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1 SAJC H2 Chemistry 2014 Prelims Paper 1 Section A For each question there are four possible answers, A, B, C and D. Choose the one you consider to be correct. 1. A solid hydrocarbon was completely combusted in a closed vessel at 120 oC. The residual gas had a volume of 64 cm3, which decreased by 24 cm3 after bubbling through a dehydrating agent. After this, 40% of the final gas volume consisted of oxygen. What is the empirical formula of the hydrocarbon? 2. A CH B CH2 C CH3 D C2H3 When cobalt metal is reacted with a solution containing cobalt(III) ions, cobalt(II) ions are formed. How many moles of Co and Co3+(aq) would result in a mixture containing both cobalt(II) and cobalt(III) ions in the mole ratio of 3:1 after the reaction had taken place? Moles of Co 3. Moles of Co3+ A 1 2 B 1 3 C 1 5 D 2 3 Which graph does not share the same general shape with the other three graphs according to the ideal gas law for a fixed mass of gas? A pV against p (at constant T) B V/T against T (at constant p) C p against V (at constant T) D pV against V (at constant T) [Turn Over 2 4. 5. Which element will possess an empty s orbital after forming a singly-charged cation? A Barium B Vanadium C Copper D Gallium Which of the following reactions will form a product with the same shape as the reactant with respect to the underlined element? 6. A 2AlCl3 Al2Cl6 B 2H2O2 2H2O + O2 C PCl3 + Cl2 PCl5 D CH3CH2CHO + [O] CH3CH2COOH Which of the following is arranged correctly in the order of decreasing melting points? A Sulfur > Chlorine > Argon B Carbon dioxide > Silicon dioxide > Germanium dioxide C Beryllium chloride > Magnesium chloride > Calcium chloride D Sodium > Magnesium > Aluminium [Turn Over 3 7. A student dissolved 0.238 mol of sodium fluoride in 300 cm3 of water. Given the following data: −1 Lattice energy of NaF Enthalpy change of hydration of F −918 kJ mol − −1 −457 kJ mol + Enthalpy change of hydration of Na −1 −390 kJ mol What would be the initial temperature of the water if the final temperature of the solution o is 20.00 C? −1 −1 Assume that the specific heat capacity of sodium fluoride solution is 4.18 J g K . o A 6.52 C B 13.48 C C 20.00 C D 33.48 C o o 8. o Ammonia gas and hydrogen chloride gas react to form ammonium chloride as shown in the equation below: NH3 (g) + HCl (g) → NH4Cl (s) ∆Ho = −176 kJ mol−1 The standard entropy change of this reaction is −284 J K−1 mol−1. Which of the following statements is not correct? A At room temperature, ∆Go = +84.4 kJ mol−1. B The reactants are less stable than the product. C The reaction is spontaneous at low temperatures and non-spontaneous at high temperatures. D There is a decrease in the degree of disorderliness. [Turn Over 4 9. The following experimental results are obtained for a reaction with the general rate equation of: rate = k [M]x [N]y [L] Experiment [M] / mol dm-3 [N] / mol dm-3 [L] / mol dm-3 Relative rate 1 0.1 0.2 0.3 1 2 0.2 0.6 1.2 72 Which of the following are possible values of x and y? 10. x y A 1 1 B 2 1 C 1 2 D 2 2 Which of the following will decrease the rate constant for a reaction? A Decrease in concentration B Decrease in temperature C Decrease in pressure D Addition of catalyst [Turn Over 5 11. Phosphorus trichloride and chlorine react according to the following equation: PCl3(g) + Cl2(g) PCl5(g) A mixture of PCl3 and Cl2 was placed in a syringe and brought to equilibrium at time t and the following graph was obtained. Which of the following accounts for the drop in [PCl3] at time t? 12. 13. A PCl5 was added to the mixture B Argon gas was added to the mixture C The syringe was heated D The plunger of the syringe was withdrawn until the volume was twice the initial volume Which of the following statements about phosphoric(V) acid is not correct? A HPO42- has a higher Kb value than H2PO4-. B PO43- can react as a base. C H2PO4- can react both as an acid and as a base. D H3PO4 has a higher pKa than HPO42-. 95 cm3 of M2+ ions is mixed with an an equal volume of X- ions at 25oC. Which of the following initial concentrations of M2+ and X- will result in the precipitation of MX2? (Ksp of MX2 = 7.67 x 10-13 mol3 dm-9) [M2+] / 10-4 mol dm-3 [X-] / 10-4 mol dm-3 A 1 1 B 1 2 C 2 1 D 2 2 [Turn Over 6 14. Use of the Data Booklet is relevant to this question. Glucose can be reacted with silver ions to form a silver mirror in the following reaction. C6H12O6 + 2Ag+ + H2O 2Ag + C6H12O7 + 2H+ Ecell = +0.75 V Which of the following metal ions cannot be used for a possible reaction with glucose? 15. A Mn3+ B V3+ C Fe3+ D Co3+ Use of the Data Booklet is relevant to this question. The following reaction does not occur under standard conditions. 2NO3─ (aq) + 8H+ (aq) + 6Cl─ (aq) 2NO (g) + 4H2O (l) + 3Cl2 (g) Which of the following can result in the reaction occuring? 16. A Decreasing the volume of the reaction vessel B Addition of nitrogen monoxide C Decreasing the pH D Addition of chlorine The properties of the oxides of four Period 3 elements W, X, Y and Z are given below. • The oxide of W is insoluble in water and in dilute acid but soluble in concentrated sodium hydroxide. • The oxide of X is amphoteric. • The oxide of Y reacts with dilute potassium hydroxide at room temperature. • The oxide of Z dissolves in water to form a solution of pH = 13. Which of the following is correct in order of increasing atomic number? A Z, X, W, Y C Z, W, X, Y B W, X, Y, Z D Z, Y, W, X [Turn Over 7 17. The following graphs show how two properties of the elements, Na to P, and their compounds, vary with proton number. 11 12 13 14 15 11 12 13 14 proton number proton number Graph 1 Graph 2 15 What properties are shown by the two graphs? Graph 1 Graph 2 A Melting point of oxide Conductivity of element B Melting point of oxide Melting point of chloride C Melting point of chloride Melting point of oxide D Melting point of element Conductivity of element [Turn Over 8 18. A student observed the reactions when sodium chloride was added separately to concentrated sulfuric acid and concentrated phosphoric acid. The experiment was repeated using sodium iodide. Reagent Observation with sodium chloride Observation with sodium iodide conc. H2SO4 white fumes evolved purple vapour evolved conc. H3PO4 white fumes evolved white fumes evolved Which deduction can be made from these observations? A The reducing power of HI is greater than that of HCl. B The bond length of HI is smaller than that of HCl. C Concentrated phosphoric acid is a stronger oxidising agent than concentrated sulfuric acid. D Basicity of concentrated phosphoric acid is greater than concentrated sulfuric acid. 19. The following data refer to cobalt as a typical transition element and calcium as an s-block element. Which of the following properties shows the correct data for both elements? Property Cobalt Calcium A Metallic radius / nm 0.150 0.117 B Electrical conductivity / relative units 10.1 50 C Melting point / oC 1495 1965 D Density / g cm-3 8.9 1.54 [Turn Over 9 20. Which of the following cannot act as a ligand? 21. A AlH4- B N2H4 C CH3OH D CO The following compound is reacted with LiAlH4 in dry ether. What is the number of stereoisomers the product would possess? A 26 B 27 C 28 D 29 [Turn Over 10 22. Which statement with regards to the reaction of monobromination of benzene is correct? A This is considered a nucleophilic substitution reaction. B The intermediate consists of 4 pi electrons. C The intermediate is planar. D Upon addition of ethanolic silver nitrate solution to bromobenzene, a cream precipitate would be observed. 23. The compound below was reacted with hot alkaline potassium manganate(VII) solution. Which statement is correct? A The organic products consist of a total of 2 sp2 hybridised carbons. B There are 3 chiral centres in the organic products. C The organic products would not form a yellow precipitate with warm alkaline iodine solution. D 24. The organic products would not form an orange precipitate with 2,4–DNPH. In the following synthesis route, which reaction mechanism is not used? CH3CH2OH CH2=CH2 CH2XCH2X CH2(NH2)CH2NH2 A electrophilic addition B elimination C nucleophilic substitution D reduction [Turn Over 11 25. A cyclic organic compound has the molecular formula C4H7NO2. Which pair of the functional groups can be present in this molecule? 