SRJC H2 Chem 2012 Prelim P2 Soln

SERANGOON JUNIOR COLLEGE General Certificate of Education Advanced Level Higher 2 CHEMISTRY 9647/02 Preliminary Examination Paper 2 Structured Questions (SPA) Suggested Solutions 1 (a) Planning In the presence of hydrogen ions, H +, bromate( V) ions, BrO 3- , oxidise bromide ions, Br–, to bromine, Br2. BrO3–(aq) + 5Br– (aq) + 6H+(aq) 3Br2(aq) + 3H2O(l) The reaction is relatively slow and can be followed by adding aqueous phenol and the indicator methyl orange to the reaction mixture. As bromine is formed, it reacts rapidly with the phenol present until the latter is used up. The free bromine now in solution bleaches the methyl orange indicator. The initial rate of the reaction can be investigated by measuring the time taken to bleach the methyl orange indicator. You are to plan a series of experiments, to determine the order of reaction with respect to the bromide ion. In addition to the standard apparatus present in a laboratory, you are provided with the following: FA 1 0.01 mol dm-3 aqueous KBr. FA 2 1.0 mol dm-3 potassium bromate(V), KBrO3. FA 3 1.0 mol dm–3 sulfuric acid, H2SO4. Aqueous phenol containing methyl orange indicator Distilled water (i) Complete the table below and outline, by means of a series of numbered steps, · the apparatus to be used · the experimental procedure · the measurements to be taken, to collect the required data.

Expt . Volume of phenol/methyl orange indicator solution / cm3 Volume of FA1 / cm3 Volume of FA2 / cm3 Volume of FA3 / cm3 Volume of distilled water / cm3 1 20.0 50.0 50.0 20.0 0.0 2 20.0 40.0 50.0 20.0 10.0 3 20.0 30.0 50.0 20.0 20.0 4 20.0 20.0 50.0 20.0 30.0 5 20.0 10.0 50.0 20.0 40.0 1. Using a measuring cylinder, add 20.0 cm3 of the phenol/indicator solution into a clean, dry conical flask. 2. Using different measuring cylinders, place 50.0 cm3 of FA 1 and 20.0 cm3 of FA 3 into the conical flask. 3. Place the conical flask on a white tile. 4. From another measuring cylinder, measure 50.0 cm3 of FA 2. 5. Add FA 2 into the conical flask, simultaneously starting the stopwatch. Swirl the conical flask carefully. 6. Stop the stopwatch when the colour of the indicator just disappears to leave a colourless solution. Record the time taken. 7. Repeat procedure 1 to 6 for experiments 2 to 5.

(ii) In order to find the order of reaction with respect to bromide, a graph of log10( 1 t) against log10(volume of KBr(aq)) can be plotted. Use the rate equation to derive a relationship between log 10( 1 t ) and log10(volume of KBr(aq)). Hence, explain how the o rder of reaction with respect to bromide can be found from the plotted graph. In these experiments, the total volume has been kept constant and only the concentration of FA 1 in the reaction mixture has been changed. The rate equation, where n is the rate order with respect to FA 1, can be simplified to rate = k’[Br-]n (where k’ = k[BrO3-]m[H+]n) · taking logarithms of the factors in this equation gi