We can use a colorimeter to follow the rate of a reaction if one of the reactants or products is coloured. This is an example of continuous monitoring as measurements are taken throughout the course of the reaction.
Iodine reacts with propanone in the presence of an acid catalyst. As the reaction proceeds the iodine is used up and the reaction mixture changes from a orange-brown colour to colourless.
I2(aq) + CH3COCH3(aq) ⇾ CH3COCH2I(aq) + H+(aq) + I–(aq)
A colorimeter measures the absorbance of particular wavelengths of light passing through a coloured solution. We can use these absorbance readings to determine the concentration of that solution since the Beer-Lambert law tells us that the concentration of a solution is proportional to the absorbance.
When we place a sample of the reaction mixture into the cuvette, the [I2] will decrease as it reacts with the propanone, and the solution will absorb less light (the absorbance readings decrease).
We can record the absorbance at set time intervals but how do we know the actual [I2] at each time interval?
We need to start by constructing a calibration curve that links different [I2] to specific absorbance readings e.g. if we prepared a series of iodine solutions at 0.010 mol dm-3, 0.008 mol dm-3, 0.006 mol dm-3, 0.004 mol dm-3, 0.002 mol dm-3 and 0.000 mol dm-3 we could measure the absorbance of each in turn, and then plot a calibration curve.
Now we are in a position to carry out the reaction between iodine, propanone and the acid catalyst in a series of experiments that will allow us to determine the order of reaction with respect to each reactant, write a rate equation and calculate a value for the rate constant, k.
Experiment 1 – to find the order of reaction with respect to iodine, we need to vary the [I2] but keep the [propanone] and [H+] constant. It is important that the [propanone] and [H+] are in excess so that any variation in the initial rate of reaction is due to the [I2].
e.g. we could start by mixing together 0.008 mol dm-3 iodine solution and 0.4 mol dm-3 hydrochloric acid. We would then add 0.4 mol dm-3 propanone and quickly transfer a sample of the reaction mixture to a cuvette and place in the colorimeter, recording the absorbance reading every 30 seconds for 6 minutes.
You can see a good video of the practical technique here.
We would then repeat the experiment, this time starting with 0.004 mol dm-3 iodine solution, 0.4 mol dm-3 hydrochloric acid and 0.4 mol dm-3 propanone. Finally we repeat the experiment with 0.002 mol dm-3 iodine solution, 0.4 mol dm-3 hydrochloric acid and 0.4 mol dm-3 propanone.
We can use the calibration curve to convert each of the absorbance readings into a concentration of iodine …
and then plot a graph of [I2] against time for each experiment. We can then calculate the gradient of each curve at t=0 to give us the initial rate of reaction for that concentration of reactant.
Finally, by plotting a graph of initial rate of reaction against [I2] we can determine the order of reaction with respect to iodine.
Take a deep breath …
Experiment 2 – to find the order of reaction with respect to propanone, we need to vary the [propanone] but keep the [I2] and [H+] constant.
e.g. we could start by mixing together 0.004 mol dm-3 iodine solution and 0.4 mol dm-3 hydrochloric acid. We would then add 1.2 mol dm-3 propanone and quickly transfer a sample of the reaction mixture to a cuvette and place in the colorimeter, recording the absorbance reading every 30 seconds for 6 minutes.
We would then repeat the experiment, this time starting with 0.004 mol dm-3 iodine solution, 0.4 mol dm-3 hydrochloric acid and 0.8 mol dm-3 propanone. Finally we repeat the experiment with 0.004 mol dm-3 iodine solution, 0.4 mol dm-3 hydrochloric acid and 0.4 mol dm-3 propanone.
Once again, we can use the calibration curve to convert each of the absorbance readings into a concentration of iodine and plot a graph of [I2] against time for each experiment, and calculate the gradient of each curve at t=0 to give us the initial rate of reaction for that concentration of reactant.
And by plotting a graph of initial rate of reaction against [propanone] we can determine the order of reaction with respect to propanone.
Take another deep breath …
Experiment 3 – to find the order of reaction with respect to the acid catalyst, we need to vary the [H+] but keep the [I2] and [propanone] constant.
e.g. we could start by mixing together 0.004 mol dm-3 iodine solution and 1.2 mol dm-3 hydrochloric acid. We would then add 0.4 mol dm-3 propanone and quickly transfer a sample of the reaction mixture to a cuvette and place in the colorimeter, recording the absorbance reading every 30 seconds for 6 minutes.
We would then repeat the experiment, this time starting with 0.004 mol dm-3 iodine solution, 0.8 mol dm-3 hydrochloric acid and 0.4 mol dm-3 propanone. Finally we repeat the experiment with 0.004 mol dm-3 iodine solution, 0.4 mol dm-3 hydrochloric acid and 0.4 mol dm-3 propanone.
Use the calibration curve to convert each of the absorbance readings into a concentration of iodine and plot a graph of [I2] against time for each experiment, and calculate the gradient of each curve at t=0 to give us the initial rate of reaction for that concentration of reactant.
The very last graph is a plot of initial rate of reaction against [H+] from which we can determine the order of reaction with respect to H+.
Kinetics experiments always take a very long time but now we have reached the end we can say that
rate = k [CH3COCH3] [H+]
You can find out more about rate expressions here 😊.