L.34 PRE-LEAVING CERTIFICATE EXAMINATION, 2012 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). The information below should be used in your calculations. Relative atomic masses: H = 1, C = 12, N= 14, O = 16, Na = 23, Ca = 40, Cr = 52 Molar volume at s.t.p. = 22.4 litres Avogadro constant = 6 10 23 mol 1 The use of the Formulae and Tables booklet approved for use in the State Examinations is permitted. A copy may be obtained from the examination superintendent. 2012 L.34 1/8 Page 1 of 8
Section A Answer at least two questions from this section [see page 1 for full instructions]. 1. In an experiment to measure the concentration of dissolved oxygen in a school aquarium a group of students took a sample of water by filling a bottle with water. They also measured the temperature of the water. In the laboratory the students followed the Winkler procedure for measuring dissolved oxygen by adding a few cm 3 of concentrated solutions of manganese sulfate (MnSO 4 ) and alkaline potassium iodide (KOH/KI) to the water in the bottle. A stopper was placed in the bottle and it was inverted a few times to mix the contents. They observed that the colour changed twice and that a precipitate formed. Next they carefully added a few cm 3 of concentrated sulfuric acid (H 2 SO 4 ) and observed that a coloured solution of iodine formed. The students then measured the concentration of the iodine by titrating it in 50 cm 3 portions against a 0.01 M solution of sodium thiosulfate (Na 2 S 2 O 3 ). The average volume of sodium thiosulfate required, when the procedure was repeated a number of times, was 5.4 cm 3. (a) Outline a precaution that should be observed when filling the bottle with water. (3) What colour changes occurred in the water after the Winkler reagents were added to the bottle? (6) Describe how the burette was prepared and filled with the sodium thiosulfate solution. (12) Name the indicator used in the titrations. State the colour changes observed in the flask from the time the indicator is added until the end point is reached. (12) The titration reaction is described by the following equation: 2S 2 O 2 + I 3 2 2I + S 4 O 2 6 Calculate the concentration of the iodine solution in moles per litre. (6) For every 1 mole of oxygen gas (O 2 ) in the water sample 2 moles of iodine (I 2 ) are formed. Calculate the concentration of dissolved oxygen in the water sample in p.p.m. (6) (g) What is the effect of increased temperature on the solubility of oxygen in water? (5) 2012 L.34 2/8 Page 2 of 8
2. A group of students prepared a sample of ethanal (CH 3 CHO) in the laboratory using the apparatus shown in the diagram. The reaction is described by the following equation: 3C 2 H 5 OH + Cr 2 O 2 7 + 8H + 3CH 3 CHO + 2Cr 3+ + 7H 2 O (a) After the apparatus had been assembled what procedure was followed involving the flask before the addition of the solution from the tap funnel? (6) State two features of the preparation that were followed to maximise the yield of ethanal and explain why each is necessary. (12) Describe the colour change in the flask after the solution in the tap funnel was added. (5) Name the organic product that would have been produced if propan-2-ol was used instead of ethanol. (3) Outline how you would show, using Tollen s reagent (ammoniacal silver nitrate), the difference between ethanal and the product named in. (12) Calculate the maximum volume of ethanal that the students could have expected to produce if they used 5.96 g of sodium dichromate (Na 2 Cr 2 O 7.2H 2 O) and the density of ethanal is 0.8 g cm 3. (12) C 2 H 5 OH + Na 2 Cr 2 O 7 + water H 2 SO 4 + water 3. To investigate the effect of concentration on the rate of a chemical reaction 100 cm 3 of a 0.10 M solution of sodium thiosulfate (Na 2 S 2 O 3 ) was measured into a conical flask and 10 cm 3 of a 1.0 M solution of hydrochloric acid (HCl) was added. The flask was quickly swirled to mix the contents and placed on top of a cross on a white sheet of paper, as shown in the diagram. The time taken for the cross to disappear when viewed from the top of the flask was measured with a stopwatch. The experiment was repeated with 100 cm 3 of the Na 2 S 2 O 3 solution at concentrations of 0.08 M, 0.06 M, 0.04 M and 0.02 M. The results are given in the table. Concentration of Na 2 S 2 O 3 (M) 0.10 0.08 0.06 0.04 0.02 Time for the cross to disappear (min) 1.51 1.82 2.27 2.94 4.76 Reaction rate (min 1 ) 0.66 0.55 0.44 0.34 0.21 (a) What other factor, not mentioned in the procedure described above, was kept constant during this investigation? (5) Describe how you would accurately prepare 100 cm 3 of the 0.08 M sodium thiosulfate solution from the 0.10 M solution. (12) How was the reaction rate calculated? (3) Plot a graph of reaction rate versus concentration of sodium thiosulfate. What does the shape of the graph show about the relationship between concentration and reaction rate? (18) Use the graph to determine the rate of reaction if a 0.01 M sodium thiosulfate solution was used and hence how long it would have taken for the cross to disappear. (6) Write a balanced equation to describe the reaction of sodium thiosulfate with hydrochloric acid. (6) 2012 L.34 3/8 Page 3 of 8
