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 gives lg(rate) = n × lg ([Br-]) + lg (k) Hence, by finding the gradient of the plotted graph, order of reaction wrt Br - can be found. (iii) The concentration of the phenol used in the experiment is very low. Suggest why this is so. [7] If too much phenol was present, it is possible that the reaction could have taken longer OR if a large amount of phenol was added the mixture may not have decolourised at all as all the bromine formed would have reacted with the phenol present.

(b) An experiment was carried out to measure the enthalpy change for the reaction of zinc with aqueous copper (II) sulfate. The equation for the reaction is: Zn (s) + CuSO4 (aq) à ZnSO4 (aq) + Cu (s) · A measuring cylinder was used to transfer separate 50 cm 3 samples of 1.25 mol dm-3 copper (II) sulfate solution into polystyrene cups. · Different weighed amounts of zinc powder were added to each sample of copper (II) sulfate. · Each mixture was stirred thoroughly and the temperature rise noted. The results of the experiments are summarised on the graph below. (i) Explain why the graph shows an initial rise in temperature and then levels off. Initially CuSO4 in excess so amount of reaction depends on amount of Zn Or more CuSO4 reacts (as more Zn added) Graph levels off because all CuSO4 used up (reject just ‘Reaction is complete’) (ii) Using the data from the graph, calculate the ΔHreaction. Q = 50 x 63.5 x 4.18 =13271.5 J Amount of CuSO4 = 50 x 1.25 1000 = 0.0625 mol ΔH = - 13271.5 0.0625 = - 212 x 103 J mol-1 (iii) Suggest a simple practical change to the method that will make the experiment more accurate. [5] Use a lid on the cup (to reduce heat loss), extra insulation for cup, weigh CuSO4 solution, use burette/pipette to measure volumes (Reject Repeat experiments OR use more accurate balance OR Smaller mass intervals) Total 12 marks 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 Mass of zinc / g Temperature / oC

2 (a) Pyruvic acid is an important component in living cells as it is involved in the aerobic process of supplying energy. The flow chart shows a series of reactions starting with compound A, which has an empirical formula of CH2. CH3CCOOH O (i) Draw the structures of compounds A, B and C in the boxes above. A B C C C H H H H C CH3 H OH CN C CH3 OH COOH H (ii) State the reagents and conditions for Steps I and II. [5] Step I: methanolic NaBH4 or H2 nickel catalyst, heat or H2 platinum, rtp Step II: I2 (aq), NaOH (aq), warm, followed by H+(aq) Cr2O72- / H+, distilation Ethanal I HCN, NaCN, cold H+, heat Ethanedioic acid II A B C Pyruvic acid H2O /H+

(b) Element D can form two different chlorides. The two chlorides of element D is commonly used in Organic Chemistry qualitative analysis to test for a specific functional group. When dissolved in a solution containing methyl orange, chlorides of element D turn the solution red. 10 cm 3 of liquid organic compound E, C nH2n+2O, is vaporised and burnt in excess oxygen. After the reaction is cooled to 25 oC, a contraction of 20 cm 3 in the gas volume was observed. When the resultant gases from the combustion was passed through aqueous sodium hydroxide, the gas volume decreased a further 20 cm 3. The vapour of E is also observed to react with the same reagents and conditions of step II mentioned in (a). (i) State the identities of element D and organic compound E. D is phosphorus. E is ethanol. (Since 10 cm 3 of vapour E combusted to give 20 cm 3 of CO2, by Avogadro’s and volume ratio, n =2 è C2H6O (ii) Hence, write an equation, if any, between one of the chlorides of element D and organic compound E. CH3CH2OH + PCl5 à CH3CH2Cl + POCl3 + HCl or 3CH3CH2OH + PCl3 à 3CH3CH2Cl + H3PO3 Total 8 marks

3 (a) (i) Both strontium and manganese are silvery metals. Write the electronic configurations of manganese and strontium. Mn: [Ar]3d54s2 Sr: [Kr]5s2 (ii) Manganese and strontium both contribute two electrons into the sea of delocalised electrons. Suggest if strontium or manganese has a higher melting point. Cationic radius of manganese is smaller as compared to strontium ions. Electrostatic forces of attraction b/w the cations and sea of delocalised electrons(metallic bonding) is stronger in Mn than in Sr . Thus more energy is required to overcome these forces of attraction. Mn has a h