26. A one ester group and one primary amine group B one ester group and one nitrile group C one primary alcohol group and one primary amide group D one tertiary alcohol group and one tertiary amine group Which statement about ethanoic acid and its derivatives is correct? A Ethanamide is soluble in water to form a neutral solution due to the formation of equal proportions of ethanoic acid and ammonia. 27. B Ethanoic acid has a relative molecular mass of 120 in liquid hexane. C Ethanoyl bromide hydrolyses in water to produce reddish brown fumes. D Ethyl ethanoate, upon reaction with 2,4-DNPH, gives an orange precipitate. Which compound would show the greatest ease of hydrolysis? A B C D [Turn Over 12 28. The compound above has a total of three nitrogen atoms (NX, NY and NZ). Which sequence show an increase in pOH? A NX, NY, NZ B NX, NZ, NY C NY, NX, NZ D NY, NZ, NX [Turn Over 13 29. Two common drugs administered when a patient experiences headache or fever are paracetamol and aspirin. A solution of the two drugs was hydrolysed using hot aqueous sodium hydroxide. Which organic product would be produced? A B CO2- O O C 30. D Which statement about proteins and amino acids is not correct? A Amino acids exist as crystalline solids at room temperature. B Hydrogen bonds can be found in secondary and tertiary structures of proteins C Denaturation of proteins leads to the breaking down of the primary structure. D Amino acids can act as buffer solutions. [Turn Over 14 Section B For each of the questions in this section, one or more of the three numbered statements 1 to 3 may be correct. Decide whether each of the statements is or is not correct (you may find it helpful to put a tick against the statements that you consider to be correct). The responses A to D should be selected on the basis of A B C D 1, 2 and 3 are 1 and 2 only are 2 and 3 only are 1 only is correct correct correct correct No other combination of statements is used as a correct response. 31. 32. 33. Which of the following are disproportionation reactions? 1 3Cl2 + 6NaOH 5NaCl + NaClO3 + 3H2O 2 IO3- + 5I- + 6H+ 3I2 + 3H2O 3 15Se + SeCl4 + 4AlCl3 2Se8[AlCl4]2 Which of the following pairs are both planar and polar? 1 OCl2 and SO2 2 XeF4 and SO3 3 IF5 and H3O+ Which of the following increase with decreasing temperature for the dissociation of water? 1 pKb 2 pH 3 pKw Temperature/ oC Kw/ mol2 dm -6 50 5.5 x 10-14 25 1.0 x 10-14 [Turn Over 15 A B C D 1, 2 and 3 are 1 and 2 only are 2 and 3 only are 1 only is correct correct correct correct No other combination of statements is used as a correct response. 34. Use of the Data Booklet is relevant to this question. Which of the following statements can be deduced from the diagram below? Fe Cu NaCl(aq) CuSO4(aq) Cell 1 1 Size of the anode in Cell 1 decreases. 2 Colour of the electrolyte in Cell 2 remains the same. 3 Effervescence is observed in both cells immediately. Cell 2 35. 0.5 mol of each of the following was added to 100 cm3 of water. Which of the following solutions are arranged correctly in the order of increasing pH? 1 SiCl4, MgCl2, NaCl 2 H2SO4, HNO3, CH3CH2COOH 3 NH3, CH3NH2, KOH [Turn Over 16 A B C D 1, 2 and 3 are 1 and 2 only are 2 and 3 only are 1 only is correct correct correct correct No other combination of statements is used as a correct response. 36. Use of the Data Booklet is relevant to this question. When iron filings are added to nitric acid, a yellow solution and nitrogen dioxide gas are formed. On the addition of ammonium thiocyanate to the resultant solution, a bloodred colouration due to an iron (III) complex is formed. Which statements are correct? 37. 1 The oxidation state of N decreases from +6 to +4 in the first reaction. 2 Redox and ligand exchange have taken place. 3 The standard cell potential of the reaction between iron filings and nitric acid is +0.85 V. The following steps represent a student’s attempt to synthesise angelic acid from but-2-ene. Unfortunately, he was unable to synthesise angelic acid. Which steps were incorrect? 1 HCl(aq), heat 2 limited Cl2, U.V. 3 Ethanolic KCN, reflux [Turn Over 17 A B C D 1, 2 and 3 are 1 and 2 only are 2 and 3 only are 1 only is correct correct correct correct No other combination of statements is used as a correct response. 38. Which of the following reagents would react with both compounds above? 39. 1 warm alkaline iodine solution 2 warm Fehling’s solution 3 2,4–DNPH Hydrogen iodide reacts with liquid hex-2-ene to form 2-iodohexane and 3-iodohexane. When hex-2-ene is mixed with bromine monochloride, BrCl, dissolved in aqueous sodium methanoate, a similar reaction occurs. Which are the possible products in the mixture? 1 40. 2 3 Deuterium, D, is the H isotope of hydrogen. Which reactions could give an organic compound having a chiral centre? 1 2 3 , CH3CHO C(CH3)2=CH2 , CH3COCH3 ~~~ END ~~~ [Turn Over 1 SAJC H2 Chemistry 2014 Prelims Paper 1 Worked Solutions 1. A solid hydrocarbon was completely combusted in a closed vessel at 120 oC. The residual gas had a volume of 64 cm3, which decreased by 24 cm3 after bubbling through a dehydrating agent. After this, 40% of the final gas volume consisted of oxygen. What is the empirical formula of the hydrocarbon? A CH B CH2 C CH3 D C2H3 Vol of H2O = 24 cm3 Vol of CO2 = 0.6 x (64 – 24) = 24 cm3 Mole ratio of CO2 : H2O = 24 : 24 = 1 : 1 Mole ratio of C : H in CxHy = 1 : 2 2. When cobalt metal is reacted with a solution containing cobalt(III) ions, cobalt(II) ions are formed. How many moles of Co and Co3+(aq) would result in a mixture containing both cobalt(II) and cobalt(III) ions in the mole ratio of 3:1 after the reaction had taken place? Moles of Co Moles of Co3+ A 1 2 B 1 3 C 1 5 D 2 3 Co + 2Co3+ 3Co2+ I 1 3 0 C -1 -2 +3 1 3 F 0 (Co3+ was in excess) [Turn Over 2 3. Which graph does not share the same general shape with the other three graphs according to the ideal gas law for a fixed mass of gas? A pV against p (at constant T) pV = nRT At constant T, pV = constant B V/T against T (at constant p) V/T pV = nRT At constant p, nR V = ( )T p C = = constant T p against V (at constant T) pV = nRT At constant T, p = (nRT) D 1 V pV against V (at constant T) pV = nRT At constant T, pV = nRT = constant V [Turn Over 3 4. Which element will possess an empty s orbital after forming a singly-charged cation? A Barium Ba+: [Xe]6s1 B Vanadium V+: [Ar]3d34s1 C Copper Cu+: [Ar]3d10 D Gallium Ga+: [Ar]4s2 5. Which of the following reactions will form a product with the same shape as the reactant with respect to the underlined element? A 2AlCl3 Al2Cl6 Trigonal planar to Tetrahedral B 2H2O2 2H2O + O2 Bent to Linear C PCl3 + Cl2 PCl5 Trigonal pyramidal to Trigonal bipyramidal D CH3CH2CHO + [O] CH3CH2COOH Trigonal planar to Trigonal planar 6. Which of the following is arranged correctly in the order of decreasing melting points? A Sulfur > Chlorine > Argon S8 has the largest electron cloud, followed by Cl2 and Ar, hence it has the most extentive id-id interactions. B Carbon dioxide > Silicon dioxide > Germanium dioxide Silicon dioxide (giant covalent structure) should have a higher m.p than carbon dioxide (simple covalent structure). C Beryllium chloride > Magnesium chloride > Calcium chloride Beryllium chloride (simple covalent structure due to high charge density of Be2+) should have a lower m.p than magnesium chloride. D Sodium > Magnesium > Aluminium Al has the largest no. of delocalised electrons, followed by Mg and Na, hence it has the strongest metallic bonding. [Turn Over 4 7. A student dissolved 0.238 mol of sodium fluoride in 300 cm3 of water. Given the following data: −1 Lattice energy of NaF Enthalpy change of hydration of F −918 kJ mol − −1 −457 kJ mol + Enthalpy change of hydration of Na −1 −390 kJ mol What would be the initial temperature of the water if the final temperature of the solution o is 20.00 C? −1 −1 Assume that the specific heat capacity of sodium fluoride solution is 4.18 J g K . o A 6.52 C B 13.48 C C 20.00 C D 33.48 C o o o ∆H = ∆H − L. E = – 457 – 390 – (–918) = +71 kJ mol-1 (endothermic) Heat absorbed, q = 71 x 0.238 = 16.898 kJ ∆T = - ! "# =- $%&'& ())(*.