NJC Prelim P3 Question Paper

NJC SH2 Preliminary Examination 9729/03/17 [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: Data Booklet Answer Paper 9729/03 Monday 11 Sep 2017 2 hours READ THESE 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 paper clips, highlighters, glue or correction fluid/tape. Section A Answers all questions. Section B Answers one question. 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. This document consists of 11 printed pages and 1 blank page.

2 NJC SH2 Preliminary Examination 9729/03/17 Section A Answer all the questions in this section. 1 (a) (i) Write an equation for the thermal decomposition of barium carbonate. [1] (ii) Describe and explain the trend observed in the ease of thermal decomposition of the carbonates of the Group 2 elements. [2] (b) Monuments made of marble or limestone, such as the Taj Mahal in India and the Mayan temples in Mexico, are eroded by acid rain containing sulfuric acid. The carbonate stone is converted into sulfate by acid rain. CaCO3(s) + H2SO4(aq)  CaSO4(s) + H2O(l) + CO2(g) The life of such monuments is now being extended by treating them with a mixture of urea, (NH2)2CO, and barium hydroxide solutions. reaction 1: After soaking in the pores of the ca rbonate stone containing water, urea reacts gradually to give ammonia and carbon dioxide. reaction 2: Carbon dioxide produced then reacts with barium hydroxide to form barium carbonate. (i) State the shape and bond angle around the C atom of (NH2)2CO. [2] (ii) Write balanced equations for reactions 1 and 2. [2] (c) When carbon dioxide gas dissolves in water, carbonic acid, H2CO3, is formed. CO2(g) CO2(aq) CO2(aq) + H2O(l) H2CO3(aq) In a school laboratory, a student was instructed to titrate 25.0 cm 3 of 0.100 mol dm –3 carbonic acid with 0.125 mol dm −3 NaOH using phenolphthalein indicator. (Ka1 of H2CO3 = 4.3 × 10−7 mol dm−3) (i) Calculate the pH of 0.100 mol dm–3 H2CO3. [1] (ii) Calculate the volume of NaOH needed to completely react with 25.0 cm 3 of 0.100 mol dm–3 H2CO3. [1] (iii) Calculate the pH of the solution when H2CO3 and HCO 3– are in equal concentrations. [1] (iv) Calculate the pH of the resulting solution when 20.0 cm3 of NaOH is added to 25.0 cm3 of H2CO3. [2] (v) Using your answers in (i) to (iv), sketch the shape of the pH curve during the titration. [2]

3 NJC SH2 Preliminary Examination 9729/03/17 [Turn over (d) Barium compounds also have other uses and soluble barium sa lts are highly poisonous. For example, b arium sulfate is used as a ‘barium meal’ in X -ray diagnostic work for patients with digestive tract problem s. On the other hand, barium carbonate is used in rat poison. The solubility products of BaCO3 and BaSO4 at 25 oC are given in the table below. Numerical value of Ksp BaCO3 5.5 × 1010 BaSO4 1.3 × 1010 (i) Calculate the solubility of barium carbonate in mol dm–3 at 25 oC. [1] (ii) When the concentration of Ba 2+ ions exceeds 0.100 mol dm –3, it is lethal. Suggest a reason why barium carbonate is poisonous when ingested by mouth whereas barium sulfate is safe. [2] (e) The values of lattice energy of CaSO 4 and BaSO 4 are − 2374 kJ mol −1 and −2480 kJ mol−1 respectively. Some standard enthalpy changes of hydration are listed below. Hhydo / kJ mol−1 Ca2+ −1577 Ba2+ −1305 SO42− −1099 Determine the enthalpy change of solution, Hsolo, for these two salts. Hence, comment on the difference in the solubilities of the two salts. [3] [Total: 20 marks]

4 NJC SH2 Preliminary Examination 9729/03/17 2 This question is about the chemistry of aluminium, iron and ruthenium, Ru. (a) Anodising is a process used to increase the thickness of the natural oxide on the surface of metal parts. Aluminium is a common metal which can be anodised as aluminium is a reactive metal that is readily oxidised by oxygen. This forms a layer of aluminium oxide, Al2O3, making it resistant to corrosion. (i) Using H 2SO4(aq) as the electrolyte and an inert electrode, draw an electrolysis set-up to show how an iPhone 7®, which is made of A l, can be anodised. [2] (ii) Write chemical equations to show the reactions at the anode during anodising. [2] (iii) The iPhone 7® has a surface area of 93.0 cm2 to be anodised. Calculate the time needed to form a 0.2 mm protective layer of Al2O3 on the iPhone 7® if a current of 2.0 A is passed through the set-up. [2] (Density of Al2O3 is 3.95 g cm3) (b) Iron metal is dissolved in limited amount s of dilute and concentrated sulfuric acid solutions and the resultant solutions undergo a series of reactions as shown below. (i) Account for the observations seen in step 1. [2] (ii) Write equations with state symbols to account for the observations in steps 2 and 3. [2] (c) When aqueous ruthenium(III) chloride is treated with zinc and aqueous ammonia, an orange compound is formed. The orange compound contains 36.9% ruthenium, 30.7% nitrogen, 25.8% chlorine and a certain percentage of hydrogen by mass. When aqueous silver nitrate is added, 1 mol of the orange compound forms 2 mol of AgCl precipitate. Given that 1 mol of the orange compound contains only 1 mol of Ru, determine the molecular formula of the orange compound. Draw a diagram of the structure of the complex ion in the orange compound. [3] H2SO4(aq) pale green solution green ppt NaOH(aq) Step 2 conc H2SO4 pale yellow solution Na2CO3(aq) Step 1 red-brown ppt and effervescence of gas that forms a white ppt with l