CHEMISTRY. DATE: 7 June 2011

Transcription

1 EUROPEAN BACCALAUREATE 2011 CHEMISTRY DATE: 7 June 2011 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. 1/16 EN

2 Question A1 Page 1/3 a) Mandelic acid, 2-hydroxy-2-phenylethanoic acid, can be obtained pure from bitter almonds. The structural formula of mandelic acid is given below: Mandelic acid can be found in certain skin care products. For example, it is used to treat acne. 100 cm 3 of an aqueous solution of mandelic acid contains 1.59 g of mandelic acid. The pk a of mandelic acid is 3.85 at 25.0 o C. i. Calculate the molar molecular mass of mandelic acid. Given: Molar atomic masses (in g mol -1 ) H : 1.01 ; C : 12.0 ; O : 16.0 ii. Calculate the concentration (in mol dm -3 ) of this mandelic acid solution. iii. Calculate the ph of this mandelic acid solution at 25.0 o C. iv. Use the structure to explain why mandelic acid is soluble in water. v. Explain why mandelic acid is more soluble in an aqueous alkaline solution. 3 marks 2/16

3 Question A1 Page 2/3 b) The concentration of mandelic acid taken from another cosmetic product is determined by titration using a solution of sodium hydroxide, NaOH(aq). ph V NaOH / cm 3 The graph above shows the titration of 25.0 cm 3 of this cosmetic product with 1.20 x 10-1 mol dm -3 sodium hydroxide solution. It is assumed that mandelic acid is the only acid in this cosmetic product. i. Give the equation for the titration using structural formulas. ii. State the ph at the equivalence point and the volume of solution of base added at the equivalence point. iii. Calculate the concentration (in mol dm -3 ) of the mandelic acid in this cosmetic product. 3/16

4 Question A1 Page 3/3 iv. Suggest an appropriate indicator from the table below for this titration. Explain your choice. Indicator ph range thymol blue bromophenol blue phenolphthalein v. What data concerning the acid can be obtained from the ph value at the half-equivalence point? c) It is necessary to use a buffer solution in order to maintain a constant ph in cosmetic products containing mandelic acid. i. Suggest how a buffer solution using mandelic acid could be prepared (it is not necessary to include a calculation). ii. In which ph region will this buffer solution be effective? iii. Give equations to show how this buffer solution works when an acid or a base is added. 4/16

5 Question A2 Page 1/3 a) Fuel cells are used in spacecraft as a source of electrical energy. Some of these cells use hydrogen, H 2 (g), and oxygen, O 2 (g), as reactants. The gases diffuse through a porous carbon electrode and react in a basic solution. Platinum is used to catalyse the reaction. Given : Standard redox potentials for the relevant couples : Couple E / V H 2 O(l) / H 2 (g) 0.83 O 2 (g) / OH - (aq) i. Identify the type of reaction which occurs at the positive electrode and give the corresponding half-equation. ii. Give the half-equation for the reaction occurring at the negative electrode. iii. Give the overall equation for the reaction occurring in the fuel cell. iv. Explain how the ph changes around each half-cell as the cell works. v. Calculate the e.m.f. of the cell under standard conditions. b) Fuel cells have recently been made using a basic solution of hydrazine, N 2 H 4 (aq), and oxygen, O 2 (g), as reactants. The relevant half-equations are: N 2 H 4 (aq) + 4OH - (aq) N 2 (g) + 4H 2 O(l) + 4e - O 2 (g) + 2H 2 O(l) + 4e - 4OH - (aq) i. Give the equation for the overall reaction taking place in the cell. 5/16

6 Question A2 Page 2/3 ii. Does hydrazine act as an oxidising agent or as a reducing agent in this reaction? Explain your answer using oxidation numbers. iii. Calculate the volume of oxygen gas used in one hour when the cell operates under standard conditions to produce a steady current of 500 ma. 3 marks Given : The molar volume of a gas under standard conditions : 24.5 dm 3 mol -1 1 Faraday = 9,65 x 10 4 C mol -1 c) A chemist wished to convert chloride ions, Cl - (aq), into chlorine, Cl 2 (g). i. From the table below, which is the most suitable reagent to perform this experiment? Explain your answer. ii. Give the overall equation for the reaction. Given: Standard redox potentials Reduction half-equation E Ө / V MnO 4 - (aq) + 8H + (aq) + 5e - Mn 2+ (aq)+ 4H 2 O(l) Cl 2 (g) + 2e - 2Cl - (aq) Cr 2 O 7 2- (aq) + 14H + (aq) + 6e - 2Cr 3+ (aq) + 7H 2 O(l) d) Bleach contains sodium hypochlorite, NaClO(aq). Sodium hypochlorite reacts with an acidified solution of potassium iodide, KI(aq), to give iodine, I 2 (aq), which can be titrated with a solution of sodium thiosulphate, Na 2 S 2 O 3 (aq) of known concentration. i. Give the equation for the reaction between ClO - (aq) ions and I - (aq) ions in acid solution to form Cl - (aq) and I 2 (aq). ii. Give the equation for the reaction between I 2 (aq) and S 2 O 3 2- (aq) ions to form I - (aq) ions and tetrathionate ions S 4 O 6 2- (aq). 6/16

