*P15* PRE-LEAVING CERTIFICATE EXAMINATION, 2008 CHEMISTRY HIGHER LEVEL TIME: 3 HOURS 400 MARKS Answer eight questions in all These must include at least two questions from Section A All questions carry equal marks (50) Information Relative atomic masses: H = 1, C = 12, N = 14, O = 16, Cu = 63.5, Mn= 55 Molar volume at s.t.p. = 22.4 litres Avogadro constant = 6 10 23 mol -1 Page 1 of 7
Section A Answer at least two questions from this section [see page 1 for full instructions] 1. In an experiment the primary standard ammonium iron (II) sulphate ((NH 4 ) 2 SO 4.FeSO 4.6H 2 O) was used to standardise a solution of potassium manganate (VII) (KMnO 4 ). 25 cm 3 of a 0.01 M ammonium iron (II) sulphate and 10 cm 3 of dilute sulphuric acid were placed in a clean dry conical flask. The potassium manganate (VII) was placed in the burette. After a number of titrations it was found that the average titre was 22 cm 3. MnO 4 + 5Fe 2+ + 8H + Mn 2+ + 5Fe 3+ + 4H 2 O (a) Explain what is meant by a primary standard solution? Why is potassium manganate (VII) not considered to be a primary standard? (9) (b) Why was sulphuric acid added to the conical flask? (5) (c) Name the piece of equipment that should be used to transfer the 25 cm 3 of ammonium iron (II) sulphate to the conical flask. (3) (d) Describe the correct procedure for rinsing this piece of equipment and transferring the solution to the conical flask (9) (e) What colour change would you observe as the solution from the burette flows into the conical flask? (6) How was the end-point detected? (3) (f) What is the role of the Mn 2+ ions produced in this reaction? (6) (g) 25 cm 3 of a 0.01 M ammonium iron (II) sulphate required 22 cm 3 of potassium manganate (VII) solution for complete reaction. Burette Potassium manganate (VII) solution Conical flask Ammonium iron (II) sulfate solution + sulfuric acid solution White tile MnO 4 + 5Fe 2+ + 8H + Mn 2+ + 5Fe 3+ + 4H 2 O Calculate the concentration of potassium manganate (VII): (i) in moles per litre. (ii) in g/litre. (9) [Relative atomic masses: K = 39, Mn =55, O = 16] 2. The diagram shows an apparatus that can be used for the preparation of ethyne gas, C 2 H 2. A liquid X is dropped onto the solid Y and the gas methylbenzene. collected in test tubes as shown. (a) Identify the liquid X and the solid Y. (8) (b) Describe the appearance of the solid Y. (3) (c) Write a balanced equation for the reaction between X and Y producing ethyne. (6) (d) Name two impurities found in the ethyne gas produced and say how these impurities may be removed. (9) Y X Ethyne gas Page 2 of 7
(e) Ethyne, C 2 H 2, is described as an unsaturated hydrocarbon. What is meant by the term unsaturated? Name the reagent used to test for unsaturation and state the type of reaction involved. (12) (f) What is the common name for ethyne and state one major use of the gas? (6) (g) In which of the two solvents water or methylbenzene is ethyne soluble? Explain why. (6) 3. Hydrogen peroxide decomposes rapidly in the presence of a manganese (IV) oxide (MnO 2 ) catalyst. (a) Write a balanced equation for the decomposition of hydrogen peroxide. (5) (b) Draw a labelled diagram of an apparatus a student could assemble to measure the rate of decomposition of hydrogen peroxide in the presence of a manganese (IV) oxide (MnO 2 ) catalyst. Indicate clearly how the reaction could be started at a time known exactly, and how the gas produced is collected and its volume measured. (12) Time/minutes 0 1 2 3 4 5 6 7 8 Volume of O 2 /cm 3 0 18.4 28.1 32.2 34.4 35.6 36.0 36.0 36.0 (c) Plot a graph to illustrate the volume of oxygen produced versus time. (12) (d) Use the graph to determine: (i) The time taken for the reaction to be complete. (ii) The percentage of oxygen produced after 2.5 minutes. (iii) The instantaneous rate of the reaction after 3 minutes. (15) (e) Explain why the rate of the reaction gets slower as the reaction proceeds. (6) Page 3 of 7
