2020 YIJC Prelim Exam Paper 3 Solutions

2020 YIJC Preliminary Examination H2 Chemistry Paper 3 (Suggested Answers) 1 Chlorine is an element in Group 17 of the Periodic Table. It exists as a diatomic gas at room temperature. Chlorine can also be found in many inorganic and organic compounds. (a) Predict the colour of the final solutions when  Cl2(aq) is added to KBr(aq)  I2(aq) is added to KCl(aq) Write equations for any reactions that occur. State clearly if no reaction occurs. [2]  Cl2 + 2Br  2Cl+ Br2 colour of final solution: orange  No reaction colour of final solution: brown (b) Describe and explain the trend in the thermal stabilities of the hydrogen halides HCl, HBr and HI. [2] The thermal stabilities of the hydrogen halides decrease down the Group from HCl to HBr to HI. This is because  the size of the halogens increases from Cl to I and the valence orbital used for bonding is larger and more diffuse;  the effectiveness of the orbitals overlap decreases;  this result in the weaker covalent bond formed between the hydrogen and halogen atoms or quoting H-X bond energies;  lesser amount of energy is required to overcome the covalent bond between the hydrogen and halogen atoms.

(c) Chlorine dioxide, C lO2 was discovered in 1811 and has been widely used for bleaching purposes in the paper industry and for treatment of drinking water. (i) ClO2 can be synthesised from the reaction between NaC lO3 and H2O2 in an acidic medium. Construct a balanced equation for the reaction between ClO3− and H2O2. [2] reduction: ClO3− + 2H+ + e−  ClO2 + H2O oxidation: H2O2  O2 + 2H+ + 2e− 2ClO3− + H2O2 + 2H+  2ClO2 + 2H2O + O2 (ii) In the process of handling C lO2, the concentration of the C lO2 in aqueous solution must not exceed 45 parts per million by mass, as it will cause irritation to the eyes and nose. 1 part per million can be expressed as 1 mg of the substance in 1 kg of water. An aqueous solution of C lO2 was prepared by adding 150 cm 3 of 1.0  103 mol dm−3 NaClO3 to 100 cm3 of 2.0  103 mol dm3 H2O2. Using your equation in (c)(i), c alculate the concentration of the C lO2(aq) in parts per million, State whether it has exceeded the safety limit. [Assume the density of the final solution is 1 g cm3.] [3] Amount of NaClO3 = (1.0  103) × 0.15 = 1.50 × 104 mol Amount of H2O2 = (2.0  103) × 0.10 = 2.00 × 104 mol Limiting reagent: NaClO3 Amount of ClO2 produced = 1.50 × 104 mol Mass of ClO2 produced = (1.50 × 104) × 67.5 = 0.010125 g = 10.125 mg Total mass of