EJC Prelim H2 Chemistry Paper 4 Solution

© EJC 9729/04/J2Prelim/25 [Turn Over 1 Investigation of redox reactions involving D-glucose D-Glucose is a sugar with molecular formula C6H12O6, which can react directly or indirectly with potassium manganate(VII), KMnO4. (a) FA 1 is a solution of potassium manganate(VII), KMnO4. FA 2 is a 1.0 mol dm−3 sulfuric acid solution, H2SO4. You are also provided with a solution of D-glucose and bench reagents. Perform the tests described in Table 1.1 and record your observations in the table. Table 1.1 test observations (i) Add 1 cm depth of FA 1 to a test -tube. Add an equal volume of aqueous sodium hydroxide. Add 1 drop of glucose solution and shake thoroughly. Allow to stand for 10 minutes, with occasional shaking. Continue with the remaining parts of Question 1. Purple FA 1 turns green, then to yellow and a brown ppt is formed in colourless solution on standing. (ii) Add 1 cm depth of FA 1 to a boiling tube. Add 2 cm depth of FA 2. Add 1 cm depth of glucose solution and warm gently. Shake well to mix. Purple FA 1 turns colourless. [2] EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2025 General Certificate of Education Advanced Level Higher 2 Paper 4 Suggested Solutions

2 © EJC 9729/04/J2Prelim/25 (b) Manganese species exist in ions of different oxidation states and colours. Suggest the identity of the final manganese-containing species in (a)(i) and in (a)(ii). final manganese-containing species in (a)(i) ................................ ................................ final manganese-containing species in (a)(ii) ................................ ............................... [1] (c) In (a)(i), D-glucose, C6H12O6, reacts with KMnO 4 in alkaline medium to give D-gluconate, C6H11O – 7. (i) Name the type of reaction that D-glucose undergoes in (a). ................................ ................................ ................................ ........................ [1] (ii) Draw the structure of the D-gluconate ion. [1] [Total: 5] MnO2 Mn2+ Oxidation

3 © EJC 9729/04/J2Prelim/25 [Turn Over 2 Determination of mass of D-glucose in a sample The Bertrand method is a classical redox titration technique used to determine the amount of reducing sugars, such as D-glucose. In this method, a sample of D-glucose, C6H12O6, is reacted with excess alkaline copper(II) solution, Cu2⁺, to form insoluble copper(I) oxide, Cu2O. equation 1 C6H12O6 + 2Cu2+ + 5OH− → C6H11O – 7 + Cu2O + 3H2O The Cu₂O is filtered and dissolved in 100 cm3 acidic iron(III) solution, Fe3+. equation 2 Cu2O + 2Fe3+ + 2H+ → 2Cu2+ + 2Fe2+ + H2O The resulting solution is then diluted to 1 dm3. The diluted solution is labelled FA 3. The amount of Fe2⁺ present in FA 3 is then determined by titration with potassium manganate(VII), KMnO4, in acidic medium. The end-point of the t