NJC_CHEM_H1 Chemistry 2008 P2 answers

National Junior College 2008 Answer Keys to H1 Prelims P2 1 (a) (i) Protons Neutrons Electrons Relative Mass 1 1 1/2000 Charge +1 0 −1 Location in atom Nucleus Nucleus Orbitals (ii) Protons: 31; Neutrons: 38; Electrons: 31 (b) (i) Isotopes are atoms of the same element with the same no. of protons and electrons but different no. of neutrons. (ii) A r of Ga = ) ( ) (71 x5 269 x5 3 + = 69.8 (iii) Both have the same charge of +3. 69Ga3+ is deflected to a larger angle towards the negative plate as it is lighter in mass. 2 (a) (i) Bond energy of NO is the energy required to break one mole of covalent bonds between N and O in a gaseous NO molecule at standard conditions. (ii) ΔHr = 4[3(+390)] + 5(+496) + 4(−607) + 6[2(−460)] = − 788 kJ mol−1 (b) (i) ΔHf θ of O2(g) is zero as it is an element at standard state. (ii) ΔHr = 4(+90) + 6(−285.8) − 4(−46) = − 1170.8 kJ mol−1 (iii) The water in (aii) is in the gaseous phase while the water in (bii) is in the liquid phase. Bond energy value in (aii) is less exothermic as energy is required to vapourise the liquid water to gas. 3 (a) (i) C H H C CH3 Br C H CH3 H C H H C CH 3 H C Br CH3 H (iii) C H H C CH 3 OH C OH CH 3 H 1

National Junior College 2008 (ii) C H H C CH3 O CH3COOH+ (iv) C H H C CH3 Cl C Cl CH 3 H Cl C H H C CH3 Cl C Cl CH 3 H Cl (b) C H H C CH3 OH C OH CH 3 HC H H C CH3 C CH3 H + [O] + H2O Purple solution of KMnO4 decolourise, brown ppt of MnO2 formed. (c) Geometric isomerism CC CH2 CH3 CH3 H CC CH2 CH3 H CH3 trans cis 4 (a) (i) 4 HCl(aq) + MnO2(s) Æ MnCl2(s) + Cl2(g) + 2H2O(l) (ii) No. of moles of MnO2 = 9 86 5 . = 0.0575 mol No. of moles of hydrochloric acid = 6.0 x 25 x 10−3 = 0.15 mol HCl is limiting reagent. Volume of Cl 2 24 x4 15 0. = 0.9 dm3 (iii) HCl is the reducing agent. (b) (i) 50 ppm would endanger human health. In 1, 000, 000 dm 3 of air, there are 50 dm3 of Cl2 In 24 dm3 of air, volume of Cl2 = 24 x 10 50 6 = 1.2 x 10−3 dm3 No. of moles of Cl2 = 24 10 x 2 13−. = 5 x 10−5 mol 2

National Junior College 2008 (ii) 4 Cl2 (g) + S2O3 2− (aq) + 5 H2O (l) Æ 8 Cl− (aq) + 2 SO4 2− (aq) + 10 H+ (aq) (iii) No. of moles of Cl2 = 24 10 x 5003− = 0.0208 mol No. of moles of S2O3 2− = 4 0208 0. = 5.208 x 10−3 mol [S2O3 2−] = 1000 x700 10 x 208 53−. =7.44 x 10−3 mol dm−3 5 (a) (i) 11 12 13 14 15 16 17 Proton no. ionic radius Ionic radius decreases from Na+ to Si4+ as no. of protons increase but the number of electrons are the same, hence, p/e ratio increases and the valence electrons are drawn nearer to the nucleus due to increasing attraction. The same exp