School of Chemistry and Physics Westville Campus, Durban. Main Examination: November 2012 CHEMICAL REACTIVITY CHEM 120 SECTION A

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1 School of Chemistry and Physics Westville Campus, Durban BARCODE CEMICAL REACTIVITY CEM 120 SECTION A Total Marks for Section A: 58 Duration: 3 ours Total Marks for Examination: 100 Internal Examiner: Dr. V. Paideya, Dr. V.O. Nyamori, Dr. M.D. Bala, Ms. G. Dawson, Mr M. Shozi Moderator: Dr. W. Van Zyl, Prof. R. Robinson, Mrs.. Govender IMPORTANT: Complete this part immediately. Student No. Seat Number Name (optional) INSTRUCTIONS: 1. Answer ALL questions. 2. Calculators may be used. 3. Your answers must be shaded in on the Multiple Choice answer sheet provided. 4. This question paper consists of 19 pages excluding cover page, periodic table and data sheet. Please check that you have them all. 5. You will also require a copy of Section B, C and D. 6. You are advised not to spend more than 1.5 hours on Section A. 7. A periodic table and a data sheet are provided.

2 INSTRUCTIONS: For each of the following questions, select the correct answer from the list provided. There is only one correct answer for each question. Indicate your answer on the multiple choice answer sheet provided. Make a dark heavy mark with a B pencil that fills the block of the appropriate letter completely. 1. The salt resulting from a strong acid (Cl) and a strong base (NaO) reacts with water to form a solution of NaCl which is: Acidic (B) Basic (C) Amphoteric (D) Neutral (1) 2. Which of the following could be added to a solution of sodium acetate to produce a buffer? (i) COO (ii) Cl (iii) COOK (iv) NaCl (i) or (ii) (B) (iv) only (C) (iii) or (iv) (D) (iii) only (1) 3. In general what will happen to the p of a buffer solution when a small amount of a strong base is added? Increase slightly (B) Decrease slightly (C) Remain exactly the same (D) Become 7.0 (1) Page 1 of 19

3 4. In the following reaction: BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + 2NaCl(aq) Ba 2+ ions react with the SO 4 2- ions in solution to form a precipitate of BaSO 4. A precipitate will form when: Q sp = [Ba 2+ ][ SO 2-4 ] (B) Q sp = K sp (C) Q sp > K sp (D) Q sp < K Sp (1) 5. A 15.0 ml sample of M Cl is titrated with a M NaO solution. What is the p of the solution before any base is added? 0.5 (B) 2.0 (C) 1.0 (D) 0.10 (1) 6. The relationship between the aqueous molar solubility, s, and K sp for the ionic solid CaF 2(s) is: s 2 = K sp (B) 2s 2 = K sp (C) s = K sp (D) 4s 3 = K sp (1) Page 2 of 19

4 7. Determine the molar solubility of SrCO 3 (K sp = 1.6 x 10-9 ) mol dm -3 (B) mol dm -3 (C) mol dm -3 (D) mol dm -3 (1) 8. If the p of the following equilibrium reaction is decreased by adding an acidic buffer: What will happen to the solubility of Mg(O) 2? Mg(O) 2 (s) Mg 2+ (aq) + 2O - (aq) Decreases due to increase in [ + ] ion (B) Increases due to increase in [ + ] ion (C) No effect (D) Decreases due to amount of O - (aq) in the system (1) 9. Consider the following table of K sp values. Compound CdCO 3 Cd(O) 2 CaF 2 AgI K sp Which compound listed below has the greatest molar solubility in water? CdCO 3 (B) Cd(O) 2 (C) CaF 2 (D) AgI (1) Page 3 of 19

