IB Chemistry

Using colorimetry to find the order of a reaction

introduction
Coloured solutions absorb some of the wavelengths in the visible region of the electromagnetic spectrum. A colorimeter can be used to measure the amount of light absorbed by a solution (the absorbance) and this is proportional to the concentration of the coloured species present. In this experiment you will use a colorimeter to investigate the reaction between bromine and methanoic acid:-
Br2 (aq) + HCOOH (aq) ↓ 2Br- (aq) + 2H+ (aq) + CO2 (g)

Bromine in aqueous solution is orange-yellow, but all the products and the methanoic acid are colourless. The colour gradually fades as the reaction proceeds. In this experiment we wish to determine the order of the reaction with respect to bromine. By using a concentration of methanoic acid which is 10 times greater than that of the bromine we can assume that the concentration of methanoic acid is constant throughout the experiment.

method
1) Set up the colorimeter according to the instructions. Select a blue or green filter.
2) While you are waiting, prepare a results table with TIME in seconds and ABSORBANCE (in relative units). You are going to read the absorbance after 15, 30, 45 seconds, etc. until you have about 9 or 10 readings.
3) Fill a sample tube with distilled water and place in the colorimeter. Adjust the controls to give zero absorbance on the scale.
4) Mix 10cm3 of 1.0M methanoic acid with 10cm3 of the bromine solution (approximately 0.1M) in a boiling tube and start a stop clock.
5) Gently shake the mixture then transfer some to clean colorimeter sample tube and place in the colorimeter. Record the absorbance every 15 seconds in your results table.

Alternatively the colorimeter could be connected to a Data Logger for automatic collection of data. The output voltage of the colorimeter (in mV) is proportional to the absorbance and, therefore, to the concentration of the bromine.

interpreting results
The rate equation involving bromine is one of the following:-
Rate = k (zero order)
Rate = k[Br2] (1st order)
Rate = k[Br2]2 (2nd order)

Decide what you would expect a graph of concentration of bromine against time to look like in each of the above cases.
Plot a graph of Absorbance against Time either by hand or by putting your data into Excel. Time should be on the x-axis (horizontal).
Remember that absorbance is proportional to concentration. Fit an appropriate trendline in Excel or do mathematical tests on your hand-drawn graph to find out which is the correct order for this reaction.