SAJC_H1 Prelims Paper 2 Solutions

Solution to H1 Paper 2 – Section A 1a(i) empirical formula = CHO2 C H O Mass/g 26.64 2.22 71.14 No. of moles 26.64/ 12 = 2.22 2.22/1 = 2.22 71.14/16 = 4.45 Simplest ratio 1 1 2 (ii) no. of mol of NaOH = mol100 . 01000 00 . 2 50=× no. of mol of dibasic acid = 0.5 × no. of mol of NaOH = 0.0500 mol Mr = 90 0500 . 0 5 . 4= (iii) Molecular formula = (CHO2)n [12 + 1 + 2(16)]n = 90 n = 2 ∴ Molecular formula is C2H2O4 (b)(i) C2H2O4 Æ 2CO2 + 2H+ + 2e (ii) no. of mol of C2H2O4 = 075 . 090 75 . 6= mol (iii) C2H2O4 : MnO4 - = 5 : 2 no of mol of MnO 4 - = 03 . 05 2 075 . 0=× mol conc. of MnO 4 - = () 2 . 1 100025 03 . 0= mol dm-3 2(a)(i) Cl2O Shape of molecule: Bent/ V-shaped Bond angle: 104° or 104.5° (ii) SO3 Shape of molecule: Trigonal planar Bond angle: 120° 1

(b)(i) Silicon oxide is a giant molecule with strong covalent bonds between Si and O atoms. Silicon chloride exists as simple covalent molecules with weak van der Waals’ forces between its molecules. More energy is needed to break the covalent bonds than the van der Waals forces, hence silicon oxide has a much higher melting point. (ii) The decrease in bond angles is explained by the decreasing electronegativity going down the group from oxygen to selenium. As electronegativity decreases, the bonding pair of electrons are less attracted to the central atom. This results in decrease in repulsion between the bond pairs and hence a smaller bond angle. 3(a)(i) Cracking produces branched chain alkanes in petroleum mixtures as they are more efficient fuels / combustion takes place more efficiently / reduces knocking in engines / prevents the premature explosion of fuels. (a)(ii) Reagent: 2,4 dinitrophenylhydrazine Condition: Warm / rtp Observation: Benzaldehyde forms an orange ppt whilst benzene will not form an orange ppt. OR Reagent: KMnO 4 Condition: H+ / heat Observation: Benzaldehyde: Purple colour turns colourless whilst benzene: Purple colour remains. OR Reagent: Tollen’s reagent Condition: Warm / rtp Observation: Benzaldehyde forms a silver mirror whilst benzene will not form a silver mirror (b) The reaction time in the engine is too short / the engine temperature is too high/knocking occurs in the engine. (c) *Other structures may be accepted if structural isomerism is shown. Other types of isomerism is unacceptable. 3(a) (a) Group III; Al (b)(i) W+: 1s2 2s2 2p6 3s2 ; X +:1s2 2s2 2p6 3s2 3p1 (ii) 2nd IE of X is lower than that of W. 3p is further away from the nucleus compared to 3s. 3p electron is better shielded compared to 3s electron. less energy required to remove 2 nd electron from X. 2

(c) The electron to be removed from Z experienced interelectronic repulsion. 4(a)(i) E: Na2O F: NaOH Reagent X: HCl / Cl2 Reagent Y: Cl2 / HCl (ii) Al2O3 + 2NaOH + 3H2O Æ 2NaAl(O