NJC_Prelim_CHEM_P2_Solutions

2015 NJC SH2 H2 Chemistry Prelim Paper 2 Solution with Examiners’ Comments 2015 SH2 H2 Chemistry Prelim Paper 2 Suggested Answers with Examiners’ Comments 1(a) Procedural format Divide the unknown into 5 separate portions - Warm a portion with Fehling’s solution: if brick red ppt forms, unknown is ethanal. - If no ppt, warm another portion with Tollen’s reagent: if silver mirror forms, unknown is benzaldehyde. - If no silver mirror, warm another portion of sample with aq I2 + NaOH(aq): if yellow ppt forms; unknown is 1-iodopentan-2-one. - If no yellow ppt, warm another portion with NaOH(aq) and test gas with moist red litmus paper: if litmus paper turns blue, unknown is propanamide. - If no alkaline gas, add Br2(aq) to last portion: if orange Br2(aq) decolourises & white ppt forms, unknown is phenylamine. Another example of plan: ethanal benzaldehyde unknown potassium manganate(VII)/ sulfuric acid; heat ethanal or benzaldehyde propanamide / 1-iodopentan-3-one / phenylamine purple solution decolourises purple MnO4 – does not decolourise To new sample, aqueous I2/ sodium hydroxide; warm yellow ppt ( CHI3) forms no yellow ppt 1-iodopentan-3-one propanamide or phenylamine To new sample, NaOH(aq); heat yellow ppt ( AgI) forms no yellow ppt Moist red litmus paper turns blue propanamide phenylamine Br2(aq) Orange solution decolourises; white ppt forms To new sample, NaOH(aq); heat; then acidify with aq HNO3; aq AgNO3

2015 NJC SH2 H2 Chemistry Prelim Paper 2 Solution with Examiners’ Comments 1(bi) N NN H NC C H H HH OO O O 1(bii) 1. Suggest suitable quantities of ethanal and 2,4 -DNPH to prepare the solid derivative (hydrazone). 2. Filter solid derivative and then dissolve in minimum volume of hot solvent (non-polar or organic) … use fluted filter paper / pre -heated funnel / Buchner funnel to remove insoluble impurities 3. Cool solution to form crystals …Then filter purified crystals using Buchner funnel. 4. Wash crystals with minimum cold solvent and dry the solid (in desiccator / press between filter papers / low temperature oven). 5. Find melting point of pure and dry crystals (m.p 165 oC). 1(c) As organic compounds are flammable, there should be NO Bunsen flame or naked flame. / Heat or warm reaction mixture in a water bath when conducting experiments. 2(ai) P: 1s2 2s2 2p6 3s2 3p3 S: 1s2 2s2 2p6 3s2 3p4 2(aii) 1st IE of S is lower than that of P: All 3p electrons in P are unpaired. Two of the 3 p electrons are paired in S with inter- electronic repulsion between the paired electrons . Thus less energy is required to remove the 1st paired electron in S 4th IE of S is lower than that of P: 4th e in S is removed from 3p subshell; while 4th e in P is removed from 3s subshell ; 3p subshell is further from the nucleus (at a higher E level) with weaker nuclear attraction. Thus less energy is required to remove this electron in S. 2(bi) p = 23