2019 NJC Prelim P3 QP

[Turn over NATIONAL JUNIOR COLLEGE SH2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9729/03 Wednesday 18 September 2019 2 hours READ THE INSTRUCTIONS FIRST Write your subject class, registration number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 11 printed pages and 1 blank page.

NJC/H2 Chem Prelim/03/2019 2 Section A Answer all the questions from this section. 1 Potassium is an element found in Period 4 of the Periodic Table. (a) (i) Write an equation for the second ionisation energy of potassium. [1] (ii) Fig 1.1 shows the second ionisation energy for the consecutive elements from Period 3 and Period 4. Fig 1.1 Suggest which of the above elements, A to H, is potassium. [2] (b) When 0.400 g of potassium dichromate(VI), K2Cr2O7, was heated using a strong flame, a yellowish green solid mixture was obtained and a gas was evolved. The gas collected ignited a glowing splint. Upon adding water to the yellowish green solid, part of the solid dissolved to give a yellow solution of K2CrO4, leaving a green solid. (i) State the identity of the gas. [1] (ii) The green solid that remained was collected and dried. It weighed 0.052 g and was found to be 68.4% chromium by mass and contains only chromium and oxygen. Determine the chemical formula of the green solid. [2] (iii) Using your answers to b(i) and b(ii), write the equation for the decomposition of potassium dichromate(VI). [1] (c) State two physical properties of chromium that differs from calcium. Explain the reasons for those differences. [2]

NJC/H2 Chem Prelim/03/2019 3 [Turn over (d) Vanadate(V) ion, VO3−, changes into pale yellow oxovanadium(V) ions, VO2+, when acid is added. Upon addition of excess zinc, the pale yellow solution changes to blue, to green and finally to violet. (i) With the aid of an equation, explain the type of reaction that occurs when vanadate(V) ion changes into oxovanadium(V) ions. [2] (ii) Explain why vanadium is able to exhibit variable oxidation states. [1] (iii) With the aid of the Data Booklet, deduce the final oxidation state of vanadium in the violet solution. [3] Solid vanadium(V) oxide, V2O5, is used as a catalyst for reaction 1. reaction 1: SO2(g) + 1 2O2(g) → SO3(g) (iv) State the type of catalysis and outline the mode of action when V2O5 is used for reaction 1. [3] (v) NO2 can also act as a catalyst for reaction 1. Write two equations to show how NO2 acts as a catalyst in reaction 1. [2] [Total : 20]

NJC/H2 Chem Prelim/03/2019 4 2 Lithium oxide, Li 2O, is used in traditional ceramic glazing to create a blue hue with copper on ceramics. (a) Using the data below and any appropriate data from the Data Booklet, construct a labelled energy level diagram to calculate the standard enthalpy change of formation of Li2O(s). / kJ mol–1 first electron affinity of oxygen –141 second electron affinity of oxygen +798 lattice energy of lithium oxide –2863 standard enthalpy change of atomisation of lithium +159 [5] (b) Lithium oxide is produced during the thermal decomposition of lithium peroxide, Li 2O2, at 450 oC. Li2O2(l) → Li2O(s) + 1 2O2(g) (i) Given that the standard enthalpy change of formation of Li 2O2(l) is –606 kJ mol –1 and using your answer in (a), calculate the enthalpy change for the thermal decomposition of Li2O2. [1] (ii) Explain why the thermal decomposition of Li2O2 is feasible. [2] (c) Table 2.1 gives the melting points of two lithium ionic compounds. compound melting point / oC Li2O 1400 Li2O2 195 Table 2.1 Explain the difference in the melting points. [2] (d) Similar to lithium peroxide, Li 2O2, lithium nitrate, LiNO 3(s), undergoes thermal decomposition to produce oxygen and a brown gas. (i) Write a balance equation, with state symbol s, for the thermal decomposition of lithium nitrate. [2] (ii) Among the Group 1 metal nitrate s, only lithium nitrate undergoes thermal decomposition. Explain why lithium nitrate can undergo thermal decomposition. [2]

NJC/H2 Chem Prelim/03/2019 5 [Turn over (e) Ethylenediamine tetraacetate, [EDTA]4−, is a chelating agent that is widely used to remove transition metal ions such as copper(II) ions from aqueous solutions. (i) Suggest a synthesis of ethylenediamine tetraacetate, [EDTA] 4–, starting from methanal and 1,2−diaminoethane (NH 2CH2CH2NH2), according to the scheme in Fig. 2.1. Deduce the structure of compound A. Include reagents and conditions for all reactions, and the structures of all other intermediate compounds, in your answer. C O H H more than one step C2H2O2ClNa A N N O O O O O O O O [EDTA]4 limited H2N NH2 Fig 2.1 [5] (ii) When aqueous ammonia is added to a solution of CuSO 4, a blue precipitate is formed which dissolves in excess aqueous ammonia to give a deep blue solution. On addition of Na 4(EDTA) solid to the mixture, the colour of the solution turns light blue. Suggest explanations for the above observations, writing equations as appropriate. [3] [Total : 22]

NJC/H2 Chem Prelim/03/2019 6 3 (a) Phenol is a white crystalline solid that is volatile. It is mildly acidic and requires careful handling as it can cause chemical burns. Table 3.1 shows the physical properties for phenol and its derivatives. compound melting point / C Ka / mol dm−3 phenol 41 1.12 × 10−10 2−nitrophenol 46 5.89 × 10−8 4−nitrophenol 114 7.08 × 10−8 Table 3.1 (i) Explain the difference in melting points between 2−nitrophenol and 4−nitrophenol. [2] (ii) Explain the difference in the acidities of phenol and 2−nitrophenol. [2] (iii) Suggest how 2,6−dibromo−4−nitrophenol can be synthesised from phenol in a 2−step synthesis. In your synthesis, you should state the reagents and conditions needed for each step, and show clearly the structure of any intermediate compounds. [3] (b) When aqueous neutral FeC l3 is added to a solution containing phenol, a violet solution containing the complex ion [Fe(C6H5O)6]3− is formed. (i) Draw the structure of the complex ion. [1] (ii) The violet complex ion [Fe(C 6H5O)6]3− is not observed in both acidic and alkaline solutions. Suggest explanations for the above observation. [2] (c) Phenol can react with compound W under suitable conditions to give bisphenol-A (BPA). OH 2 + + H2O BPA compound W C CH3 CH3 HO OH The use of the table of characteristic infra -red absorption frequencies for some selected bonds in the Data Booklet is relevant to this question. Infra-red absorptions can be used to identity functional groups in organic compounds. For example, phenol shows absorption at 3200−3600 cm−1 due to the O−H bond. The analysis of compound W shows absorptions at 1670−1740 cm−1. Identify the bond present in compound W and suggest its structure .