5 10. When a non-volatile solute is dissolved in a volatile solvent, which property is greater for the solution than for the solvent? Boiling point (B) Freezing point (C) Rate of evaporation (D) Vapour pressure (1) 11. What is the molality of a solution made by dissolving 36.0 g of glucose (C 6 12 O 6 = g mol -1 ) in 64.0 g of 2 O? (B) 3.12 (C) (D) (2) 12. Determine the freezing point of a 1.7 m ethylene glycol solution. Pure water has a freezing point depression constant K f = 1.86 C m -1 and freezes at 0 C. 0.9 C (B) -0.9 C (C) 3.2 C (D) -3.2 C (1) 13. What is the oxidation number of rhenium in Ca(ReO 4 ) 2? +1 (B) +3 (C) +6 (D) +7 (1) Page 4 of 19

6 14. When potassium permanganate, KMnO 4, is added to an acidified solution of oxalic acid, 2 C 2 O 4, the products are CO 2 gas and Mn 2+ ions. What is the reducing agent in this reaction? KMnO 4 (B) 2 C 2 O 4 (C) 3 O + (D) CO 2 (1) 15. If a voltaic cell has a positive E value, what can be concluded about the values of G and K eq? G < 0, K eq < 1 (B) G < 0, K eq > 1 (C) G > 0, K eq < 1 (D) G > 0, K eq > 1 (1) 16. Which of the following is a redox reaction? (i) K 2 Cr 2 O 7 + CaCl 2 CaCr 2 O 7 + 2KCl (ii) Ag + + Br AgBr (iii) Fe + S FeS (i) only (B) (ii) only (C) (iii) only (D) (i) and (iii) (1) Page 5 of 19

7 17. One of the differences between a voltaic cell and an electrolytic cell is that in an electrolytic cell a non-spontaneous reaction is forced to occur. (B) electrons flow towards the anode. (C) O 2 gas is produced at the cathode. (D) oxidation occurs at the cathode. (1) 18. For the following electrochemical cell, determine which reaction occurs at the cathode: Cu(s) Cu 2+ (aq) Ag + (aq) Ag(s) Ag + (aq) + e Ag(s) (B) Cu(s) Cu 2+ (aq) + 2e (C) Cu 2+ (aq) + 2e Cu(s) (D) Ag(s) Ag + (aq) + e (1) 19. Which substance would be the most soluble in petrol? NaCl (B) exane (C) Water (D) Cl (1) Page 6 of 19

8 20. In the context of alkane structure, which of the following observations is not correct using the model of hybridisation? Carbon atoms in a compound can make single bonds to 4 adjacent atoms. (B) The bond angles from alkane carbons to adjacent bonded atoms are approximately 109. (C) Each carbon valence orbital contributes one electron when it forms a bond. (D) The spherical 2s orbital of carbon atom is not involved in bonding. (1) 21. Indicate all of the following molecular structures that are constitutional isomers of each other? (I) 3 C C 2 C C 2 (II) 3 C C C (III) 3 C C 2 C (IV) C 2 C 2 C 2 C C 3 (V) 3 C C 2 C (VI) 3 C C C 2 (I), (II), (IV) and (VI) (B) (I) and (V) (C) (I), (II), (III), (IV) and (VI) (D) (I), (II) and (VI) (1) Page 7 of 19

9 22. What is the correct IUPAC name for the following alkane? 2-ethyl-3-methylpentane (B) 4,5-dimethylheptane (C) 5-ethyl-4-methylpentane (D) 3,4-dimethylheptane (1) 23. Which statement about hydrocarbons is false? Alkanes can be produced by hydrogenating alkenes. (B) The smallest alkane to have structural (constitutional) isomers has 4 carbon atoms. (C) Cyclic alkanes are structural isomers of alkenes. (D) Alkanes are more reactive than alkenes. (1) 24. Consider the three isomeric alkanes i.e. n-hexane, 2,3-dimethylbutane, and 2- methylpentane. Which of the following correctly lists these compounds in order of increasing boiling point? n-hexane < 2,3-dimethylbutane < 2-methylpentane (B) 2-methylpentane < 2,3-dimethylbutane < n-hexane (C) n-hexane < 2-methylpentane < 2,3-dimethylbutane (D) 2,3-dimethylbutane < 2-methylpentane < n-hexane (1) Page 8 of 19

