2019 MI Prelim H2 Chem P2 QP

Class Adm No Candidate Name: This question paper consists of 16 printed pages and 2 blank pages. 2019 Preliminary Exams Pre-University 3 H2 CHEMISTRY 9729/02 Paper 2 Structured Questions 16 Sept 2019 2 hours Candidates answer on the Question paper. Additional materials: Data Booklet READ THESE INSTRUCTIONS FIRST Do not turn over this question paper until you are told to do so Write your name, class and admission number on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Question 1 2 3 4 5 Total Marks 7 23 16 14 15 75

2 1 (a) Zinc-air batteries worked by oxidising zinc with oxygen from the air. At the cathode, oxygen converts to hydroxide ions. At the anode, zinc reacts with the hydroxide ions to form zincate, [Zn(OH)4]2-. (i) Construct an ion-electron equation for the reaction that take s place at each electrode under alkaline conditions. [2] Cathode: ……………………………………………………………………………………… Anode: ………………………………………………………………………………………… (ii) At 298 K, the standard electrode potential of the [Zn(OH)4]2-(aq)|Zn(s) half-cell is -1.25 V. Calculate the cell voltage using relevant data from the Data Booklet. [1] (iii) Using relevant data from the Data Booklet, deduce if oxygen gas can be replaced with chlorine gas. [2] …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… (b) Another type of battery known as the nickel -cadmium battery is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. The electrode reaction equations for the discharging process under alkaline conditions is given below. Cathode: Cd + 2OH- → Cd(OH)2 + 2e- Anode: 2NiO(OH) + 2H2O + 2e- → 2Ni(OH)2 + 2OH- (i) Construct the overall equation for the reaction that takes place during charging. [1] ……………………………………………………………………………………………………

3 [Turn over (ii) Suggest a disadvantage of using nickel-cadmium battery. [1] …………………………………………………………………………………………………… …………………………………………………………………………………………………… [Total: 7]

4 2 Paramagnetism and diamagnetism are different forms of magnetism. Paramagnetic materials are weakly attracted by an externally applied magnetic field and form magnetic fields in the direction of the applied magnetic field. In contrast, diamagnetic materials are repelled from magnetic fields and form magnetic fields in the direction opposite to that of the applied magnetic field. Transition metals are mostly paramagnetic or diamagnetic. The magnetic property is characterised by the presence of unpaired electrons in paramagnetic compounds and absence of un paired electrons in diamagnetic compounds. The table below shows the magnetic property of metal complexes. Formula of complex Magnetic property of complex Fe(CO)5 Paramagnetic [Fe(H2O)6]2+ Paramagnetic [Fe(H2O)6]3+ Paramagnetic [ZnCl4]2- Diamagnetic [V(H2O)6]3+ [Sc(H2O)3(OH)3] (a) (i) Write the electronic configurations of V in [V(H2O)6]3+ and Sc in [Sc(H2O)3(OH)3]. [2] V in [V(H2O)6]3+: ………………………………………………………………………………… Sc in [Sc(H2O)3(OH)3]: ………………………………………………………………………… (ii) State the magnetic property of [V(H2O)6]3+ and [Sc(H2O)3(OH)3] in the above table. [1]

5 [Turn over (iii) Draw labelled diagrams to show the shapes of the d orbitals of the metal ion in [V(H2O)6]3+. Include the relative energy of the d-orbitals in the complex. [3] d-orbitals of lower energy:…………………………………………………………………….. d-orbitals of higher energy: ………………………………………………………………………….. (iv) Explain why transition metal can show variable oxidation states. [1] …………………………………………………………………………………………………… …………………………………………………………………………………………………… (b) Sodium reacts with iron pentacarbonyl to produce a salt known as sodium tetracarbonylferrate, Na2Fe(CO)4. 2Na + Fe(CO)5 ⟶ Na2Fe(CO)4 + CO ------- eqn (1) (i) Explain what is meant by the coordination number of a complex using Fe(CO) 5 as an example. [1] …………………………………………………………………………………………………… …………………………………………………………………………………………………… (ii) Draw the structure of Fe(CO)5 and state its shape. [2] Shape: …………………………………………………………………………………………...

6 (iii) Deduce the oxidising and reducing agent in eqn (1). [2] …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… (iv) Explain why carbon monoxide, CO, is poisonous. [2] …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… …………………………………………………………………………………………………… (v) The tetracarbonylferrate dianion acts as a nucleophile and react with alkyl halide by the SN2 mechanism to form a new C-Fe bond. [Fe(CO)4]2- + CH3Br ⟶ [Fe(CH3)(CO)4]- + Br – ------- eqn (2) Suggest a mechanism between Fe(CO) 42- and CH3Br. State clearly any intermediates that may be formed and use curly arrows to indicate the movement of electron pairs. [3]

7 [Turn over (vi) State the rate equation for the reaction in eqn (2). [1] …………………………………………………………………………………………………… (vii) I Sketch the graph of [Fe(CH3)(CO)4]– against time. II Sketch the graph of rate against [CH3Br] given that [Fe(CO)4]2- is in excess. [2] (c) The melting point of sodium tetracarbonylferrate, Na 2Fe(CO)4 is lower than that of sodium oxide. With reference to the structure and bonding, explain the difference in melting point.[3] ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… ………………………………………………………………………………………………………… [Total: 23] [Fe(CH3)(CO)4] – / mol dm-3 Time/s Rate / mol dm-3 s-1 [CH3Br] / mol dm-3

8 3 (a) Phosgene, COCl2, is essential in the manufacturing of everyday products, including medical products and footwear. Phosgene is produced by combining carbon monoxide and chlorine with a catalyst. CO(g) + Cl2(g) → COCl2(g) (i) Draw a ‘dot-and-cross’ diagram of COCl2 and state the bond angle a bout the central atom. [2] Bond angle: ……………………….. (ii) Using relevant data from the Data Booklet , c onstruct a fully labelled energy level diagram to calculate the enthalpy change of the above reaction. [3]

9 [Turn over (iii) Explain what is meant by