7 Question A2 Page 3/3 A 25.0 cm 3 sample of the bleach was diluted to give 250 cm 3 of solution cm 3 of this solution was added to an excess of acidified potassium iodide solution, KI(aq). The volume of 2.00 x 10-1 mol dm -3 sodium thiosulphate solution required to titrate this last solution was 18.5 cm 3. iii. Determine the concentration of sodium hypochlorite in this sample of bleach. 4 marks 7/16

8 Question A3 Page 1/2 a) Ammonia, NH 3 (g), is manufactured by the Haber-Bosch process. In this process nitrogen, N 2 (g) and hydrogen, H 2 (g) react at high pressure in the presence of a catalyst. Explain whether this is a redox reaction or an acid-base reaction according to Brønsted-Lowry theory. b) Sodium carbonate, Na 2 CO 3 (s), can be obtained industrially by the Solvay process. During this process carbon dioxide, CO 2 (g), is passed through a saturated solution of sodium chloride, NaCl(aq), and ammonia to produce ammonium chloride, NH 4 Cl(aq), and sodium hydrogencarbonate, NaHCO 3 (s). The sodium hydrogencarbonate is decomposed by heating to give sodium carbonate and carbon dioxide among the products. i. Give the equation for the reaction which leads to the formation of sodium hydrogencarbonate and ammonium chloride. ii. Give the equation for the thermal decomposition of sodium hydrogencarbonate. c) i. Give the equation for the preparation of ammonia from ammonium chloride, NH 4 Cl(s) and calcium oxide, CaO(s). ii. Calculate the ph of an aqueous solution of ammonia containing 4.89 g of ammonia in 500 cm 3 of solution at 25.0 o C. 3 marks Given: The pk b of NH 3 (aq) at 25.0 o C : 4.75 Molar atomic masses (in g mol -1 ): H : 1.01; N : 14.0 iii. State whether an aqueous solution of ammonium chloride is acidic, neutral or basic. Give the relevant equation. 8/16

9 Question A3 Page 2/2 d) In 1906, Ostwald developed a method for manufacturing nitric acid, HNO 3 (aq), in three successive steps. In the first step, ammonia is combusted in oxygen, O 2 (g), from the air at a high temperature (700 o C) in the presence of a catalyst to obtain nitrogen monoxide, NO(g), and water. In the second step, after cooling the nitrogen monoxide to 200 o C, it is oxidised to nitrogen dioxide, NO 2 (g). In the third step, nitrogen dioxide reacts with water to form nitric acid and nitrogen monoxide. i. Give the relevant equations for the three steps to produce nitric acid. 4 marks ii. A factory produces 5.00 x 10 5 kg of pure nitric acid each day. Calculate the volume of ammonia required under standard conditions to obtain this mass of acid each day. 4 marks Given : The molar volume of a gas under standard conditions : 24.5 dm 3 mol -1 Molar atomic masses (in g mol -1 ): H : 1.01 ; N : 14.0 ; O : 16.0 e) Silver, Ag(s), can react with nitric acid, HNO 3 (aq), to give a solution of silver nitrate, AgNO 3 (aq), and nitrogen monoxide, NO(g). i. Give the overall equation for this redox reaction. ii. Use the redox potentials given in the following table to explain why gold, Au(s), does not react with nitric acid. Given: Standard redox potentials: Couple E Ө / V Ag + (aq) / Ag(s) NO 3 - (aq) / NO(g) Au 3+ (aq) / Au(s) /16

10 Question B1 Page 1/2 a) Aldehydes can be readily converted to nitriles by reacting with compound X. Nitriles can then be hydrolysed by boiling with hydrochloric acid, HCl(aq) to give carboxylic acids as shown below: X HC l Consider the following reactions: A X B HC l C where C represents the compound CH 3 CH(OH)COOH i. Give the IUPAC name and write the structural formula of compound A. ii. Give the IUPAC name of compound C. iii. Give the formula of the reagent X involved in the conversion of A to B. iv. Give the mechanism for the conversion of A to B. Use curly arrows to show the movement of pairs of electrons. 3 marks Compound A can be prepared by oxidising another compound. v. Give the IUPAC name and write the structural formula of this other compound. vi. Give the IUPAC name of the product obtained by oxidising compound A with an acidified solution of potassium dichromate, K 2 Cr 2 O 7 (aq). vii. State, with an explanation, whether compound C is a stronger or weaker acid than propanoic acid. 3 marks When propanoic acid is dissolved in a non polar solvent it appears to have a molar molecular mass which is double the expected value. viii. Suggest a reason, based on its molecular structure, to explain this behaviour. 10/16

