CHEMISTRY. DATE: 11 th June 2010

Transcription

1 EUROPEAN BACCALAUREATE 2010 CHEMISTRY DATE: 11 th June 2010 DURATION OF THE EXAM : 3 hours (180 minutes) PERMITTED MATERIAL : Calculator (not graphical and not programmable) INSTRUCTIONS : Answer two A questions and two B questions. Indicate which four questions you have answered by putting crosses in the appropriate place on the sheet supplied. Use a separate answer sheet for each of the four main questions. Page 1/16 EN

2 Question A1 Page 1/2 a) Certain flowers such as hydrangeas, display various colours due to the presence of natural coloured pigments. The appearance of the colours red, mauve, violet and blue are due to the presence of anthocyanins in the petals. The violet colour is attributed to the molecule shown below, which is represented as HA in the questions which follow. HA can be present as two acid-base couples H 2 A + (aq) / HA(aq) with pk a1 = 4.3 and HA(aq) / A (aq) with pk a2 = 7.0. H 2 A + (aq) is red, HA(aq) is violet and A (aq) is blue. The value of pk w = i. Define an acid according to Brønsted theory. ii. Identify the acid and the base forms for each of the two couples using the simplified formulas given above. iii. Write the equation showing the reaction when the acid HA(aq) reacts as an acid with water. iv. Give the expression for the acid dissociation constant K a2 for HA(aq). v. Determine the value of K a2 b) The ph of a solution containing HA(aq) is i. From the expression for K a2 calculate the ratio [A - (aq)] eq [HA(aq)] eq ii. Deduce the predominant species present and state the colour of the solution. Page 2/16

3 Question A1 Page 2/2 c) A - (aq) and HA(aq) can react as Brønsted bases. i. Give the equation for the reaction of HA(aq) behaving as a base with water. ii. Give the expression for the base dissociation constant K b1 for this reaction. iii. Prove that K a1 x K b1 = K w iv. Reproduce the diagram below. pk a1 pk a ph Show on the diagram where the species H 2 A + (aq), HA(aq) and A (aq) predominate and show the colours of the flowers as a function of ph. v. Explain why the flowers of hydrangeas can change colour according to the nature of the soil. d) Magnesium ammonium phosphate is a sparingly soluble salt which can be formed in urine and is also known as uric acid stone in medicine. The formation of the salt can be described by the following simplified equation. Mg 2+ (aq) + NH 4 + (aq) + HPO 4 2- (aq) + H 2 O(l) MgNH 4 PO 4 (s) + H 3 O + (aq) i. Describe the effect of changing the ph on the formation of the salt. ii. Using the values of K a and K b, write the equation for the dominant reaction that takes place when the HPO 4 2- (aq) ion is present in water. iii. By referring only to the reaction in d) ii. Calculate the ph of a solution of hydrogenphosphate ions, HPO 4 2- (aq), when the initial concentration of the ions [HPO 4 2- (aq)] is 1.50 x 10-3 mol dm -3. Given : K a (HPO 4 2- ): K b (HPO 4 2- ): Page 3/16

4 Question A2 Page 1/2 a) Two half-cells involving the couples Fe 3+ (aq)/fe(s) and Fe 2+ (aq)/fe(s) are joined together to produce an electrochemical cell. N.B. The values needed can be found at the end of part c). i. Write the half-equations for the chemical reactions taking place at the electrodes. ii. Give the overall equation for the reaction taking place in the cell. iii. Calculate the electromotive force (e.m.f.) of the cell operating under standard conditions. iv. Draw a labelled diagram of the cell and show the direction of electron flow in the external circuit. b) Potassium manganate(vii), KMnO 4 (aq), is an oxidising agent. Explain why it is not used to determine quantitatively the amount of Fe(II) ions in a solution of Fe(II) chloride? c) Equal volumes of equimolar solutions of potassium manganate(vii), KMnO 4 (aq), manganese(ii) sulphate, MnSO 4 (aq), potassium dichromate(vi), K 2 Cr 2 O 7 (aq), and chromium(iii) sulphate, Cr 2 (SO 4 ) 3 (aq) are mixed and allowed to come to equilibrium. i. State which ion is oxidised and which ion is reduced. ii. Using the half-equations, write the equation for the overall reaction. Given : Standard redox potentials : Couple E / V MnO - 4 (aq), H + (aq) / Mn 2+ (aq) Cl 2 (g) / Cl - (aq) Cr 2 O 2-7 (aq), H + (aq) / Cr 3+ (aq) Fe 3+ (aq) / Fe 2+ (aq) Fe 3+ (aq) / Fe(s) Fe 2+ (aq) / Fe(s) Page 4/16

