CHEMISTRY HIGHER LEVEL

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1 *P15* PRE-LEAVING CERTIFICATE EXAMINATION, 2007 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 = mol -1 Page 1 of 7

2 Section A Answer at least two questions from this section [see page 1 for full instructions]. 1. In an experiment to standardise a solution of potassium manganate (VII) (KMnO 4 ) the primary standard ammonium iron (II) sulphate ((NH 4 ) 2 SO 4.FeSO 4.6H 2 O) was used. 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 H + Mn Fe H 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) How is the conical flask cleaned? (d) Describe two procedures applied to the conical flask during the titration to ensure an accurate result. (e) What colour change would you observe as the solution from the burette flows into the conical flask? How was the end-point detected? (3) (f) What is the role of the Mn 2+ ions produced in this reaction? (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. MnO 4 + 5Fe H + Mn Fe H 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] Page 2 of 7

3 2. Ethanoic acid (CH 3 COOH) is prepared using ethanol (C 2 OH) and acidified sodium dichromate (Na 2 Cr 2 O 7.2H 2 O). The first arrangement of apparatus is shown in the diagram. -- 3C 2 OH +2Cr 2 O 7 +16H + 3CH 3 COOH + 4Cr H 2 O (a) What other substance should be placed in the flask Explain why? (5) (b) State two features of the preparation that are necessary to maximise the yield of ethanoic acid and explain the reason for each. (12) (c) Describe and account for the colour change which is observed during the preparation? (9) (d) Describe how the product ethanoic acid may be isolated from the mixture in the flask? (e) Name two impurities which might be present alongside the ethanoic acid. (f) Write a balanced equation for the reaction between ethanoic acid and sodium carbonate (Na 2 CO 3 )? How would you identify the gas produced by the reaction? (12) 3. To investigate the effect of temperature on a reaction rate a student measured 100 cm 3 of 0.01 M solution of sodium thiosulfate into a conical flask, added 10 cm 3 of 1.0 M hydrochloric acid, heated it to 30 0 C and then placed it on top of a cross drawn on a sheet of white paper as shown in the diagram. The student noted the time taken for the cross to become obscured by a pale yellow precipitate formed in the solution. The reciprocal of time (1/time) was used as a measure of the initial rate of the reaction. The experiment was repeated a number of times at various temperatures and the results were recorded in the table below Temperature in o C Time taken for cross to disappear. Rate= I/Time 10 o C 2min 44sec o C 2min 05sec o C 1min 12sec o C 40sec (a) Identify the pale yellow precipitate formed in the flask. (5) (b) Plot a graph between the rate (1/Time) and the temperature, putting the temperature on the horizontal axis (X axis). (9) (c) What conclusion may be drawn from the graph about the effect of temperature on the rate of a chemical reaction? (d) Explain why increasing temperature has a significant effect on the rate of reaction. (e) Use the graph to estimate the time taken for the cross to disappear if the reaction was carried out at 35 o C. (9) (f) State and explain three other factors that have an effect on the rate of a chemical reaction. (15) Page 3 of 7

4 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) What is the oxidation number of sulphur in (i) H 2 SO 4 and (ii) Na 2 S 2 O 3.? (b) What is an ideal gas? (c) Draw and label the energy profile diagram for an endothermic reaction. (d) State two differences between Mendeleev s periodic table and the modern periodic table. (e) State the shape of the following molecules (i) PH 3 and (ii) AlCl 3 (f) Name two additives or two types of additives used to increase the octane number of a fuel. (g) On what principle is the analytical technique atomic absorption spectrometry based? (h) Define the heat of combustion? (i) Describe a simple test to test for polarity in a liquid. (j) Give the systematic name of the compound CH 3 C(CH 3 ) 2 CH 2 CH(CH 3 )CH 3. (k) Answer part A or B A Distinguish between a batch and a continuous production process. OR B Give an example of (i) an ionic crystal and (ii) a covalent macromolecular crystal. [Refer to the data provided in the mathematical tables pages for this question]. 5. (a) Distinguish between intermolecular and intramolecular bonding. (8) (b) Name two types of intermolecular bonding. Explain how each is formed and give an example of where they are found. (18) (c) Define electronegativity. Use the electronegativity values provided in the mathematical tables page 46 to predict the bond type in the following compounds. (i) CaCl 2 (ii) NH 3 (iii) PH 3 (9) State the shape of the PH 3 molecule. (3) 6. C 3 H 6 C 2 CHO C 3 H 7 O H C 2 COOH A B C D (a) Name the compounds labelled A, B, C and D and identify the homologous series to which each compound belongs. (12) (b) Which of the compounds has (i) two planar carbons (ii) only tetrahedrally bonded carbons. (8) (c) Name the type of reaction represented by the following conversions and name the suitable reagent(s) used in each conversion? (i) Compound C to Compound A (ii) Compound B to compound D (12) (d) Compound B exists as an isomer which belongs to a different homologous series. Name the isomer and the homologous series. (e) Describe a test which may be used to distinguish between compound B and the isomer mentioned in (d) above. (12) Page 4 of 7

