Zhonghua Prelim P2 Ans

2024 ZHSS Pure Chem (6092) Preliminary Examination Section A (70 m) 1 (a) Put a tick (√) if a redox reaction has occurred. (i) a copper strip added to zinc nitrate solution (ii) aqueous chlorine added to potassium iodide solution √ (iii) hydrogen gas passed over heated copper(II) oxide √ (iv) Dilute sulfuric acid reacts with lithium hydroxide ( 1m each) 2 (b) Difference: Reaction rate is slower when used with aqueous bromine. Explain: Chlorine is more reactive than bromine/ gains electrons more readily (and displaces iodine from iodide solution faster); 1 1 (c) CuO + H2  Cu + H2O 1 [5] 2 (a)(i)  Silicate has an orderly arrangement, while soda-lime glass does not have an orderly arrangement.  Each oxygen atom is covalently bonded to 2 Si atoms in silicate, but not all oxygen atoms are covalently bonded to 2 Si atoms in soda-lime glass;  Silicate contains only covalent bonds, but soda-lime glass contains covalent and ionic bonds; Any two points 2 (ii) Conducts electricity only in molten state. There are more free moving/mobile ions that act as charge carriers 1 1 (b)(i) Potassium hydroxide has a giant lattice structure; A large amount of energy is needed to overcomes the strong forces of attraction between the oppositely charged ions; Chlorine has a simple molecular/ covalent structure where a small amount of energy is needed to overcome weak intermolecular forces; structure 1m, bonding & energy 2 m 1 1 1 (ii) hydroxide ion, OH- O 2.6, H 1 2 9m

3 (a) The greater the solubility of a solute in the mobile solvent, the faster the solute moves up the paper, the higher the spot is on the paper. 1 (b)(i) 2 spots (no mark awarded) Pigments X and Y have the same Rf value of 0.6 and thus travelled the distance up the chromatogram appearing as one spot. Pigment Z has smaller Rf value of 0.1 and thus travelled a shorter distance up the chromatogram, appearing as another different spot. 1 1 (ii) Pigment Z and Y would not have been separated from the mixture as they are insoluble in pure water and pure ethanol respectively. 1 1 5m 4 (a) Concentration of CuSO4 decreases from 1.00 mol/dm3 until there is no more Cu2+ ions in solution as Cu2+ ions is reduced to form Cu at the cathode. Cu2+(aq) +2e  Cu(s) 1 1 (b) Use aqueous sodium hydroxide, blue ppt insoluble in excess if there is copper(II) ions present OR Use aqueous ammonia solution, blue ppt soluble in excess to form a dark blue solution. 1 (c) Amount of Cu deposited = 200/1000 X 1.00 = 0.200 mol (1) Mass of Cu = 0.200 X 64 =12.8 g [1] Increase in mass of the iron object = 12.8 g 2 (d)(i) 1

(ii) solution name of products of electrolysis ionic equation for reacti