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You will follow the concentration change of one of the reactants by the method of comparative visual colorimetry. From data obtained experimentally plot graphically the change of the reactant concentration in dependence on time. 

1. Making of the comparative colorimetric scale of bromine solution Measure with a syringe into 10 identical test-tubes the following quantities of bromine water (0.01-molar): into the first one –10.0 cm3 ; 2nd – 9.0 cm3; 3rd – 8.0 cm3, ....... 9th – 2.0 cm3; into the tenth one – 1.0cm3. Then add to all the test-tubes (except the first one) distilled water to reach a total volume of 10.0 cm3 in each. Seal the test-tubes with stoppers and mix the solutions. Put the test-tubes in a stand with a white background. Finally calculate the concentration (in mol dm-3) of bromine in the solutions in all testtubes. 

2. Reaction of bromine solution with formic acid Carry out the reaction by mixing 100.0 cm3 of bromine solution with 1.0 cm3 of 1.00- molar solution of formic acid. Immediately after mixing transfer 10.0 cm3 of the resulting solution to the test-tube identical with that used for colorimetric scale. By comparing the colour shade of the reaction mixture (in one-minute intervals) with that of the solutions in the scale, investigate changes of bromine concentration in dependence on time. Put the data in a table containing time (t) and concentration of Br2

Task: Plot the bromine concentration in dependence on time a read the half-time of the reaction from the diagram.

1. Write the equation for the reaction of bromine with formic acid assuming that the reactants are in stoichiometric amounts. 

2. In analytical chemistry a volumetric solution of bromine can be prepared by dissolving a mixture of bromate and bromide in acid medium. Explain this mode of preparation by means of a chemical equation in ionic form. 

1 Answer

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1. HCOOH (aq) + Br2 (aq) → CO2 (g) + 2H+ (aq) +2Br(aq) 

2. BrO-3 + 5Br− + 6H+ →3 Br2 + 3H2

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