$&) = - 13.48oC Initial T = 20 + 13.48 = 33.48oC [Turn Over 5 8. Ammonia gas and hydrogen chloride gas react to form ammonium chloride as shown in the equation below: NH3 (g) + HCl (g) → NH4Cl (s) ∆Ho = −176 kJ mol−1 The standard entropy change of this reaction is −284 J K−1 mol−1. Which of the following statements is not correct? A At room temperature, ∆Go = +84.4 kJ mol−1. ∆Go = ∆Ho - T∆So = - 176 – 298(-0.284) = -91.4 kJ mol−1 B The reactants are less stable than the product. C The reaction is spontaneous at low temperatures and non-spontaneous at high temperatures. At low T, |∆H| , |T∆S|, since both ∆H and ∆S are negative, ∆G < 0. D There is a decrease in the degree of disorderliness. There is a decrease in no. of gaseous moles from 2 to 0. [Turn Over 6 9. The following experimental results are obtained for a reaction with the general rate equation of: rate = k [M]x [N]y [L] Experiment [M] / mol dm-3 [N] / mol dm-3 [L] / mol dm-3 Relative rate 1 0.1 0.2 0.3 1 2 0.2 0.6 1.2 72 Which of the following are possible values of x and y? x y A 1 1 B 2 1 C 1 2 D 2 2 From expt 1 to 2, [M] increases by 2 times, [N] increases by 3 times and [L] increases by 4 times, rate increases by 72 times. Order of reaction wrt [L] is 1. A: 21 x 31 x 41 = 24 B: 22 x 31 x 41 = 48 C: 21 x 32 x 41 = 72 D: 22 x 32 x 41 = 144 [Turn Over 7 10. Which of the following will decrease the rate constant for a reaction? A Decrease in concentration B Decrease in temperature C Decrease in pressure D Addition of catalyst From Kinetics lecture notes, Change Effect on rate constant, k Effect on EA Increase in concentration No effect No effect Increase in pressure (gaseous system) No effect No effect Increase in temperature Increased No effect Use of catalyst Increased Decreased [Turn Over 8 11. Phosphorus trichloride and chlorine react according to the following equation: PCl3(g) + Cl2(g) PCl5(g) A mixture of PCl3 and Cl2 was placed in a syringe and brought to equilibrium at time t and the following graph was obtained. Which of the following accounts for the drop in [PCl3] at time t? A PCl5 was added to the mixture B Argon gas was added to the mixture C The syringe was heated D The plunger of the syringe was withdrawn until the volume was twice the initial volume Volume increases, resulting in an immediate drop in pressure. Equilibrium then shifts left to increase the pressure by favouring the backward reaction which produces less gaseous molecules. 12. Which of the following statements about phosphoric(V) acid is not correct? A HPO42- has a higher Kb value than H2PO4-. Ka of H3PO4 > Ka of H2PO4- > Ka of HPO42Kb of H2PO4- < Kb of HPO42- B since Kb = Kw / Ka 3- PO4 can react as a base. In presence of acid, PO43- + H+ HPO42- C H2PO4- can react both as an acid and as a base. In presence of acid, H2PO4- + H+ H3PO4 In presence of base, H2PO4- + OH- HPO42- + H2O D H3PO4 has a higher pKa than HPO42-. Ka of H3PO4 > Ka of H2PO4- > Ka of HPO42pKa of H3PO4 < pKa of H2PO4- < pKa of HPO42- [Turn Over 9 13. 95 cm3 of M2+ ions is mixed with an an equal volume of X- ions at 25oC. Which of the following initial concentrations of M2+ and X- will result in the precipitation of MX2? (Ksp of MX2 = 7.67 x 10-13 mol3 dm-9) [M2+] / 10-4 mol dm-3 A [X-] / 10-4 mol dm-3 1 1 I.P = [M ][X ] = (0.5 x 10 )(0.5 x 10-4)2 = 1.25 x 10-13 mol3 dm-9 < Ksp B 2+ - -4 2+ - -4 2+ - -4 2+ - -4 1 2 I.P = [M ][X ] = (0.5 x 10 )(1 x 10-4)2 = 5 x 10-13 mol3 dm-9 < Ksp C 2 1 I.P = [M ][X ] = (1 x 10 )(0.5 x 10-4)2 = 2.5 x 10-12 mol3 dm-9 < Ksp D 2 2 I.P = [M ][X ] = (1 x 10 )(1 x 10-4)2 = 1 x 10-12 mol3 dm-9 > Ksp Precipitation occurs 14. Use of the Data Booklet is relevant to this question. Glucose can be reacted with silver ions to form a silver mirror in the following reaction. C6H12O6 + 2Ag+ + H2O 2Ag + C6H12O7 + 2H+ Ecell = +0.75 V Which of the following metal ions cannot be used for a possible reaction with glucose? A Mn3+ Mn3+ + e- B V2+ E = -0.26 V Fe3+ Fe3+ + e- D E = +1.49 V V3+ V3+ + e- C Mn2+ Fe2+ E = +0.77 V Co2+ E = +1.82 V Co3+ Co3+ + e- Ag+ is reduced to Ag while glucose is oxidised. Ecell = Ered - Eox = +0.75 V Eox = 0.80 - 0.75 V = + 0.05 V For a feasible reaction, Ecell < 0, hence Ered (Mx+/M) should be more positive than +0.05 V. [Turn Over 10 15. Use of the Data Booklet is relevant to this question. The following reaction does not occur under standard conditions. 2NO3─ (aq) + 8H+ (aq) + 6Cl─ (aq) 2NO (g) + 4H2O (l) + 3Cl2 (g) Which of the following can result in the reaction occuring? A Decreasing the volume of the reaction vessel B Addition of nitrogen monoxide C Decreasing the pH Increase in [H+] will cause equilibrium to shift right, favouring the forward reaction. D 16. Addition of chlorine The properties of the oxides of four Period 3 elements W, X, Y and Z are given below. • The oxide of W is insoluble in water and in dilute acid but soluble in concentrated sodium hydroxide. • The oxide of X is amphoteric. • The oxide of Y reacts with dilute potassium hydroxide at room temperature. • The oxide of Z dissolves in water to form a solution of pH = 13. Which of the following is correct in order of increasing atomic number? A Z, X, W, Y C Z, W, X, Y B W, X, Y, Z D Z, Y, W, X W is Silicon. X is Aluminium. Y is either Phosphorus or Sulfur. Z is Sodium. [Turn Over 11 17. The following graphs show how two properties of the elements, Na to P, and their compounds, vary with proton number. 11 12 13 14 15 11 12 13 14 proton number proton number Graph 1 Graph 2 15 What properties are shown by the two graphs? Graph 1 Graph 2 A Melting point of oxide Conductivity of element B Melting point of oxide Melting point of chloride C Melting point of chloride Melting point of oxide D NaCl & MgCl2 – giant ionic NaO, MgO & Al2O3 – giant ionic AlCl3, SiCl4 and PCl5 – simple covalent Si2O3 – giant covalent Melting point of element P4O10 – simple covalent Conductivity of element [Turn Over 12 18. A student observed the reactions when sodium chloride was added separately to concentrated sulfuric acid and concentrated phosphoric acid. The experiment was repeated using sodium iodide. Reagent Observation with sodium chloride Observation with sodium iodide conc. H2SO4 white fumes evolved purple vapour evolved conc. H3PO4 white fumes evolved white fumes evolved Which deduction can be made from these observations? A The reducing power of HI is greater than that of HCl. NaCl + H2SO4 HCl (white fumes) + NaHSO4 (Cl- is not oxidised at all.) NaI + H2SO4 HI + NaHSO4 HI produced is readily oxidised by conc. H2SO4 to I2, hence I- has a greater ease of oxidation and reducing power than Cl-. 8HI + H2SO4 4I2 + H2S + 4H2O B The bond length of HI is smaller than that of HCl. False, as HCl has higher bond strength (smaller bond length) than HI due to smaller size of Cl and more effective orbital overlap. C Concentrated phosphoric acid is a stronger oxidising agent than concentrated sulfuric acid. H3PO4 is a non-oxidising acid. D Basicity of concentrated phosphoric acid is greater than concentrated sulfuric acid. True, but cannot be deduced from the above observations. [Turn Over 13 19. The following data refer to cobalt as a typical transition element and calcium as an sblock element. Which of the following properties shows the correct data for both elements? A Property Cobalt Calcium Metallic radius / nm 0.150 0.117 Co has more protons than Ca, hence the nuclear charge is higher than Ca. Co also has more electrons than Ca but the increase in shielding effect is minimal. Although the additional 2 electrons in Co are added in an inner sub-shell, they still provide a relatively poor shielding for the outermost 4s electrons. Hence, the valence 4s electrons in Co experiences a greater effective nuclear charge and are attracted more strongly to the nucleus than those in Ca, resulting in a smaller radius. B Electrical conductivity / relative units 10.1 50 Cobalt should have higher conductivity than calcium due to more delocalised electrons from 3d and 4s. C Melting point / oC 1495 1965 Cobalt should have higher m.p than calcium. For cobalt, both 3d and 4s electrons are delocalised to form a ‘sea’ of electrons for strong metallic bonding. Hence, strong electrostatic forces holding the metallic cations (which has a larger cationic charge and smaller cationic radius) together. D Density / g cm-3 8.9 1.54 While the size of the atom, measured by the metallic radius, decreases slightly from scandium to zinc, the relative atomic mass increases considerably. This results in an increase in density from scandium to zinc. The d-block metals are, in general, denser than the s-block metals. [Turn Over 14 20. Which of the following cannot act as a ligand? A AlH4- No available lone pair for dative bonding B N2H4 C CH3OH D 21. CO The following compound is reacted with LiAlH4 in dry ether. What is the number of stereoisomers the product would