Section B [See page 1 for instructions regarding the number of questions to be answered.] 4. Answer eight of the following items (a),,, etc. (50) (a) Name the scientist who first measured the e/m of the electron. What does e refer to? Identify the atom or ion represented by the following electron configurations: (i) [1s 2, 2s 2, 2p 6 ], (ii) [1s 2, 2s 2, 2p 6 ] 2+. Which type of nuclear radiation has the greatest penetrative power? Name the ion that causes heated ammonium molybdate to form a yellow precipitate when concentrated nitric acid is added. Calculate the percentage by mass of nitrogen in urea (CO(NH 2 ) 2 ). (g) (h) Write a balanced equation for the reaction of calcium carbide and water that occurs in the production of ethyne. Draw the structural formula of ethyl methanoate. Name the analytical technique based on the measurement of the absorption of radiation by the bonds present in a compound. (i) What is the oxidation number of (i) chlorine in NaOCl and (ii) sulfur in S 2 O 3 2? (j) Calculate the ph of a solution that contains 1 g per litre of sodium hydroxide (NaOH). (k) Answer part A or part B. A B Identify a group of compounds used to replace CFCs as refrigerant gases. or Name the allotrope of carbon that consists of 60 atoms bonded together in an arrangement that resembles a football. 5. (a) Define electronegativity. Name the scientist who devised the scale of electronegativity. (8) State two reasons why electronegativity values increase going across a period in the periodic table. (6) Use electronegativity values to predict the type of bonding in (i) phosphorus(iii) chloride (PCl 3 ), (ii) calcium chloride (CaCl 2 ), (iii) carbon dioxide (CO 2 ). (9) Use a dot and cross diagram to show the bonding in a calcium chloride (CaCl 2 ) molecule. (9) Describe the shape of a molecule of (i) carbon dioxide, (ii) water. (6) Use your knowledge of bonding and molecular shape to suggest reasons why the boiling point of carbon dioxide ( 57 C) is significantly lower than the boiling point of water (100 C). (12) 2012 L.34 4/8 Page 4 of 8
6. (a) Name each of these gaseous hydrocarbons: (i) the main gas in natural gas, (ii) the gas in LPG which has C 4 molecules, (iii) a gas that when mixed with oxygen forms a flame for welding and cutting. (8) Why are mercaptans added to natural gas and to LPG? (3) Petrol is one of the major products of an oil refinery. (i) Name the fraction, other than light gasoline, that is used in petrol. (ii) Name the reforming process that is required to produce petrol from the fractions named in (i). (iii) Name another group of compounds that is produced by the same process named in (ii). (9) Cyclohexane and benzene are hydrocarbons with high octane numbers. Explain what is meant by the octane number of a fuel. State the molecular formula and draw the molecular structure of (i) cyclohexane, (ii) benzene. (15) Ethanol (C 2 H 5 OH) is a fuel that can be produced by fermentation of sugar and when it burns it produces carbon dioxide and water. Calculate the heat of combustion of ethanol, given that the heats of formation of carbon dioxide, ethanol and water are 394, 278 and 286 kj mol 1, respectively. (15) 7. (a) EDTA is used as a chelating agent in the measurement of hardness of water. What do the letters EDTA stand for? (5) Hardness in water is caused by dissolved salts of calcium, one of which is calcium hydrogencarbonate. Show, by giving a balanced equation, how boiling water that contains calcium hydrogencarbonate softens hard water. What kind of hardness is not affected by boiling? (12) Outline two processes that are used to remove suspended solids in the treatment of water for domestic use. (12) Elevated levels of phosphate and nitrate can lead to eutrophication in rivers and lakes. (i) What is the visible evidence of eutrophication? (ii) How are nitrates and phosphates removed in sewage treatment? (6) Write an equation for the ionic product (K w ) of water. The ionic product of pure water at 10 C is 0.293 10 14. Calculate the ph of the water. Is it acidic, basic or neutral? (15) 8. Study the reaction scheme and answer the questions that follow. W X Y C 2 H 5 OH C 2 H 4 C 2 H 4 Cl 2 C 2 H 3 Cl A B C D (a) Name compounds B, C and D. (9) Which of the four compounds is most soluble in water? Justify your answer. (6) Conversion W can be carried out in a school laboratory. Name the catalyst that is used in this conversion. Describe a test to show that B is unsaturated. (9) Classify each of the conversions W, X, Y as an addition, an elimination or a substitution reaction. (9) Conversion X forms an intermediate species. Draw the molecular structure of this species. If this conversion takes place in the presence of water name a possible additional product formed. (12) Compound D is polymerised to form a waterproof material. What is the common name of this substance? (5) 2012 L.34 5/8 Page 5 of 8
