog plot you simply plot ln(initial rate) vs. ln(concentration).
If y = ax^{b}, ln y = ln a + b*ln x, so the slope of the graph gives the exponent b.
But it's still wrong if it is meant to be zero order.
Are you drawing tangents by eye? That is notoriously difficult to do reliably.
What do you get if you estimate initial rate = {titre(30 s)  titre(0 s)}/30 ?