possess? A 26 B 27 C 28 D 29 [Turn Over 15 is the product, with no geometric isomers 22. Which statement with regards to the reaction of monobromination of benzene is correct? A This is considered a nucleophilic substitution reaction. This should be electrophilic substitution B The intermediate consists of 4 pi electrons. C The intermediate is planar. The carbon with four bonds has a tetrahedral shape D Upon addition of ethanolic silver nitrate solution to bromobenzene, a cream precipitate would be observed. Bromine is directly bonded to the benzene and hence would not likely undergo nucleophilic substitution to form Br–. Hence no precipitation would occur. [Turn Over 16 23. The compound below was reacted with hot alkaline potassium manganate(VII) solution. Which statement is correct? A The organic products consist of a total of 2 sp2 hybridised carbons. There is a total of 3 sp2 hybridised carbons. B There are 3 chiral centres in the organic products. There are no chiral centres in the products. C The organic products would not form a yellow precipitate with warm alkaline iodine solution. D The organic products would not form an orange precipitate with 2,4–DNPH. There are 2 ketone functional groups in the products which would form an orange precipitate with 2,4-DNPH. [Turn Over 17 24. In the following synthesis route, which reaction mechanism is not used? CH3CH2OH CH2=CH2 CH2XCH2X CH2(NH2)CH2NH2 25. A electrophilic addition B elimination C nucleophilic substitution D reduction A cyclic organic compound has the molecular formula C4H7NO2. Which pair of the functional groups can be present in this molecule? A one ester group and one primary amine group B one ester group and one nitrile group C one primary alcohol group and one primary amide group D one tertiary alcohol group and one tertiary amine group [Turn Over 18 26. Which statement about ethanoic acid and its derivatives is correct? A Ethanamide is soluble in water to form a neutral solution due to the formation of equal proportions of ethanoic acid and ammonia. Ethanamide is soluble in water and remains as ethanamide molecules. B Ethanoic acid has a relative molecular mass of 120 in liquid hexane. Ethanoic acid exist as a dimer in non-polar solvents. C Ethanoyl bromide hydrolyses in water to produce reddish brown fumes. HBr are white fumes. D Ethyl ethanoate, upon reaction with 2,4-DNPH, gives an orange precipitate. Esters do not undergo condensation reactions with 2,4-DNPH. 27. Which compound would show the greatest ease of hydrolysis? O A O Cl Cl C B HO I D OH I A – presence of ethanoyl chloride (an acyl chloride) which hydrolyses in water immediately B – alkyl bromide undergoes hydrolysis slower than acyl chlorides C – alkyl iodides undergoes hydrolysis faster than alkyl bromide due to the weaker C – I bond but still slower as compared to acyl chlorides. D – chlorine is directly bonded to carbon which is part of an alkene. This results in the C–Cl bond possessing partial double bond character, hence hydrolysis is unlikely to take place. [Turn Over 19 28. The compound above has a total of three nitrogen atoms (NX, NY and NZ). Which sequence show an increase in pOH? A NX, NY, NZ B NX, NZ, NY C NY, NX, NZ D NY, NZ, NX NX is the nitrogen in an amide, which is neutral. NY is the nitrogen in a tertiary amine. NZ is the nitrogen in a N-substituted phenylamine, which has its electrons delocalised into the benzene ring. [Turn Over 20 29. Two common drugs administered when a patient experiences headache or fever are paracetamol and aspirin. A solution of the two drugs was hydrolysed using hot aqueous sodium hydroxide. Which organic product would be produced? A B C D The products are: [Turn Over 21 30. Which statement about proteins and amino acids is not correct? A Amino acids exist as crystalline solids at room temperature. Amino acids exists as zwitterions which have strong ionic bonds and hence exist as crystalline solids at room temperature. B Hydrogen bonds can be found in secondary and tertiary structures of proteins. Hydrogen bonds between peptide linkages form the secondary structure and hydrogen bonds between R-groups form the unique 3D conformation in the tertiary structure. C Denaturation of proteins leads to the breaking down of the primary structure. Disruption of primary structure occurs during hydrolysis. Denaturation is the process where secondary, tertiary and quaternary structures are disrupted. D Amino acids can act as buffer solutions. Amino acids, upon dissolving in water, would form zwitterion, which has both acidic and basic functional groups and hence could maintain the pH of the solution when small amounts of acid or alkali are added. 31. Which of the following are disproportionation reactions? 1 3Cl2 + 6NaOH 5NaCl + NaClO3 + 3H2O 2 IO3- + 5I- + 6H+ 3I2 + 3H2O 3 15Se + SeCl4 + 4AlCl3 2Se8[AlCl4]2 Disproportionation reaction is a redox reaction in which a species is simultaneously reduced and oxidised to form two different products. The second and third reactions show two species in different oxidation states forming one product with another oxidation state. [Turn Over 22 32. Which of the following pairs are both planar and polar? 1 OCl2 and SO2 Both ben and non-polar 2 XeF4 and SO3 Square planar Non-polar 3 IF5 and H3O+ Square pyramidal Polar 33. Trigonal planar Non-polar Trigonal pyramidal Polar Which of the following increase with decreasing temperature for the dissociation of water? 1 Temperature/ oC Kw/ mol2 dm -6 50 5.5 x 10-14 25 1.0 x 10-14 pKb At lower T, Kw , [OH-] , Kb , pKb 2 pH At lower T, Kw , [H+] , pH 3 pKw At lower T, Kw , pKw [Turn Over 23 34. Use of the Data Booklet is relevant to this question. Which of the following statements can be deduced from the diagram below? Fe Cu NaCl(aq) CuSO4(aq) Cell 1 1 2 3 Cell 2 Size of the anode in Cell 1 decreases. At the anode: Cl2 + 2e2ClE = +1.36 V + O2 + 4H + 4e 2H2O E = +1.23 V 2+ Fe + 2e Fe E = -0.44 V 3+ Fe + 3e Fe E = -0.04 V Fe has the most negative E , so it will get oxidised at the anode. Colour of the electrolyte in Cell 2 remains the same. Electroplating of copper in Cell 2. Anode: Cu Cu2+ + 2eCathode: Cu2+ + 2e- Cu Effervescence is observed in both cells immediately. [Turn Over 24 35. 0.5 mol of each of the following was added to 100 cm3 of water. Which of the following solutions are arranged correctly in the order of increasing pH? 1 SiCl4, MgCl2, NaCl SiCl4 (pH = 2 due to HCl fumes), MgCl2 (pH = 6.5 due to high polarising power of Mg2+), NaCl (pH = 7) 2 H2SO4, HNO3, CH3CH2COOH H2SO4 (strong dibasic acid), HNO3 (strong monobasic acid), CH3CH2COOH (weak organic acid) 3 NH3, CH3NH2, KOH CH3NH2 is more basic than NH3 due to the electron-donating methyl group which increases the availability of the lone pair of e- on N. KOH is a strong base. 36. Use of the Data Booklet is relevant to this question. When iron filings are added to nitric acid, a yellow solution and nitrogen dioxide gas are formed. On the addition of ammonium thiocyanate to the resultant solution, a bloodred colouration due to an iron (III) complex is formed. Which statements are correct? 1 The oxidation state of N decreases from +6 to +4 in the first reaction. It should be from +5 to +4. 2 Redox and ligand exchange have taken place. Redox: Fe is oxidised to Fe3+ and NO3- is reduced to NO2. Ligand exchange: [Fe(H2O)6]3+ + SCN- [Fe(SCN)(H2O)5]2+ + H2O yellow blood-red 3 The standard cell potential of the reaction between iron filings and nitric acid is +0.85 V. Fe3+ + 3e- E = -0.04 V Fe NO3- + 2H+ + e- NO2 + H2O E = +0.81 V Ecell = +0.81 – (-0.04) = +0.85V [Turn Over 25 37. The following steps represent a student’s attempt to synthesise angelic acid from but-2-ene. Unfortunately, he was unable to synthesise angelic acid. Which steps were incorrect? 1 HCl(aq), heat 2 limited Cl2, U.V. 3 Ethanolic KCN reflux [Turn Over 26 38. Which of the following reagents would react with both compounds mentioned above? 1 warm alkaline iodine solution 2 warm Fehling’s solution 3 2,4–DNPH Neither compounds would work with Fehling’s solution as neither have the aliphatic aldehyde function group, compound B is an acyl iodide which is not a carbonyl compound and hence would not react with 2,4-DNPH. [Turn Over 27 39. Hydrogen iodide reacts with liquid hex-2-ene to form 2-iodohexane and 3-iodohexane. When hex-2-ene is mixed with bromine monochloride, BrCl, dissolved in aqueous sodium methanoate, a similar reaction occurs. Which are the possible products in the mixture? 