Rates of reaction - kinetics
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I cannot find the units of absorbance anywhere. Can you please tell me. I am plotting absorbance vs time graph to find the rate when doing colorimetry to plot the Arrhenius equation. Also how to do error bars on graphs and if it is necessary? Lastly, what effect it would have on reaction if you use iodine in water instead of iodine in potassium iodide. Thank you for your help.
Ulex writes
Absorbance is essentially a ratio; it is meter reading of solution/meter reading using pure water. You may not have actually taken a meter reading using pure water but by not altering the lighting conditions through your colorimeter, it is as if you had. Ratios have no units.
According to the Lambert-Beer Law, absorbance is proportional to concentration. Your rates of reaction are therefore in units of time-1. It does not matter what these units actually are because you are plotting the logarithms of the rates and you are looking for the gradient of the resulting graph. Using different units for concentration (and therefore rate) simply moves the graph up or down without altering its gradient.
Personally I would not use error bars here. Instead I would take the points as plotted and draw three straight lines – one of best fit, one with the maximum gradient you could reasonably draw through the points and one with the minimum gradient. Calculate activation energies using all three values of the gradient. Doing this gives an estimate of your likely error.
Iodine is virtually insoluble in pure water so it would be next to impossible to do reactions with aqueous iodine. The situation is actually a bit more complicated than this; iodine reacts slightly with water to give iodate(I) ions and iodide ions. The latter then react with iodine molecules to give the familiar red-brown colour of iodine in iodide!
Always carry out a risk assessment and check with your teacher before starting any practical work.
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updated: 01 May 2007