5 Question A2 Page 2/2 d) The most important ores of aluminium and iron comprise of Al 2 O 3 and Fe 2 O 3 respectively. Aluminium is obtained from its ore by electrolysis. Iron is obtained from its ore by heating with carbon monoxide, CO(g). During this process the carbon monoxide is converted into carbon dioxide, CO 2 (g). i. Give the equation for the reduction of aluminium ions during the electrolysis of aluminium(iii) oxide. The unbalanced equation below represents the reaction between iron(iii) oxide and carbon monoxide: Fe 2 O 3 (s) + CO(g) Fe(s) + CO 2 (g) ii. Determine the oxidation numbers of carbon and iron. iii. Give the balanced equation for the reaction. iv. Calculate the mass of iron that could be obtained from 4.50 kg of iron ore containing 80.0% by mass of Fe 2 O 3 assuming that the remaining 20.0% contains no iron. v. Calculate the charge in coulombs (C) necessary to produce 2.00 kg of aluminium during the electrolysis of its oxide. vi. If a current of 2.00 x 10 4 A is used calculate the time it takes to produce 2.00 kg of aluminium by electrolysis of Al 2 O 3. Given: Molar atomic masses (in g mol -1 ): O: 16.0 Al: 27.0 Fe: Faraday (F): 9.65 x 10 4 C mol -1 Page 5/16

6 Question A3 Page 1/3 a) A patent for a zinc bromide rechargeable battery was registered with the European Patent Office in The following experiment shows the recharging process of this battery: an aqueous solution of zinc bromide is electrolysed using graphite electrodes. i. Draw a labelled diagram of the cell during the recharging process. ii. Give the half-equations for the reactions taking place at each of the electrodes and specify the polarity of the electrode associated with each half-equation. iii. Give the overall equation for the reaction occurring during the recharging of the battery. iv. Calculate the electromotive force (e.m.f.) when the battery is discharging under standard conditions. v. In the design given in the patent the container of the battery is made of poly(ethene) and the electrodes are made of graphite. Explain the choice of these two materials based on their chemical properties. Given: Standard redox potentials Couple E Ө / V Zn 2+ (aq) / Zn(s) 0.76 Br 2 (l) / Br - (aq) Cl 2 (g) / Cl - (aq) b) Swimming pools can be disinfected using a product called bromine sticks. The active ingredient in this product has the formula C 5 H 6 BrClN 2 O 2. i. Calculate the percentage by mass of bromine in the active ingredient. ii. One product of the disinfection process is bromide ions. Use the relevant standard redox potentials to explain what might happen if the pool has also been treated with chlorine, Cl 2 (g). Include the relevant equation in your answer. Given: Molar atomic masses (in g mol -1 ): H : 1.01 C : 12.0 N : 14.0 O : 16.0 Cl : 35.5 Br : 79.9 Page 6/16

7 Question A3 Page 2/3 c) The ideal ph for swimming pool water is between 7.2 and 7.6. In order to maintain the correct ph compounds containing hydrogencarbonate ions, HCO 3 - (aq), can be added. i. Write the equations which show how hydrogencarbonate ions can control the ph within the region given. Excess acid can be removed by running the water over a marble, i.e. a calcium carbonate, CaCO 3 (s), bed. ii. Write an equation to show the neutralisation action of marble with an acid. iii. Calculate the mass of calcium carbonate required to increase the ph of 2.00 x 10 3 m 3 of pool water from 6.5 to 7.2. Given: Molar atomic masses (in g mol -1 ): C : 12.0 O : 16.0 Ca : 40.1 d) Small amounts of ammonium ions, NH 4 + (aq), are present in good quality fresh water. However, ammonium ions react with water to form ammonia which is toxic to aquatic life. i. Give the equation for this reaction and state the expression for the acid dissociation constant, K a, for ammonium ions. The graph below shows how the ratio of the concentrations [NH 4 + (aq)]/[nh 3 (aq)] changes with temperature, θ (in o C), and ph value. 4 marks Page 7/16

8 Question A3 Page 3/3 ii. Use the graph to explain what happens to the quality of the water if the temperature θ increases within the ph range 8.0 to iii. Use the graph to calculate the value for the acid dissociation constant of ammonium ions, K a, at temperature θ = 0 o C. Page 8/16

9 Question B1 Page 1/2 a) Many aldehydes have very strong and characteristic smells. This is the case for cinnamaldehyde, the major component of cinnamon, and vanillin responsible for the characteristic smell of vanilla pods. A molecule of cinnamaldehye contains one benzene ring and also contains one carbon to carbon double bond. i. A sample of cinnamaldehyde contains 81.8% carbon, 6.10% hydrogen and 12.1% oxygen by mass. Determine the empirical formula of cinnamaldehyde. ii. The molar mass of cinnamaldehyde is 132 g mol -1. Determine the molecular formula of cinnamaldehyde. iii. Give one possible structural formula of cinnamaldehyde. Given: Molar atomic masses (in g mol -1 ): H: 1.01 C: 12.0 O: 16.0 b) The structural formula of vanillin is given below: i. The molecule of vanillin contains two other functional groups apart from an aldehyde. State the names of these two other groups. ii. Vanillin can react with ethanol in the presence of concentrated sulphuric acid. Give the equation for this reaction using structural formulas. iii. Vanillin can also react with ethanoic acid. Give the equation for this reaction using structural formulas. iv. When vanillin reacts with Fehling s solution it forms a carboxylate anion. State what will be observed during the reaction and give the equation for the reaction. v. Vanillin can react with an acidified solution of hydrogen cyanide, HCN(aq). Give the mechanism for this reaction. 4 marks Page 9/16