9. (a) State Le Châtelier s principle. (5) In the manufacture of ammonia, nitrogen gas reacts with hydrogen gas according to the following equation: N 2(g) + 3H 2(g) 2NH 3(g) ΔH = 92.4 kj Use Le Châtelier s principle to predict the kind of pressure (high or low) that would maximise the yield of ammonia. (6) Outline the effect on the rate of production of ammonia and the equilibrium composition if the reaction is carried out (i) at a high temperature, (ii) with a catalyst. (12) Write an expression for the equilibrium constant (K c ) for the above reaction. (6) When 5 moles of nitrogen and 15 moles of hydrogen were placed in a 2 litre vessel at a particular temperature it was found that the equilibrium mixture contained 6 moles of ammonia. Calculate the value of the equilibrium constant (K c ) for the reaction. (18) What effect, if any, will an increase in pressure have on the value of K c? (3) 10. Answer any two of the parts (a), and. (2 25) (a) Define a mole of a substance. (7) Limewater, calcium hydroxide (Ca(OH) 2 ), reacts with carbon dioxide gas according to the following balanced equation: Ca(OH) 2 + CO 2 CaCO 3 + H 2 O (i) How many moles of calcium hydroxide are present in 1 litre of a 5% (w/v) solution of limewater? (6) (ii) What volume of carbon dioxide will react with 100 cm 3 of the solution of limewater at s.t.p.? (6) (iii) What mass of calcium carbonate will be formed in that reaction? (6) Describe how you would carry out a flame test to verify that a compound contains a barium salt. (9) Explain how the atoms of an element emit particular colours of light when the element is heated in a flame. (9) Outline how a solution of barium chloride (BaCl 2 ) may be used to distinguish between a solution containing sulfate ions and a solution containing sulfite ions. (7) What is the principle on which the mass spectrometer is based? (9) Calculate, to two decimal places, the relative atomic mass of a sample of magnesium shown by mass spectrometry to be composed of 79% magnesium 24, 10% magnesium 25 and 11% magnesium 26. (9) Magnesium 23 is unstable. Write an equation to show the change that occurs when magnesium 23 emits a beta particle. (7) 2012 L.34 6/8 Page 6 of 8
11. Answer any two of the parts (a), and. (2 25) (a) (i) Give two assumptions of the kinetic theory of gases. (6) (ii) State Boyle s law. (7) (iii) Write an expression for the combined gas law and use it to calculate the volume of a gas at s.t.p. when it occupies 100 cm 3 at a temperature of 20 C and at a pressure of 98,000 Pa. How many molecules of the gas are present in this quantity? (12) Define reduction in terms of electron transfer. (4) During the electrolysis, using inert electrodes, of sodium sulfate (Na 2 SO 4 ) solution to which some universal indicator (green) was added, a blue colour and a red colour were observed to form at separate electrodes. (i) Name a suitable material for the electrodes. (3) (ii) Name the electrode at which the blue colour formed. (3) (iii) Explain, using a balanced equation, the formation of the blue colour at this electrode. (9) (iv) Describe in terms of oxidation and reduction what happens when a piece of zinc is placed in a solution of copper sulfate. (6) Answer either part A or part B. A Iron is extracted from iron ore in a blast furnace. Haematite (Fe 2 O 3 ) is a common iron ore. Coke and limestone are added to the ore and hot air is blown through the hot mixture. (i) Iron is a transition metal. State one property of transition metals. (4) (ii) Coke plays two roles in the extraction process. What are these roles? Write balanced equations to explain these roles. (12) (iii) Outline, with the aid of balanced equations, the function of the limestone in the extraction process. (6) (iv) Objects made of iron may be protected from corrosion by use of a sacrificial anode, such as magnesium. Explain how this works. (3) B or Select one of the manufacturing processes below and answer the questions which follow: ammonia manufacture magnesium oxide manufacture nitric acid manufacture (i) Outline how two of the feedstock components are treated prior to their use in the manufacturing process you have chosen. Write a chemical equation to describe one of these processes. (12) (ii) Write a balanced equation for the reaction that is involved in the manufacture of the main product of the process you have chosen. (6) (iii) What is the main commercial use for the main product of the process you have chosen? Why is this product suitable for this purpose? (7) 2012 L.34 7/8 Page 7 of 8
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