11 Question B1 Page 2/2 b) Compound D has a molar molecular mass of 118 g mol -1. The percentage composition by mass of this compound is: C: 50.8 % ; H: 8.50 %; O: 40.7 %. i. Show that the molecular formula of compound D is C 5 H 10 O 3. Given : Molar atomic masses (in g mol -1 ): H : 1.01 ; C: 12.0 ; O : 16.0 Boiling compound D with sodium hydroxide solution followed by neutralisation with acid to give compound, C, with the formula CH 3 CH(OH)COOH, and an alcohol E. ii. Give the structural formulas and the IUPAC names of compounds D and E. iii. Give the equation for the reversible reaction between compounds C and E to form compound D. iv. State the necessary experimental conditions for the reaction described in b)iii. above to take place. 4 marks 11/16

12 a) Consider the following amines: Question B2 Page 1/3 dopamine tyramine adrenalin noradrenalin i. For each of the amines state whether it is a primary, secondary or tertiary amine. ii. State the name of another functional group possessed by all these four amines. iii. Explain whether noradrenalin or tyramine will be more soluble in water. iv. Explain why the pk b of noradrenalin (pk b = 5.42) is higher than the pk b of adrenalin (pk b = 4.84).. 12/16

13 b) Consider the following amino acids: Question B2 Page 2/3 glycine (Gly) alanine (Ala) phenylalanine (Phe) i. Explain why the pk a of the COOH functional group of alanine is lower than that of propanoic acid. ii. Glycine and propanoic acid have virtually identical molar molecular masses. Explain why the melting point of glycine is very much higher than the melting point of propanoic acid. Alanine can show optical activity and amphoteric behaviour. iii. Define the two underlined terms. iv. Show the three-dimensional representations of the two enantiomers of alanine. v. Explain why glycine does not exhibit optical activity. vi. Give appropriate equations to illustrate the amphoteric behaviour of alanine in aqueous solution. The primary structure of a tripepetide is Ala-Gly-Phe vii. Give the simplified structural formula of this tripeptide formed from the condensation of these three amino acids. 13/16

14 Question B2 Page 3/3 c) Kevlar is an example of a synthetic polymer formed from the reaction of 1,4- diaminobenzene and benzene-1,4-dicarboxylic acid. i. Give the structural formulas of 1,4-diaminobenzene and benzene-1,4- dicarboxylic acid ii. Give the simplified structural formula of the repeating unit of Kevlar. iii. Explain why Kevlar is water resistant but is dissolved by acid. 14/16

15 Question B3 Page 1/2 a) One of the essential fatty acids is linoleic acid. This acid cannot be synthesised by the human body and therefore must come from food. The formula of linoleic acid is shown below: CH 3 (CH 2 ) 4 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH The degree of unsaturation of linoleic acid can be determined by the addition of iodine, I 2 (aq). The iodine number represents the mass (in grams) of iodine which adds to 100 g of an unsaturated compound. i. Give the equation for the reaction of iodine with linoleic acid. ii. Calculate the iodine number for linoleic acid. iii. Give the simplified formula of the triglyceride formed between propane-1,2,3-triol and linoleic acid. iv. Show that the iodine number of the triglyceride obtained in a) iii. is marks Given : Molar molecular masses (in g mol -1 ): linoleic acid : 280 ; triglyceride of linoleic acid : 878 Molar atomic mass (in g mol -1 ): I : 127 Hydrogenation can be used to decrease the degree of unsaturation of certain oils during the production of margarine. Generally triglcerides in margarine have an iodine number of about 85. v. Assuming that the triglycerides present in margarine have a similar molar molecular mass to the triglyceride of linoleic acid, state the number of C=C double bonds present in a molecule of a margarine triglyceride. vi. Explain the difference in melting points between the starting oil and the partially hydrogenated fat. 15/16

16 Question B3 Page 2/2 b) Complete hydrogenation of the triglyceride of linoleic acid leads to the formation of the triglyceride of octadecanoic acid. This product can be used to make soap. i. Give the equation for the saponification reaction using structural formulas and give the IUPAC names of the products of the reaction. ii. Calculate the volume of 10.0 mol dm -3 potassium hydroxide solution, KOH(aq), required to saponify 1.00 x 10 3 kg of the triglyceride of octadecanoic acid. 4 marks Given: Molar molecular mass (in g mol -1 ): triglyceride of octadecanoic acid : 890 c) In aqueous solution soaps form an emulsion with fats. i. With the help of a labelled diagram explain how soap acts as an emulsifier. ii. Explain why hard water prevents soap from working. iii. Why do synthetic anionic detergents work effectively in hard water conditions? iv. State the disadvantages of using synthetic detergents. 3 marks 16/16