1 40. 2 3 Deuterium, D, is the .$H isotope of hydrogen. Which reactions could give an organic compound having a chiral centre? 1 /01,1345 CH3CHO 677777778 The product is made of optical isomers of CH3CH(OH)CO2– 2 /09 C(CH3)2=CH2 6777778 The product is made of C(CH3)2ClCH2D 3 13:/; ,0/< 4/ CH3COCH3 677777777778 The product is made of CH3CD(OD)CH3 ~~~ END ~~~ [Turn Over SAJC H2 Chemistry 2014 Prelims Paper 2 1 Answer all the questions 1 Planning (P) Iron(II) sulfate is one of the most readily available sources of iron(II). Hence it is used to make iron supplement tablets to treat anaemia and other iron deficiencies. Women are at a higher risk for iron-deficiency anaemia than men, especially pregnant women. However, excess iron in the body can lead to health problems, and in severe cases, death. The percentage by mass of iron in an iron supplement tablet can be determined by titrating it with acidified potassium manganate(VII). (a) Using the Data Booklet, write an ionic equation for the reaction between iron(II) sulfate and acidified potassium manganate(VII). _________________________________________________________________ [1] (b) If the concentration of a standard solution of potassium manganate(VII) is 0.500 mol dm‒3, calculate the range of mass of powdered iron supplement tablets needed for a suitable and reliable titration. You may assume that the percentage mass of iron in iron supplement tablets is between 80% to 90%. [2] [Turn over 2 1 (c) Write a plan for the titration that you would carry out to calculate the mass of iron present in the tablet. You are given a 0.500 mol dm‒3 standard solution of potassium manganate(VII) and excess powdered iron tablets. You may use the reagents and apparatus normally found in a school or college laboratory. In your plan, you should give: Details, including quantities of all reagents and capacities of apparatus used; The essential details of the titration procedure. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ [6] [Turn over 3 1 d) Some women take multi-vitamin pills to supplement their diet which often also contain mineral ions such as Cu2+. However, too much Cu2+ in our body causes various health problems. In a manufacturing error, a batch of multi-vitamin pills contained too much Cu2+ and was also contaminated accidentally with a small amount of Ba2+ ions which are poisonous. A sample from this batch of multi-vitamin pills is dissolved to form a solution. You are to devise a series of chemical tests which can be conducted under laboratory conditions using common laboratory reagents and apparatus to separate the two cations above, such that each cation is present in a separate precipitate. Record your plan in the table below. Test Expected observation Identity of cation in precipitate [3] [Total: 12 marks] [Turn over 4 2 Phosphorus tribromide is a colourless liquid that is commonly used in laboratories for the manufacture of alkyl bromide from alcohols. (a) (i) Draw a dot and cross diagram of phosphorus tribromide. (ii) Draw the structure for the compound formed when phosphorus tribromide reacts with boron tribromide (iii) Suggest whether phosphorus tribromide or boron tribromide has a higher boiling point. Explain. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ [5] (b) Phosphorus tribromide reacts in a similar way as phosphorus trichloride. Write an equation, with state symbols, to show the reaction of phosphorus tribromide with water. _________________________________________________________________ [1] [Turn over 5 2 (c) Using the information from the table below and any relevant data from the Data Booklet, construct an energy level diagram to calculate the bond energy of the P-Br bond. ∆H/ kJmol-1 Atomisation energy of phosphorus +315 Enthalpy change of formation of phosphorus tribromide -185 Enthalpy change of vapourisation of phosphorus +39 tribromide [4] [Turn over 6 2 (d) The general equation for the formation of alkyl bromide from alcohols using phosphorus tribromide is: PBr3 + 3ROH 3RBr +H3PO3 The reaction mechanism for the above reaction using ethanol and phosphorus tribromide is shown below. (i) State the type of reaction for the overall reaction _____________________________________________________________ (ii) Complete the mechanism above by using partial charges, lone pairs and curly arrows to represent the electron flow. (iii) Suggest a reason why this reaction method cannot be used to prepare C(CH3)3Br. _____________________________________________________________ _____________________________________________________________ (iv) Using the above mechanism in 2(d), suggest the organic compound formed when ethanoic acid is reacted with phosphorus tribromide. _____________________________________________________________ [7] [Turn over 7 2 (e) PBr3 is used in the following reaction scheme to produce a sweet-smelling compound D. (i) Draw the structures of A to D in the spaces provided above. [Turn over 8 2 (e) (ii) Give the reagents and conditions for Steps II, III, IV and VI. Step II: ____________________________________________________ Step III:____________________________________________________ Step IV: ____________________________________________________ Step VI: _________________________________________________________ [8] [Total: 25 marks] 3 Hydrogen peroxide exhibits oxidising and reducing properties, depending on pH. In acidic solutions, H2O2 is one of the most powerful oxidising agents known and is used commonly as a bleaching agent and disinfectant. In one of its reactions, it oxidises potassium iodide to form iodine and water. (a) With reference to the Data Booklet, write the balanced equation for the reaction between hydrogen peroxide and potassium iodide. _____________________________________________________________ [1] [Turn over 9 3 (b) To study the kinetics of the above reaction, two sets of separate experiments were performed, in which the initial concentrations of each of the reactants were varied in turn, the other two being kept constant. The results are shown below. Graph 1 Graph 2 [H ] [I-] 0.10 0.10 0.08 0.08 + 0.06 W 0.04 X 0.02 0.06 0.04 Y 0.02 Z s 2s time t 2t time W: [H2O2] = 1.0 mol dm-3 and [I-] = 1.0 mol dm-3 X: [H2O2] = 2.0 mol dm-3 and [I-] = 1.0 mol dm -3 Y: [H2O2] = 1.0 mol dm-3 and [H+] = 1.0 mol dm-3 Z: [H2O2] = 2.0 mol dm -3 and [H+] = 1.0 mol dm-3 Use the data above to deduce the rate equation and give the units for the rate constant. [5] [Turn over 10 3 (c) Two different mechanisms have been proposed for this reaction. Mechanism A: H2O2 + I- H2O + OI(slow) + (fast) OI + H HOI HOI + H+ + I- I2 + H2O (fast) Mechanism B: H2O2 + I- + H+ H2O + HIO (slow) HIO + I- I2 + OH(fast) OH- + H+ H2O (fast) Suggest with reasons which of the two mechanisms, A or B, fits the kinetic data in (b). _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ [2] (d) Draw the energy profile diagram for the reaction based on mechanism A. [3] [Total: 11 marks] [Turn over 11 4 Cobalt has been used for centuries to impart a rich blue color to glass and ceramics. (a) A complex of cobalt is prepared by the following method: Air is bubbled through an aqueous solution containing CoCl2, NH3 and NH4Cl. The resulting solution was evaporated and crystals of the cobalt compound, P can be isolated. P has an octahedral cation, and is found to have the following composition by mass: Co: 25.2% ; N: 24.0% ; H: 5.1% ; Cl : 45.7% (i) Given the molar mass of P is 233.4, calculate the molecular formula of P and suggest the ligands that are bonded to cobalt. (ii) On adding an excess of aqueous solution of silver nitrate to an aqueous solution of 0.01 mol of P, 1.43 g of silver chloride is precipitated. Given that P has no overall dipole moment, deduce the structural formula of P. Draw the full structure of its cation, showing clearly the arrangement of the ligands that surround cobalt. [6] [Turn over 12 4 (b) A student carried out the following experiment in the laboratory to investigate the chemistry of cobalt compounds. Procedures Step 1 5 g of the potassium sodium tartrate was weighed into a 250 cm3 beaker. 