10 Question B1 Page 2/2 c) Vanillin can react with a basic solution of silver nitrate. The simplified equation for this reaction is given below: C 6 H 3 OHOCH 3 CHO(aq) + 2Ag + (aq) + 3OH - (aq) C 6 H 3 OHOCH 3 COO - (aq) + 2Ag(s) + 2H 2 O(l) A sample of a food additive containing vanillin has a mass of 2.40 x 10-3 g x 10-3 g of silver are formed when the sample undergoes the reaction above. i. Calculate the percentage by mass of vanillin in the sample. ii. State the assumption made during the calculation. Given: Molar atomic mass (in g mol -1 ): Ag: 108 Molar molecular mass of vanillin (in g mol -1 ): 152 Page 10/16

11 Question B2 Page 1/3 a) The reaction scheme below shows the synthesis of a coloured compound, B, starting with 4-aminobenzenesulphonic acid, H 2 N-C 6 H 4 -SO 3 H. i. Refer to the group enclosed by the box to name the chemical family to which compound A belongs. ii. Name the type of reaction for the synthesis of compound B and state the chemical family to which this coloured compound belongs. iii. Explain, by referring to its molecular structure, why compound B is coloured. Page 11/16

12 Question B2 Page 2/3 b) Consider the following nitrogen-containing compounds: A. (CH 3 ) 3 N B. C 6 H 5 NH 2 C. CH 3 (CH 2 ) 2 NH 2 D. CH 3 CH 2 NHCH 3 E. NH 3 i. For each amine state whether it is primary, secondary or tertiary. ii. Name compounds B and C. iii. Among the five compounds above, compound B, C 6 H 5 NH 2, is the weakest base. Explain why. c) Captopril is a medicine used to control high blood pressure. It possesses an amide group and can be synthesised from the two molecules A and B below: i. Name the oxygen-containing functional group in molecule A. ii. Does molecule A exhibits optical isomerism? If so, draw the molecule and identify the chiral carbon atom with an asterisk (*). Explain your answer. iii. Knowing that one of the products is hydrogen chloride, HCl(g), and using structural formulas, write the overall equation for the reaction between A and B to form Captopril. Page 12/16

13 Question B2 Page 3/3 d) The IUPAC name of the amino acid leucine is 2-amino-4-methylpentanoic acid. i. Give the simplified structural formula for a molecule of leucine. ii. Explain the term isoelectric point of an amino acid. iii. The isoelectric point of leucine is 6.0. Give the simplified structural formulas of the major species present in aqueous solutions of leucine with the following ph values. 1 o ) ph = o ) ph = o ) ph = 13.0 iv. By referring to its molecular structure explain why leucine is a solid at room temperature (25 o C). Page 13/16

14 Question B3 Page 1/3 a) The structure formulas below are of the molecules of two lipids, X and Y. One of the lipids is a fat (solid) the other is an oil (liquid). X Y i. Name the functional group common to compounds X and Y. ii. Name the two reactants necessary to make compound X. iii. Indicate which of the two compounds X or Y is an oil. Explain why it is a liquid at room temperature. iv. Oils, such as vegetable oils, can be modified chemically to make margarine, that is they can be turned into fats (solids). Name the chemical reagent necessary to carry out this transformation. b) Compound Y can react with iodine, I 2 (aq). During this reaction the brown colour of the iodine solution disappears. i. Explain how compound Y reacts with iodine. ii. Calculate the mass of compound Y that reacts completely with 100 g of iodine. Given: Molar atomic mass (in g mol -1 ): I: 127 Molar molecular mass of compound Y (in g mol -1 ): 878 Page 14/16

15 Question B3 Page 2/3 c) Compound X can react with potassium hydroxide, KOH(aq), to form a soap. i. Give the equation for this reaction using simplified structural formulas and name the type of reaction that takes place. ii. Using a diagram, explain the detergent properties of soaps. iii. Explain why the soap obtained in c) i. is not so efficient in hard water, i.e. in the presence of a relatively high concentration of calcium ions, Ca 2+ (aq). d) After a certain time fats and oils turn rancid. Generally these unpleasant smells are due to the oxidation of the fatty acids. The addition of molecules called antioxidants can help prevent rancidity. The structural formulas of two antioxidants are shown below: BHT THBQ i. Name a functional group that is common to these two compounds. ii. Explain, in terms of their molecular structures, why both of these antioxidants are soluble in fats and oils. iii. Only one of these compounds is also slightly soluble in water. State which of the two compounds this is and explain this difference in solubity. Page 15/16

16 Question B3 Page 3/3 e) Ascorbic acid, known as Vitamin C, is another example of an antioxidant. It has the following structure: When it functions as an antioxidant the ascorbic acid molecule loses two hydrogen ions, H + (aq) from the two hydroxyl groups, OH, attached to the ring. Using structural formulas write the corresponding half-equation for this oxidation process. Page 16/16