10 25. Which of the following is the staggered conformation for rotation about the C 1 -C 2 bond in the following structure? 3 C C 2 C C C 2 (B) 3 C C 2 3 C (C) 3 C (D) 3 C 3 C 3 C (1) 26. Is the carbon indicated by the asterisk in the structure below primary (1 ), secondary (2 ), tertiary (3 ) or quaternary (4 )? * 1 (B) 2 (C) 3 (D) 4 (1) Page 9 of 19

11 27. Which of the following reaction is a chain propagation step in the free radical chlorination of methane? (1) 28. Why is cis-trans isomerism possible in dibromocyclobutanes but not in dibromobutanes? (B) (C) (D) It is impossible for a dibromocyclobutane to have a plane of symmetry. Dibromocyclobutanes are two dimensional but dibromobutanes only have one dimension. Dibromocyclobutanes can have substituents whereas dibromobutanes do not. Single C C bonds in the dibromocyclobutane ring cannot rotate freely whereas they can in dibromobutanes. (1) 29. Which of the following structures are cis-trans isomers of each other? (I) (II) (III) (IV) I and III (B) I and II (C) II and IV (D) I and IV (1) Page 10 of 19

12 30. Which of the following compounds contains a stereogenic centre? (B) Cl (C) (D) (1) 31. Which of the compounds is an enantiomer of the following compound? 3 C Br C 2 Br C 2 Br Br 2 C Br (B) Br 3 C Br C Br2 C 3 (C) (D) Br (2) 2 C Br Page 11 of 19

13 32. What is the correct IUPAC name for the compound below? (4R,5S)-4,5-dimethylheptane (B) (3R,4R)-3,4-dimethylheptane (C) (3S,4S)-3,4-dimethylheptane (D) (4S,5R)-4,5-dimethylheptane (2) 33. Which of the following is the most stable cation? 3 C C (B) 3 C C (C) F C F (D) C (1) F 34. Which of the following are the elimination products of the reaction shown below? C 2 Br + O? C 2 =CBr + 2 O (B) OC 2 C 2 Br (C) C 2 O + Br (D) C 2 =C 2 + Br + 2 O (1) Page 12 of 19

14 35. What is the correct IUPAC name of the following compound? O Cl 3 C (B) (C) 2-methyl-5-chlorocyclohexanol 5-chloro-2-methylcyclohexanol 3-chloro-6-methylcyclohexanol (D) 3-chloro-2-methylcyclohexanol (1) 36. What is the main product of the following reaction? O + N CrO 3 Cl? (B) Cl O (C) (D) O O (1) Page 13 of 19

15 (D) N 3, O 2, F, Na +, Mg 2+ (1) 37. What is the product of the following reaction? C C 1. O 3 2. ( ) 2 S? 3 C 3 C 3 C O C C O (B) C C O 3 C O (C) 3 C C O (D) O C (1) 38. What is the structure of rhombic sulfur? Discrete S 2 molecules (B) Discrete S atoms (C) Crown shaped S 8 molecules (D) Polymeric S n chains (1) 39. Arrange the ions N 3, O 2, Mg 2+, Na +, and F in order of increasing ionic radius, starting with the smallest first. N 3, Mg 2+, O 2, Na +, F (B) Mg 2+, Na +, F, O 2-, N 3 (C) N 3, O 2, Mg 2+, F, Na + Page 14 of 19