5 7. (a) What is meant by the term equilibrium? Why is it described as a dynamic state? (8) The formation of ammonia from its elements is shown in the following reversible reaction. This is an important process in the fertilizer industry. N 2(g) + 3H 2(g) 2NH 3 (g) H = 92.4 kj mol -1 (b) What is the name given to this industrial process? (3) (c) Write the equilibrium constant expression (K ) for the reaction above. c (d) In an experiment 4.0 moles of nitrogen and 12 moles of hydrogen were mixed in a 10 litre vessel at a certain temperature. It was found that there was 4.0 moles of ammonia in the equilibrium mixture. Calculate the concentration of each gas at equilibrium in moles/litre? Calculate the value of the equilibrium constant (K c ) at this temperature? (12) (e) How would a catalyst affect this reaction? (3) (f) State the ideal conditions needed to maximize the yield of ammonia. (g) What conditions are actually used and explain the reason for each? (12) 8. (a) Define a conjugate acid base pair and a weak acid in terms of the Bronsted-Lowry theory of acids and bases? (8) Identify one species acting as a base and also identify its conjugate acid in the following system. - HSO 3 + H 3 O + H 2 SO 3 + H 2 O (b) Define ph? What are the limitations of the ph scale? Explain the difference between a strong acid and a concentrated acid. (c) Calculate the ph of a solution containing 1.7g of ammonia (NH 3 ) in 500 cm 3 of water, given that the K b for ammonia is (12) A bottle of wine contains a 12% (v/v) concentration of alcohol. What volume of alcohol is contained in 250 cm 3 of the wine? 9. (a) What is meant by the Biochemical Oxygen Demand (B.O.D)? (5) 10.0 cm 3 of water from a stream was diluted to 1 litre with distilled water and then divided into two portions. The dissolved oxygen concentration of one of the portions was measured immediately and was found to be 8.4 p.p.m. After storing the other portion for a period of time and under certain conditions for the determination of B.O.D the dissolved oxygen concentration was found to be 4.6 p.p.m. (i) For how long and under what conditions was the second portion stored? (ii) Calculate the B.O.D of the undiluted stream water. (iii) What unit of concentration is also used for p.p.m.? (3) (b) Water treatment for domestic use involves several stages. Name the stage where each of the following occurs and describe what happens in each. (i) Micro-organisms are killed. (ii) Suspended solids are removed before filtering. (iii) Protects against tooth decay. (18) Distinguish between the primary and secondary stages of sewage treatment. (12) Page 5 of 7

6 10. Answer any two of the parts (a), (b) and (c). (a) What is radioactivity? (4) Explain what is meant by an alpha particle. (3) Use a suitable equation to show how an alpha particle is emitted from 241 Am 95 Compare alpha, beta and gamma radiation under the following headings (i) Charge (ii) Mass (iii) Penetrating power. (12) (b) Describe a test which could be carried out to detect the presence of nitrate ions in an aqueous solution. (15) In the tertiary treatment of sewage nitrates and phosphates are removed from water. Why is this removal necessary? What is the term used to describe the enrichment of water by these compounds? (4) (c) The following reaction is used in the formation of nitrogen monoxide. 3Cu (s) + 8 HNO 3 (aq) 3Cu(NO 3 ) 2 (aq) + 2 NO (g) + 4H 2 O (aq) 100 cm 3 of 2.0 M solution of nitric acid was reacted with 6.0g of copper. (i) Show clearly that the copper is in excess in this reaction. (10) (ii) What volume of nitrogen monoxide measured at s.t.p was produced? (iii) How many water molecules were produced if there is only a 75% yield of all products? (9) [Relative atomic masses: Cu = 63.5 ] Page 6 of 7

7 11. Answer any two of the parts (a), (b) and (c). (a) Define the heat of formation and kilogram calorific value. (12) Ethanol burns according to the following equation 2C 2 OH + 6O 2 4CO 2 + 6H 2 O Given the heats of formation of ethanol, carbon dioxide and water are 288, 394 and -286 kj mol -1 respectively, calculate the heat of combustion of ethanol. (13) (b) Explain the terms (i) energy level (ii) orbital Write the electronic configuration of a neutral copper atom. Explain why the first ionisation energy of nitrogen (1400 kjmol 1 ) is greater than that of oxygen (1310 kjmol 1 ) Describe Rutherford s experiment which led to the discovery of the nucleus. (7) (c) Answer part A or part B A Explain the term greenhouse effect. (7) Name three significant greenhouse gases? Explain the source of one of these gases. (3) Name one gas in the atmosphere which is not a greenhouse gas? (3) Explain the role of the ocean in controlling the greenhouse effect. B Aluminium is extracted from alumina by electrolysis using the following setup (see diagram) Cryolite is added to the alumina before electrolysis. What is the formula of alumina? Why is the cryolite added? What materials are used for (i) the anode and (ii) the cathode? Write the equation for the reaction which occurs at the cathode. (3) State one use of aluminium. (4) or Page 7 of 7

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