60 cm3 of distilled water was added and the solution was stirred. Step 2 20 cm3 of hydrogen peroxide solution was added to the solution in step 1. Effervescence was observed after 5 hours. The student repeated the experiment with the addition of 2 cm3 of cobalt(II) chloride solution to the mixture. He recorded the following observations: The solution in the beaker turned from colourless to pink when cobalt(II) chloride solution was added. The solution then turned green and vigorous effervescence was observed immediately. When the effervescence stopped, the solution turned back to pink. What was the role of cobalt(II) chloride solution in the experiment? Give two reasons to support your answer using the information above. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ [3] [Turn over 13 4 (c) When a crystal of NH4SCN was added to cobalt(II) chloride solution, the solution changed colour and the visible absorption spectrum for the resulting solution was shown below: Relative absorbance 300 400 500 600 700 800 Wavelength (nm) White light spectrum red Wavelength(nm) 700 (i) green 600 500 blue 400 violet 300 What is the colour of the resulting solution? _____________________________________________________________ (ii) Explain why the resulting solution is coloured. _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ _____________________________________________________________ [4] [Total: 13marks] [Turn over 14 5 This question involves various cyclic compounds and their reactions. (a) In the Shapiro reaction, a carbonyl group reacts in the presence of an electrophile to form an alkene. A simplified example of the Shapiro reaction is shown below. where E represents an electrophile Shapiro reaction is used on Q. Q (i) State the number of chiral centers in Q Number of chiral centers = ____________________ (ii) In the diagram below, circle the sp2 hybridised carbons. [Turn over 15 5 (a) (iii) Draw the structures of the product formed when Q reacts with each of the following reagents in the Shapiro reaction. H2O (iv) Chloroethane Cyclic esters R and S are possible products of the Shapiro reaction. R S Suggest reagents and conditions to distinguish between esters R and S. State the observations for each ester. Reagents and conditions: ______________________________________________________________ Observations: ______________________________________________________________ ______________________________________________________________ [6] [Turn over 16 (b) The structure of a dicarboxylic acid J is shown below. J (i) The pKa values of the two acidic groups in J are 2.2 and 6.4. In the diagram above, circle the acidic group which has a pKa value of 2.2. Explain your answer. ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ K is an isomer of J. K (ii) State the type of isomerism exhibited in J and K. ______________________________________________________________ [Turn over 17 5 (b) (iii) With the aid of a diagram, explain why J has a more stable mono-anion than the mono-anion of K. ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ [5] [Total: 11 marks] ~~~END~~~ [Turn over SAJC H2 Chemistry 2014 Prelims Paper 2 Worked Solutions 1 (a) 5Fe2+ + MnO4‒ + 8H+ 5Fe3+ + Mn2+ + 4H2O [1] (b) If volume of titre is assumed to be 25 cm3: Amount of KMnO4 required = 0.5 x 0.025 = 0.0125 mol Amount of Fe2+ = 0.0125 x 5 = 0.0625 mol Mass of Fe2+ = 0.0625 x 55.8 = 3.4875 g Maximum mass of tablet = (100/80) x 3.4875 = 4.36 g Minimum mass of tablet = (100/90) x 3.4875 = 3.88 g [2] (c) • Weigh a dry and clean weighing bottle. • Add iron supplement tablet into the weighing bottle and weigh the bottle + tablet. • Tip the tablet into a small beaker and reweigh the emptied weighing bottle to determine the actual mass of tablet used. • Add excess dilute sulfuric acid to the small beaker containing the tablet. • Stir with a glass rod to dissolve the tablet. • Transfer the solution with several washings into a clean 250 cm3 volumetric flask. • Make up to the mark with distilled water. • Stopper the volumetric flask and shake well to obtain a homogeneous solution. • Pipette 25.0 cm3 of the iron solution prepared into a 250 cm3 conical flask. • Fill the burette the standard solution of KMnO4. • Titrate the iron solution against KMnO4 from the burette, with continuous swirling. • Stop when one drop of solution from the burette causes a colour change from colourless to pale pink. [6] (d) Step Expected observation Identity of cation in ppt Add NaOH(aq) Pale blue ppt formed in Cu2+ dropwise until excess. colourless solution. 3 Pour 2 cm of the solution into a test tube. Filter the mixture into a separate test tube. Add excess H2SO4(aq) White ppt formed. [½] Ba2+ to the filtrate. * Can identify Ba2+ first [3] 2 a) i) ii) iii) Both compounds are simple covalent. Phosphorus tribromide is polar with permanent dipole-permanent dipole while boron tribromide is non polar with induced dipole-induced dipole. More energy required to break the stronger pd-pd interactions of phosphorus tribromide so PBr3 has a higher boiling point. [5] b) PBr3(l) +3H2O(l) H3PO3(aq) +3HBr(aq) [1] -1 c) Energy/ kJ mol P(g) + 3Br(g) 3/2 x (+193) P(g) + 3/2Br2(l) +315 3 x BE P(s) + 3/2Br2(l) 0 PBr3(g) -185 +39 PBr3(l) BE = +250 kJmol-1 [4] d) i) Nucleophilic substitution ii) iii) To form C(CH3)3Br would require the starting alcohol to be a tertiary alcohol which is not feasible for a SN2 reaction due to steric hindrance/large bulky groups/electron donating methyl groups. iv) e) CH3COBr [7] (i) (ii) [8] Total: 25 marks 3 a) H2O2 +2 I- + 2H+ 2H2O + I2 b) From graph 1, since graph is a straight line graph, rate of reaction is constant when [H+] changes. Therefore zero order wrt [H+]. From graph 1, when [I-] is constant, [H2O2] is doubled, rate (gradient) is doubled from 0.02/2s to 0.02/s. Therefore first order wrt [H2O2]. From graph 2, When [H+] = 1.0 mol dm−3, [H2O2] = 2.0 mol dm−3, t1/2 is constant at ½ t. Therefore first order wrt [I-]. Rate = k [H2O2] [I-] c) units for k: mol-1dm3s-1 Mechanism A. Step 1 is the rate determining step as the stoichiometric coefficient of the reactants matches the power of the [H2O2] and [I-] in the rate equation. d) Energy ∆H Reaction Pathway [Total: 11 marks] 4 a) i) Co N H Cl % 25.2 24.0 5.1 45.7 Mole ratio 0.428 1.71 5.1 1.29 Simplest ratio 1 4 12 3 n (CoN4H12Cl3 ) = 233.4, n=1 molecular formula = CoN4H12Cl3 . Ligands: NH3 and Clii) 0.01 mol of P contains 9.965 x 10-3 mol of free chloride ions 1mol of P contains 1 mol of free chloride ions formula of P : [Co(NH3)4Cl2]+ Cl- [6] b) i) Catalyst The reaction becomes faster when it is added as seen from the vigorous effervescences The solution turns from pink to green and back to pink colour as the catalyst is regenerated 4c) i) Violet ii) In the isolated gas phase, the d orbitals of the Co2+ is degenerate. In the complex, partially filled 3d orbitals split into two groups with a small energy gap between them. When light shines, the complex absorbs light energy from the visible light spectrum to promote electrons from the lower to the higher energy group, (d-d transition). The light not absorbed will be reflected and seen as the colour of the complex. [7] [Total: 13marks] 5 a) i) ii) Number of chiral centers = 2 iii) iv) H2O Chloroethane Reagents and conditions: KMnO4, aq. H2SO4, heat Observations: S decolourise purple KMnO4 but purple KMnO4 remains in R *K2Cr2O7 accepted. [6] (b) J (i) It is further from the electron donating cyclopentane group which destabilise the carboxylate anion. (ii) 5 (b) Geometric isomer/ cis-trans (iii) Anion after dissociation is stabilised by intra hydrogen bonding of the carboxylate group with –OH [5] [Total: 11 marks] ~~~END~~~ SAJC H2 Chemistry 2014 Prelims Paper 3 1 (a) 1 Phosphorus pentachloride, PCl5, is a useful reagent in organic synthesis. It can undergo auto-ionisation in polar solvents according to the following equilibrium: 2PCl5 PCl4+ + PCl6‒ Draw the dot-and-cross diagrams for PCl4+ and PCl6‒ ions and state the shapes around the central atom for each ion. [3] (b) PCl5 exists in equilibrium with PCl3 and chlorine gas as shown below: PCl5 (g) PCl3 (g) + Cl2 (g) ∆H = +124 kJ mol‒1 At equilibrium, at 200 °C and a total pressure of 5 atm, 40% of PCl5 is dissociated. (i) Write an expression for Kp for the above equilibrium. (ii) Calculate a value for Kp at 200 °C and state its units. (iii) Predict and explain the effect of increasing the temperature on the value of Kp. [5] (c) Aluminium oxide is insoluble in water. However, it is able to dissolve in an aqueous solution of PCl5. (i) Explain why aluminium oxide is insoluble in water. (ii) Explain why aluminium oxide can dissolve in an aqueous solution of PCl5 and write