Section B [See page 1 for instructions regarding the number of questions to be answered] 4. Answer eight of the following items (a), (b), (c), etc. (50) (a) State Charles Law. (b) Define reduction in terms of electron transfer. (c) How could you test for the presence of the nitrate ion in aqueous solution? (d) Atomic Absorption Spectrometry is used to detect heavy metals e.g. lead in water. What is the underlying principle in this analytical technique. (e) Account for the trend in the size of the atomic radius going across a period in the periodic table. (f) How many atoms are present in 560 cm 3 of ammonia at s.t.p. (g) Draw an energy profile diagram of an endothermic reaction and indicate on the diagram the activation energy. (h) Give the systematic name of the compound CH 3 C(CH 3 ) 2 CH 2 CH(CH 3 )CH 3. (i) What are alpha particles? (j) Write the (i) conjugate acid and (ii) the conjugate base of HSO 4. (k) Answer part A or B. A How does nitrogen fixation occur in nature? B Give an example of (i) an ionic crystal and (ii) a covalent macromolecular crystal. 5. (a) Define fi rst ionization energy. (5) Account fully for the trends in first ionisation energies of elements across the second period of the periodic table (i.e. Li to Ne). (15) Account for the trend in first ionisation energies of the elements going down Group II of the periodic table, i.e. the alkaline-earth metals. (6) (b) What contribution did Dobereiner make to the systematic arrangement of the elements? (6) State two ways in which Mendeleev s periodic table of elements differs from that of Moseley. (6) (c) Define relative atomic mass. (6) Calculate the relative atomic mass of chlorine given the following isotopic composition: chlorine-35 = 75% and chlorine-37 = 25%. (6) Page 4 of 7
6. (a) Define heat of reaction. (5) The combustion of 2,2,4-trimethylpentane may be described by the following balanced equation: 2C 8 H 18 (l) + 25O 2 (g) 16CO 2 (g) + 18H 2 O(l) Given that the heats of formation of 2,2,4-trimethylpentane, carbon dioxide and water are 259, 394 and 286 kj mol 1, respectively, calculate the heat of combustion of 2,2,4-trimethylpentane. (15) (b) What is the octane number of a fuel? (6) The organic compound 2,2,4-trimethylpentane is one of the reference hydrocarbons used in measuring the octane number of a fuel. Name the other reference hydrocarbon. (6) What structural feature of 2,2,4-trimethylpentane gives it such a high octane number. Give one other structural feature which increases the octane rating of a hydrocarbon. (6) (c) There are two structural isomers of the hydrocarbon of formula C 4 H 8. In the case of each of these isomers, draw the structure of the molecule and give its systematic IUPAC name. (12) 7. X Y C 2 H 4 C 2 OH C 2 ONa A B Z C C 2 COOC 2 D (a) Name the compounds A, B, C and D. (12) (b) Identify the type of reaction represented by the letters X and Z. (6) (c) Draw compound D and indicate on the diagram a carbon atom which has a planar geometry and a carbon atom which has a tetrahedral geometry. (12) (d) Name the reactant added to B in order to form the compound D. (6) (e) Compound A reacts with hydrochloric acid to form C 2 Cl. Describe the mechanism of the reaction involved in this conversion. (14) Page 5 of 7
