NYJC H2 Chem Prelim 08 P2 Answers

NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 Answers CANDIDATE NAME CLASS TUTOR’S NAME CHEMISTRY 9746/02 Paper 2 Structured 17 September 2008 1 hour 30 minutes Candidates answer on the Question Paper Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. 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. For Examiner’s Use 1 2 3 4 5 6 7 8 Total This document consists of 14 printed pages and 0 blank page. [Turn over

2 For Examiner's Use Answer all questions. 1 Aluminium chloride sublimes at 178 oC at standard pressure. The diagrams below show how the volume of aluminium chlori de varies with temperature as well as how its electrical conductivity varies with temperature at the stated constant pressures. H2 Chemistry 9746/02/NYJC J2/08 PX [Turn Over (a) Explain why aluminium chloride has zero electrical conductivity at regions D and E. For region D and E, aluminium chloride exists as simple molecules with no mobile electrons, hence do not conduct electricity. [2] 178 195 Temperature/ oC Volume of aluminium chloride Constant pressure of 2.5 atm Constant pressure of 1atm 178 195 C D Constant pressure of 2.5 atm Constant pressure of 2.5 atm Temperature/ oC Electrical conductivity 0 E 178 195 178 195 Temperature/ oC Volumeof aluminium chloride A B Constant pressure of 1 atm 195178

3 For Examiner's Use (b) Draw the displayed formula of the al uminium chloride molecule at regions A and B. Region A Region B H2 Chemistry 9746/02/NYJC J2/08 PX [Turn Over [2] (c) Suggest the physical state of t he aluminium chloride at region C . Explain your answer and hence state the significance of 195 oC. Liquid state because at 2.5 atm, the higher pressure prevents the molecules from vaporizing. Hence 195oC is the boiling point. [2] [Total: 6] 2(a) The following table shows the first ionisation energies of phosphorus, sulphur and chlorine. Element Ionisation Energy / kJ mol-1 P 1060 S 1000 Cl 1260 (i) Write an equation to represent the first ionization energy of chlorine. Cl(g) → Cl+(g) + e− ..................................................................................... [1] (ii) Why is the ionisation energy of sulphur lower than that of phosphorus? P: 1s2 2s2 2p6 3s2 3px 1 3py 1 3pz 1; S: 1s2 2s2 2p6 3s2 3px 2 3py 1 3pz 1 .............. Less energy required to remove an electron from 3p x orbital of S because of interelectron repulsion arising from two electrons occupying the same orbital. ....................................................................................................................... ....................................................................................................................... ................................................................................................................... [2] Al Cl Cl Cl: Al Cl Cl :Cl Al Cl Cl Cl

4 For Examiner's Use (b) The graphs below show the pH of the so lutions obtained after reaction (if any) of chlorides and oxides of some Period 3 elements with water. pH of solution 7 0 Na Mg Al Si P S oxides chlorides (i) Explain why aqueous sodium chloride ha s a pH of 7. Include any relevant equations in your answer. NaCl(s) dissolves to form hydrated ions; Na +(aq) has low charge density; does not undergo hydrolysis. NaCl(s) + aq → Na+(aq) + Cl−(aq) ............................................................. [2] (ii) Use the graph above and data from the Data Booklet to predict the pH of aqueous BeCl2. pH ≈ 3 or any pH < 7 wit h reasoning/data (any one : diagonal relationship; charge density; Group II) [2] (iii) Give the formula of an oxide of sul phur and give the oxidation state of sulphur in this oxide. Give a balanced equation to account for the pH of its aqueous solution. Formula of oxide: SO2 or SO3 ................................................................. Oxidation state of sulphur in oxide: +4 or +6 ................................................. Equation: SO2 + H2O → H2SO3 or SO3 + H2O → H2SO4 ........................... [3] [Total: 10] H2 Chemistry 9746/02/NYJC J2/08 PX [Turn Over

5 For Examiner's Use 3 Silver chromate(VI), Ag2CrO4 is only sparingly soluble in water. (a) Write an expression for the solubility product, Ksp, of silver chromate(VI). Ksp = [Ag+]2 [CrO4 2-] .................................................................................................[1] (b) The solubility of silver chromate(VI) in water at 25 oC is 0.0302 g dm-3. For a saturated solution of silver chromate(VI) at 25 oC, calculate (i) the concentration, in mol dm-3, of chromate(VI) ions, [CrO4 2-] = 0.0302 332.0 = 9.096 x 10 -5 mol dm-3 = 9.10 x 10-5 mol dm-3 (3 sf) (ii) the concentration, in mol dm -3, of silver ions, [Ag+] = 2 x 9.10 x 10 −5 = 1.82 x 10-4 mol dm-3 (iii) the value of Ksp of silver chromate(VI), stating the units. Ksp = [Ag+]2 [CrO4 2-] = (1.82 x 10-4)2 (9.10 x 10−5) = 3.014 x 10-12 = 3.01 x 10 -12 mol3 dm-9 (3 sf) [4] (b) Will a precipitate form when equal volumes of solutions containing 3 x 10-3 mol dm-3 silver nitrate and 5 x 10 -3 mol dm-3 of potassium chromate (VI) are mixed together? Explain your answer, with the aid of relevant calculations. Ionic product, [Ag+]2 [CrO4 2-] = (3 x 10 -3/2)2 (5 x 10-3/2) = 5.625 x 10 -9 mol3 dm-9 Since ionic product > Ksp, hence ppt will be formed. [2] [Total: 7] H2 Chemistry 9746/02/NYJC J2/08 PX [Turn Over

6 For Examiner's Use 4(a) (i) Write a balanced equation for the reaction of heat on calcium nitrate. Ca(NO3)2 → CaO + 2NO2 + ½O2 (ii) Explain, with the help of equations including state symbols, what happens when a small amount of water is added initially to the white solid obtained after heating in (a)(i), followed by an excess of water. CaO(s) + H2O(l) → Ca(OH)2(s) When a small amount of water is added, solid calcium hydroxide is formed initially. When excess water is added, limewater, or aqueous calcium hydroxide is formed. Ca(OH) 2(s) + aq → Ca(OH)2(aq) [3] (b) A 4.50 g sample of a carbonate of a Group II metal, X, lost 1.34 g in mass when heated strongly. Identify the metal. XCO 3(s) → XO(s) + CO2(g) Original mass loss in mass Amount of CO 2 = 1.34/44.0 = 0.03045 mol Ratio of XCO 3 : CO2 = 1 : 1 Amount of XCO 3 = 0.03045 = 4.50/Mr of XCO3 M r of XCO3 = 147.8 Or 147.8 = A r of X + 12.0 + 3(16.0) A r of X = 87.8 ⇒ X is strontium. [3] [Total: 6] 5 J (Cr2H8N2O7) is an orange crystalline solid that is readily soluble in water to give an orange solution. When aqueous sodium hydr oxide is added to a sample of the solution, the solution tu