balanced equations for any reactions that occur. [4] [Turn Over 2 1 (d) 5-hydroxy-2-coumaranone can be synthesised using compound F as the starting material. The structural formulae of 5-hydroxy-2-coumaranone and compound F are shown below. HO O O 5-hydroxy-2-coumaranone Compound F Suggest a synthesis to convert compound F into 5-hydroxy-2-coumaranone in not more than five steps. State all reagents and conditions used and all intermediates formed. [4] (e) G and H have the following structure: OH CO2H O CHO O Compound G Compound H Describe a chemical test which would distinguish between F and G and a different chemical test to distinguish between F and H. [4] [Total: 20] [Turn Over 3 2. (a) Hydroxyapatite, Ca5(PO4)3OH, is a group of phosphate minerals which can be found in the teeth and bones of the human body. Thus, it is commonly used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. (i) Write an equation to illustrate what is meant by the term enthalpy change of formation of hydroxyapatite. (ii) Using the following information and with the aid of the Data Booklet, construct a Born Haber cycle to calculate the enthalpy change of lattice energy of hydroxyapatite. Value / kJ mol-1 (iii) ∆Hformation Ca5(PO4)3OH (s) -12 969 ∆Hformation OH- (g) -230 ∆Hformation PO43- (g) -1913 ∆Hatomisation Ca (s) +178.2 Hydroxyapatite thermally decomposes to produce calcium phosphate, calcium oxide and steam. Write a balanced equation to show the thermal decomposition of hydroxyapatite. Hence, calculate the mass of calcium oxide that could be obtained by decomposing 9 g of hydroxyapatite. (iv) Al2PO4(OH)3 is a recently discovered phosphate mineral found in high pressure metamorphic rocks. Suggest, using the Data Booklet, why it has been found that Al2PO4(OH)3 decomposes at a lower temperature than hydroxyapatite. [8] [Turn Over 4 2 (b) An organophosphate is the general name for esters of phosphoric acid. Acephate is an organophosphate which is used in many insecticides and herbicides. Acephates are acutely toxic where recent studies suggest a possible link to adverse effects in the neurobehavioral development of foetuses and children. (i) Experts have recommended that the concentration of acephate in fruits should not exceed 7 ppm (1 ppm = 1 mg of acephate in 1 kg of fruits). Calculate the maximum mass in grams of acephate that a fruit weighing 50 g can contain before it is deemed hazardous. (ii) Alkyl phosphates are phosphate esters that are formed when 1 mole of phosphoric acid combines stepwise with 3 moles of an alcohol as shown in the reaction below. State the type of reaction in the formation of the phosphate esters. [Turn Over 5 2 (b) (iii) Alkyl phosphates play a central role in biochemistry. Phosphate ester linkages compose the backbone of the nucleic acid, RNA, which performs multiple vital roles such as genetic expression. Ribose, an alcohol, forms part of the RNA backbone. Using the segment of the RNA molecule above and (b)(ii), suggest the structural formula of the alcohol which reacted with phosphoric acid. (iv) Name three functional groups in the segment of the RNA molecule. [6] [Turn Over 6 2 (c) An aldose is an alcohol that contains only one aldehyde group per molecule. The Killiani-Fischer synthesis lengthens an aldose carbon chain by adding one carbon to the aldehyde end of the aldose. An example of the synthesis of tetrose from triose is illustrated below. (i) Outline the mechanism of the reaction in Step I. (ii) Tetrose is reacted with NaBH4 to form compound X. Tetrose is reacted with acidified potassium dichromate to form compound Y. Using the structural formulae of X and Y, write balanced equations to explain each of the above reactions. [6] [Total: 20 marks] [Turn Over 7 3 (a) Electrophoresis is a technique used to separate a mixture of amino acids. Often, this process is carried out using a gel which is soaked in a buffer solution to ensure pH is kept at a relatively constant value. A small well is etched on the gel and the solution containing the mixture of amino acids is dropped carefully in the well. An electric field is then applied across the gel. The table below shows three amino acids to be separated by electrophoresis and their isoelectric points. Amino acid Formula of side chain Isoelectric point (R in RCH(NH2)CO2H) Cysteine (Cys) ‒CH2SH 5.07 Lysine (Lys) ‒(CH2)4NH2 9.74 Serine (Ser) ‒CH2OH 5.58 (i) Explain what is meant by a buffer solution. (ii) Draw the predominant species of each amino acid when the pH of the buffer solution is 7.20. (iii) Illustrate two ways in which vinegar might interact with a protein that contains the three amino acids in the table above to bring about denaturation. You may find that including sketches or diagrams will help you in your answer. [8] (b) Keratin is an important protein which is a major component in hair. Keratin contains large amounts of cysteine which gives strength to our hair. Perming hair involves creating permanent waves in hair. This process includes forming new disulfide bonds via oxidation. Hydrogen peroxide is often used as the oxidising agent. (i) Briefly explain how the presence of cysteine in keratin strengthens hair. (ii) Using the Data Booklet, write the half-equations involved in the formation of new disulfide bonds using hydrogen peroxide. [3] [Turn Over 8 3 (c) Hydrogen peroxide can also act as a reducing agent. It is used to react with sodium hypochlorite, NaClO, to produce oxygen gas. The resulting solution is then reacted with aqueous silver nitrate and a white precipitate is seen. The white precipitate disappears on addition of dilute aqueous ammonia. (i) Using the Data Booklet, write the overall equation that produces oxygen gas. (ii) Identify the white precipitate and explain its subsequent disappearance with the aid of relevant equations. (iii) Sodium hypochlorite can be prepared using chlorine in the laboratory. Suggest a reason why this preparation should not involve heating. (iv) A 250 cm3 solution contains 0.05 mol dm‒3 silver nitrate and 0.04 mol dm‒3 copper(I) nitrate. By carefully adding solid NaCl, it is possible to precipitate Ag+ while keeping Cu+ in solution. Compound Ksp value AgCl 1.8 x 10‒10 CuCl 1.2 x 10‒6 Using the data in the table above, determine the maximum mass of NaCl that can be added before Cu+ starts to precipitate. [9] [Total: 20] [Turn Over 9 4 (a) Electrosynthesis in organic chemistry is the synthesis of chemical compounds in an electrochemical cell using inert electrodes. In 1932, Gordon Alles reported his investigations into the electrolysis of an aqueous solution of nitroalkene to an amine. 1-phenyl-2-aminopropane can be obtained from the cathode by the following reaction. (i) Suggest the products of the reaction at the anode. Hence, construct an equation for the overall reaction. (ii) Calculate the mass of 1-phenyl-2-aminopropane produced when a current of 0.25 A is passed through an aqueous solution of phenyl nitropropylene for a day. [4] (b) On completion of the electrosynthesis reaction, the purification of 1-phenyl-2-aminopropane is effected by exactly neutralising the impure product with aqueous hydrochloric acid. (i) Write a balanced chemical equation between 1-phenyl-2-aminopropane and hydrochloric acid. (ii) 1-phenyl-2-aminopropane crystals are dissolved in ethanol and made up to 250 cm3 in a volumetric flask. 25 cm3 of this solution is then pipetted and required 27.4 cm3 of 0.05 mol dm-3 aqueous hydrochloric acid for reaction. Calculate the concentration of the solution of 1-phenyl-2-aminopropane in g dm-3. (iii) Hence, determine the percentage purity of the 1-phenyl-2-aminopropane crystals produced during the electrosynthesis reaction in (a)(ii). [4] [Turn Over 10 4 (c) (i) 1,4-diaminobutane is a bidentate ligand which can be synthesised from a suitable nitroalkene. 