16 40. Elemental iodine (I 2 ) is a solid at room temperature. What is the major attractive force that exists among different I 2 molecules in the solid? Ionic-dipole interactions (B) London dispersion forces (C) Ion-ion interaction (D) Dipole-dipole attractions (1) 41. When the following equation is balanced, what is the stoichiometric coefficient of oxygen? PbS(s) + O 2 (g) PbO(s) + SO 2 (g) 3 (B) 5 (C) 4 (D) 1 (1) 42. Consider the following reaction employed in the laboratory preparation of Br: W PBr 3 (l) + X 2 O(l) Y 3 PO 3 (aq) + Z Br(g) (B) (C) (D) The stoichiometric ratios in the equation are W = 1, X = 1, Y = 1, Z = 1 and the bromine-containing reactant and product are ionic. The stoichiometric ratios in the equation are W = 3, X = 1, Y = 3, Z = 1 and the bromine-containing reactant and product are ionic. The stoichiometric ratios in the equation are W = 1, X = 3, Y = 3, Z = 1 and the bromine-containing reactant and product are covalent. The stoichiometric ratios in the equation are W = 1, X = 3, Y = 1, Z = 3 and the bromine-containing reactant and product are covalent. (1) Page 15 of 19

17 43. Below is a portion of the periodic table. Which statement below about the atomic radii of the following selected elements is correct? Consider all elements given carefully C Si P S Cl Ar Ge F Br Sn I (B) (C) (D) The atomic radii of the Group 17 elements increased down a group because the electronegativity increases in the same manner. The atomic radii increase from Si to Cl (Period 3) because it generally increases with increasing atomic number across a period. The atomic radii for the Group 14 elements decreases down the group because the elements change from non-metal (C) to metal (Sn) and metal atoms attract electrons better than non-metal atoms. The atomic radii from Si to Ar decrease because the valence electrons experience a greater Z eff across a period. (2) 44. Which of the following correctly represents the second ionization of calcium? Ca(g) Ca + (g) + e (B) Ca + (g) + e Ca 2+ (g) (C) Ca + (g) + e Ca(g) (D) Ca + (g) Ca 2+ (g) + e (1) Page 16 of 19

18 45. Arrange the following ions (O 2, Mg 2+,F, Na +.) in order of increasing difficulty of removal of an e (ionization): O 2 < Mg 2+ < F < Na + (B) Mg 2+ < O 2 < Na + < F (C) O 2 < F < Na + < Mg 2+ (D) Mg 2+ < Na + < F < O 2 (1) 46. ow does LiO behave when dissolved in water? As an acid that forms Li + and O ions. (B) As a base that forms Li + and O ions. (C) As a base that forms LiO and + ions. (D) As an acid that forms LiO and + ions. (1) 47. Predict the products of a reaction between Ba(NO 3 ) 2 (aq) and K 2 SO 4 (aq). Ba(s), KNO 3 (aq) and K 2 SO 3 (aq) (B) BaSO 4 (s) and KNO 3 (aq) (C) BaSO 3 (s) and KNO 3 (aq) (D) BaSO 3 (s) and KNO 2 (aq) (1) 48. Why do alkali metals tend to be more reactive than alkaline earth metals? alkali metals have lower melting points (B) alkali metals have lower densities (C) alkali metals have greater electron affinities (D) alkali metals have lower ionization energies (1) Page 17 of 19

19 49. Which of the following is the strongest acid? 2 SO 3 (B) 2 SeO 4 (C) 2 SeO 3 (D) 2 SO 4 (1) 50. Which one of the following elements has an allotrope that is produced in the upper atmosphere by lightning? elium (B) Sulphur (C) Chlorine (D) Oxygen (1) 51. ow can the superoxide ion be distinguished from the peroxide ion and the oxide ion? The superoxide ion is an acid. (B) The superoxide ion is a base. (C) The superoxide ion is diamagnetic (D) The superoxide ion is paramagnetic. (1) 52. Element M reacts with chlorine to form a compound with the formula MCl 2. Element M is more reactive than magnesium and has a smaller radius than barium. This element is. Be (B) Sr (C) Se (D) Na (1) Page 18 of 19

20 53. What is the name of the commercial process for the preparation of sulfuric acid? Ostwald process (B) aber process (C) Contact process (D) Mond process (1) 54. Atoms of which element, indicated by a letter on the periodic table, are expected to have the largest atomic radius? A) A B) B C) C D) D (1) [Total Marks: 58] END OF SECTION A Page 19 of 19