8. (a) Distinguish between the primary and secondary stages of sewage treatment. (12) Sewage treatments plants may also carry out a tertiary process which reduces the levels of certain substances. Name these substances and explain why their removal is necessary. (8) (b) In the treatment of water for drinking, aluminium sulphate is added. State the purpose of this addition and name the process involved. (9) (c) Explain how an acid-base indicator XOH which is itself a weak base functions. (9) (d) Calculate the ph of a 0.2M ammonia (NH 3 ) solution given that the dissociation constant K b for ammonia is 1.8 10-5. What is the ph of a sodium hydroxide (NaOH) solution of the same concentration. (12) 9. (a) State Le Chátelier s principle. (5) The following reversible chemical reaction shows the formation of sulphur trioxide. 2SO 2 + O 2 2SO 3 H = - 196 kj mol -1 Use Le Chátelier s principle to predict the levels (high or low) of temperature and pressure to maximize the yield of sulfur trioxide when equilibrium is established. (6) State giving reasons the effect that the following will have on the yield of sulfur trioxide (i) raising the temperature, (ii) decreasing the pressure and (iii) decrease in the concentration of sulfur dioxide. (15) (b) Write the equilibrium constant expression for the reaction above. (6) State what effect the following equations will have on the value of the equilibrium constant: (i) SO 2 + ½O 2 SO 3 (ii) 2SO 3 2SO 2 + O 2 (6) (c) In the following equilibrium the forward reaction is endothermic. Co(H 2 O) 6 + 4Cl CoCl 4 + 6H 2 O pink blue (i) (ii) State and explain the colour change observed when the solution is placed in ice. (6) Other than heating, mention one way of reversing the change caused by cooling the reaction mixture. (6) Page 6 of 7
10. Answer any two of the parts (a), (b) and (c). (a) Write out the electronic configuration (s, p, etc.) of a neutral chromium atom. (4) Describe using a dot and cross diagram the bonding in the ammonia molecule. (9) What is the shape of the ammonia molecule? (6) Which of the following angles 107 0, 109 0, and 111 0 would you expect to be closest to the bond angle found in the ammonia molecule? (6) (b) What is a catalyst? (4) Name the two theories of catalysis. (6) State which of these theories occur in the catalytic converter in a car. (6) Name one element used as a catalyst in a catalytic converter. (3) State and explain the effect leaded petrol has on a catalytic converter. (6) (c) Ethanal is prepared in the laboratory according to the following equation: 3C 2 OH + Cr 2 O 7 2 + 8H + 3CH 3 CHO + 2Cr 3+ + 7H 2 O If 7.4g of sodium dichromate (Na 2 Cr 2 O 7.2H 2 O) and 5.6g of ethanol (C 2 OH) are used in the reaction determine which of the two reagents are present in excess. (13) After purification 1.2g of ethanal was obtained, calculate the percentage yield. (12) [Relative atomic masses: Na = 23: Cr =52: O =16: H = 1 C = 12: ] 11. Answer any two of the parts (a), (b) and (c). (a) Describe a test which could be carried out to identify the presence of the phosphate ion in aqueous solution. (15) What test could be carried out to distinguish between the two phosphate salts copper phosphate (Cu 3 (PO 4 ) 2 ) and sodium phosphate (Na 3 PO 4 )? (4) What observation would be made in this test? (6) (b) Explain the term electrolysis. (7) A solution of sodium sulphate was electrolysed using the following apparatus. Write the balanced half equations for the reactions that occur at the anode and the cathode. (12) If universal indicator is added to the solution at the start of the experiment describe the colour change that occurs at each electrode. (6) (c) Answer part A or part B. Anode Cathode A. What is the approximate percentage of (i) nitrogen and (ii) oxygen in air? (6) Explain how oxygen and nitrogen are produced for industrial use. (13) State any two uses of (i) nitrogen gas and (ii) oxygen gas. (6) B. What is corrosion? Explain in terms of electron transfer what occurs when iron corrodes. (9) Name two substances that accelerate the corrosion of iron. Give a reason for each. (12) Name one method that may be used to prevent the corrosion of iron. (4) Page 7 of 7