1,4-diaminobutane Using information from (a), suggest the structural formula of this nitroalkene. (ii) What do you understand by the term bidentate ligand? (iii) Addition of 1,4-diaminobutane to an aqueous solution containing Cu2+ ion causes the formation of a violet complex. Account for this change in colour of the copper-containing compounds. (iv) Suggest the formula of this violet complex ion. [5] (d) (i) 2-aminopropane can be synthesised using NH3 and a suitable chloroalkane. Draw the displayed formula of this chloroalkane. (ii) A by-product in the formation of 2-aminopropane from (d)(i) gives an immediate precipitate with ethanolic silver nitrate and is able to conduct electricity in the molten and aqueous state. Suggest a structure for this by-product given that its molecular formula is C12H28NCl. [2] [Turn Over 11 4 (e) Amines react with carbonyl compounds to yield imines as shown in the following reaction. One of the pathways by which organisms make alanine involves the addition of ammonia to alpha-keto acids such as pyruvic acid. (i) Using the above information, suggest the structural formula of imine W. (ii) With reference to structure and bonding, explain why alanine has a higher melting point than pyruvic acid. (iii) Propose a two-step synthetic pathway for the synthesis of pyruvic acid from propene. [5] [Total: 20 marks] [Turn Over 12 5 (a) J has the following structural formula: The pKa1 and pKa2 values of J are 3.25 and 7.60 respectively. (i) Calculate the pH of a 0.025 mol dm‒3 solution of J (ignore the effect of pKa2 on the pH). (ii) 30 cm3 of 0.025 mol dm‒3 NaOH is added to 10.0 cm3 of 0.025 mol dm‒3 of J. Draw the structural formulae of the predominant organic species when 15 cm3 of NaOH is added. (iii) Sketch the shape of the pH curve you would expect to obtain from (ii) and briefly describe how you have calculated the various key points on the curve. [8] (b) A, C10H14O2NI, dissolves in aqueous HNO3. A is formed, along with a brown precipitate, when B reacts with potassium manganate(VII) at 0°C. On addition of ethanolic silver nitrate to A, a yellow precipitate is formed. When aqueous bromine is added to A, the orange-red solution decolourises and a white precipitate, C, C10H11O2NIBr3, is formed. 0.125 mol of A produces 2800 cm3 of gas at s.t.p. when sodium metal is added. D, C10H13O2NICl, is produced when potassium dichromate and hot dilute hydrochloric acid is added to A. D reacts with 2,4-DNPH but not with Tollen’s reagent. E and a yellow precipitate are formed when warm aqueous iodine and aqueous sodium hydroxide are added to both A and D separately. Deduce the structures of A to E, and explain the reactions described. [12] [Total: 20] ~~~ END OF PAPER ~~~ [Turn Over SUGGESTED ANSWERS SAJC PRELIM 2014 PAPER 3 (H2 CHEMISTRY) 1 (a) [3] (b) (i) Kp = (ii) PCl5 PCl3 Cl2 I / mol Z 0 0 C / mol -0.4 z +0.4 z +0.4 z E / mol 0.6 z 0.4 z 0.4 z Total moles = 1.4 z PPCl5 = (0.6 z / 1.4 z) x 5 = 2.143 atm PPCl3 = PCl2 = (0.4 z / 1.4 z) x 5 = 1.429 atm Kp = 0.953 atm (iii) When temperature increases, the equilibrium will favour the endothermic reaction to absorb the excess heat / to decrease the temperature. Hence, rateforward increases more than ratebackward. Hence, Kp increases. [5] (c) (i) The energy released from forming ion-dipole interaction between aluminium oxide and water is insufficient to overcome ionic bonds in aluminium oxide. (ii) Aluminium oxide is amphoteric. Aqueous solution of PCl5 is acidic. PCl5 + 4 H2O H3PO4 + 5 HCl Al2O3 + 6HCl →2AlCl3 + 3H2O [4] 1 [Turn Over (d) [4] (e) Between F and G Add Tollen’s reagent to both compounds and warm. No silver mirror seen for compound F, while silver mirror is seen for G. OR Add neutral FeCl3 F forms a violet solution (complex) whilst G forms no violet solution. OR Add Br2 (aq) F orange will turn to colourless with white ppt and G will remain orange. OR Add 2, 4-DNPH. Orange ppt formed for G and no orange ppt formed for F. Between F and H Add 2, 4-DNPH. Orange ppt formed for H and no orange ppt formed for F. OR Add hot aq H2SO4 K2Cr2O7 F turns from orange to green whilst H remains orange. [4] [Total: 20] 2 [Turn Over 2. (a) (i) 5 Ca(s) + 3 P(s) + 13/2 O2 (g) + ½ H2 (g) Ca5(PO4)3OH (s) (ii) 5(178.2) + 5(590+1150) – 230 – 3(1913) + x = - 12969 x = -16 591 kJ mol-1 (iii) 2Ca (PO ) OH 3Ca (PO ) + CaO + H O 5 4 3 3 4 2 2 Moles of hydroxyapatite = 9 / 502.5 = 0.01791 mol Moles of calcium oxide = 8.96 x 10-3 mol Mass of calcium oxide = 8.96 x 10-3 x (40.1+16) = 0.502 g (iv) Quote: Ca2+: 0.099 nm and Al3+: 0.050 Al3+ ion has a higher charge density, and higher polarising power than Ca2+. Thus, there is more distortion of the electron cloud of the anion, hence requiring less energy to break. [8] (b) (i) Given that 1 kg of fruits can contain 7mg of acephate, 1g of fruit = 7 x 10-6 g Hence, 50 g of fruit = 3.5 x 10-4 g of acephate (ii) Nucleophilic substitution / Condensation 3 [Turn Over (b) (iii) (iv) Alkene, (secondary or tertiary) amide, secondary alcohol OR ether [6] (c) (i) (ii) [6] [Total: 20 marks] 4 [Turn Over 3 (a) (i) A buffer solution is a solution whose pH remains almost unchanged when a small amount of H+ or OH- is added to it. (ii) Cysteine Lysine Serine H H2N H COO H 2N CH2SH COO CH2OH (iv) AND [8] (b) (i) Cysteine can form disulfide linkages which are strong covalent bonds which are hard to break. (ii) 2 -CH2SH -CH2S-SCH2- + 2H+ + 2eH2O2 + 2H+ + 2e- 2H2O [3] (c) (i) H2O2 O2 + 2H+ + 2e- 2e- + 2H+ + ClO- Cl- + H2O Overall eqn: H2O2 + ClO- O2 + H2O + Cl(ii) White ppt is AgCl. AgCl + 2NH3 [Ag(NH3)2]+ + ClAgCl Ag+ + Cl- (Eqm 1) The decrease in [Ag+] due to formation of complex shifts eqm 1 to the right. Also, decrease in [Ag+] also decreases I.P. of AgCl which becomes lower than Ksp of AgCl, hence ppt dissolves. 5 [Turn Over (iii) ClO- can undergo disproportionation at higher temperatures to form ClO3-. (iv) For CuCl: [Cu+][Cl‒] = 1.2 x 10‒6 [Cl‒] = (1.2 x 10‒6) / 0.04 = 3.00 x 10‒5 mol dm‒3 For AgCl @ the point where IP CuCl = Ksp CuCl Hence, [Ag+][Cl‒] = 1.8 x 10‒10 [Ag+] = 6.00 x 10‒6 mol dm‒3 No of moles of AgCl formed = 250 / 1000 x 0.05 – 250 / 1000 x 6.00 x 10‒6 = 0.012499 mol Total moles of Cl- needed = Cl- in AgCl + Cl- in saturated solution = 0.012499 + 250 / 1000 x 3.00 x 10-5 = 0.012507 mol Maximum mass of NaCl that can be added = 0.012507 x (35.5 + 23) = 0.732g [9] [Total: 20] 4 (a) (i) Product are H+ and O2 Overall equation (ii) Q = (0.25) x (24 x 60 x 60) = 21, 600 C From the above half equation, 1 mole of 1-phenyl-2-aminopropane requires 8F (772 000C). Mol of 1-phenyl-2-aminopropane formed by 21,600 C = 0.0280 mol Mass of 1-phenyl-2-aminopropane = 0.0280 x [ 9 (12) + 13 (1) + 14 (1) ] = 0.280 x 135 = 3.78 g [4] (b) (i) 6 [Turn Over (ii) Moles of HCl = moles of 1-phenyl-2-aminopropane in 25 cm3 = 1.37 x 10-3. moles of 1-phenyl-2-aminopropane in 250 cm3 = 1.37 x 10-2 mol [1-phenyl-2-aminopropane] in g dm-3 = (4 x 1.37 x 10-2 x 135) = 7.40 g dm-3 (iii) Mass in 250 cm3 = 1.85 g percentage purity of the 1-phenyl-2-aminopropane crystals = 1.85 / 3.78 x 100 = 49.0 % [4] (c) (i) (ii) Bidentate ligand can form 2 dative bonds to a central atom or ion. (iii) When 1,4-diaminobutane is added, the colour of the complex changes from blue to violet as there is a ligand exchange of water by 1,4-diaminobutane. The energy absorbed for the excitation of d electrons changes as different ligands are bonded to Cu2+. (iv) [5] (d) (i) 7 [Turn Over (ii) [2] (e) NH (i) C CH3 (ii) COOH Pyruvic is a simple covalent molecule whilst alanine is a zwitterion with giant ionic lattice structure. Intermolecular forces between the pyruvic acid molecules is hydrogen bonding while in alanine, it’s held by strong electrostatic forces of attraction between oppositely charged ions. More energy is needed to break the stronger ionic bonds in alanine than the hydrogen bond in the acid. (iii) [5] 5 (a) (i) [H+] = (10-3.25 x 0.025)1/2 = 3.7494 x 10-3 mol dm-3 pH = 2.43 (ii) (ii) [8] 8 [Turn Over (b) Observations Deductions A dissolves in aqueous HNO3. A contains amine or phenylamine group. Neutralisation B reacts with cold KMnO4 to Mild oxidation. form A. B contains C=C. A is a diol. Brown ppt is MnO2. A forms yellow ppt on addition of Nucleophilic substitution. ethanolic silver nitrate. A contains iodoalkane. Yellow ppt is AgI A decolourises aqueous Electrophilic substitution. bromine and white ppt is formed. A is phenylamine. B contains 3 Br atoms. A produces 0.125 mol of gas Gas is H2. when sodium metal is added. Redox reaction. 1 mol of A produces 1 mol of H2. Two –OH group present in A. D is produced when hot acidified Oxidation. K2Cr2O7 is added to A. Primary or secondary alcohol present in A. Carboxylic acid or ketone in D D reacts with 2,4-DNPH but not D contains ketone. with Tollen’s. Secondary alcohol present in A. (no double award) D undergoes condensation. A and D both form yellow ppt with aqueous alkaline iodine. A contains and D contains . E has a RCOOYellow ppt is CHI3. Oxidation. A: B: 9 [Turn Over C: D: E: [12] [Total: 20] END OF PAPER 10 [Turn Over