Edexcel Jun 2010 Paper 5 Q21 (with explained solutions) |

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Edexcel Jun 2010 Paper 5 Q21

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PMT SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 Brass is an alloy of copper, zinc and, in some cases, other metals too. There are over 30 varieties of brass for different applications. The amount of copper in a brass can be found as follows:A weighed sample of brass is reacted with the minimum amount of concentrated nitric acid. The solution is neutralized, a portion of it pipetted into a conical flask, and excess potassium iodide solution is added. The iodine produced is titrated with a solution of sodium thiosulfate of known concentration. (a) The ionic equation for the reaction between copper metal and concentrated nitric acid is shown below. Cu(s) + 2NO 3 (aq) + 4H+ (aq)Cu2+(aq) + 2NO2(g) + 2H2O(l) (i) Give the oxidation numbers of the copper and nitrogen in both the reactants and products. (2) Copper: fromtoNitrogen: fromto(ii) Write the two half-equations that can be combined to give the ionic equation shown above. (2) 10 *N36507A01028*<br />
PMT (iii) Explain why the standard electrode potentials for the two ionic half-equations that you have written give an incorrect value for Ecell for this reaction as described above. (2)(b) The solution produced contains a mixture of zinc ions and copper ions. (i) State TWO observations that you would see if concentrated ammonia solution were to be added, drop by drop, to the solution until in excess. (2)(ii) Copper ions can be separated from the zinc ions in the solution by adding sodium hydroxide solution in excess, followed by filtration of the mixture. Write equations, including state symbols, for the THREE reactions that occur. (3) Equation 1 Equation 2 Equation 3 *N36507A01128* 11 Turn over<br />
PMT *(iii) Give examples of amphoteric behaviour and ligand exchange, by reference to the reactions of zinc compounds. (3)(c) A sample of Admiralty Brass of mass 3.00 g was treated with nitric acid and made up to a neutral solution of volume 250 cm3. Excess potassium iodide was added to 25.0 cm3 portions of this solution, and the liberated iodine was titrated with sodium thiosulfate solution, concentration 0.100 mol dm3. The mean titre was 33.10 cm3. (i) Write the ionic equation for the reaction between thiosulfate ions and iodine. *(ii) The equation for the reaction between copper(II) ions and iodide ions is shown below. 2Cu2+ + 4I2CuI + I2 Hence calculate the percentage by mass of copper in Admiralty Brass. Give your answer to three significant figures. (1) (6) 12 *N36507A01228*<br />
PMT (iii) When setting up the burette, a student failed to fill the jet of the burette. Explain the effect that this would have on the value of the first titre. If this first titre was included in the calculation of the mean titre, what effect would this have on the value for the percentage of copper in the brass? (2)(Total for Question 21 = 23 marks) *N36507A01328* 13 Turn over<br />

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Section B Question Number 21 (a)(i) Question Number 21(a)(ii) Acceptable Answers Copper: 0 to +2/2+/2+/II/2 (1) Nitrogen: +5/5+/5+/V/5 to +4/4+/4+/IV/4 (1) Acceptable Answers CuCu2+ + 2e() OR Cu2e()Cu2+ (1) Cu[(H2O)6]2+ OK if 6 waters shown on l.h.s. Reject Reject Mark Mark+ 2H+ + e()NO2 + H2O+ 4H+ + 2e()N2O4 + 2H2O (1)+ 4H+ + 2e()2NO2 + 2H2O (1) NO3 OR 2NO3 OR 2NO3 Ignore the full equation if it is given as well Allow equations written as reverse of above Ignore state symbols even if wrong AllowforAcceptable Answers Reject (electrode potential) values are for standard conditions (1) nitric acid is concentrated / not 1 mol dm-3 / not 1 M (1) Allow temperature not stated for second mark NO3 Any reference to loss of NO2are not 1 mol dm-3 Mark Question Number 21(a)(iii) 5 6CH0501 1006<br />
Question Number 21(b)(i) Question Number 21(b)(ii) Mark Mark Acceptable Answers Reject initially a (pale/light) blue precipitate (1) Allow blue solid Ignore white precipitate (re-dissolves in excess to form) a (deep) blue solution (1) Stand alone mark Accept any shade of blue except greenish-blue Any colour (other than blue) precipitate in blue solution Reject Acceptable Answers 2(aq) (1) 2 2-(aq)+ 2H2O(l) Cu2+(aq) + 2OH(aq)Cu(OH)2(s) (1) Zn2+(aq) + 2OH(aq)Zn(OH)2(s) (1) Zn(OH)2(s) + 2OH(aq)Zn(OH)4 If two previous equations combined correctly then (1) only : Zn2+ + 4OHZn(OH)4 Allow Zn(OH)2(s) + 2OH(aq)ZnO2 OR Zn(OH)2(s) + 4OH(aq)Zn(OH)6 OR equivalent non-ionic equations, including those with Zn2+ + 2NaOH etc OR Correct balanced equations starting with hexaqua or tetraqua cations ALLOW the hydroxides to be shown as e.g. Zn(OH)2(H2O)4 (s) provided that the whole equation balances. Penalise missing /incorrect state symbols on product once only. Ignore other state symbols 4(aq) 6 6CH0501 1006<br />
Question Number 21(b)(iii) QWC Mark Acceptable Answers Reject Reference to zinc ions or zinc metal Do not allow deprotonation First 2 marks: zinc hydroxide/oxide amphoteric because it reacts with alkali (to give a solution of a zincate) (1) and reacts with acid (to give a salt) (1) zinc hydroxide is / acts as both an acid and an alkaliscores (1) only Third mark: hexaquazinc or hydrated zinc ions exchanged water for ammonia or other named ligand (1) OR Zn(H2O)6 2+ + 4NH3etc (1) Allow any number of ammonias from 1 to 6 Allow balanced equations, ionic or full. Ligand exchange reaction must start with a complex ion Note: If zinc mentioned initially but equation refers to a correct compound then credit should be given If equations wrong but words are correct then ignore equations 7 6CH0501 1006<br />
Question Number 21(c)(i) Question Number 21(c)(ii) QWC Question Number 21(c)(iii) Acceptable Answers Reject Mark Non-ionic equation. Mark Reject 70.06 or 70.0 Reject Mark 22I + S4O6 I2 + 2S2O3 Ignore state symbols even if wrong. 2 Acceptable Answers = 0.0331 dm3 x 0.1 mol dm 3 = 0.00331 mol (1) Amount thiosulphate = amount of copper(II) ions in 25 cm3 portion (1)amount Cu = 10 x 0.00331= 0.0331 mol in total (1)mass Cu = 0.0331 mol x 63.5 g mol-1 (1) = 2.102 g% copper = (2.102 x 100) 3.00 (1) = 70.1% (1) to 3 s.f. only Mark consequentially but if % > 100 then (-1) If equation in (i) is incorrect but used correctly in part (ii) then all marks can be scored unless answer > 100% Correct answer can score 6 marks irrespective of the stoichiometry of the equation in (c)(i) If candidates uses 64 for molar mass of Cu final answer will be 70.6; scores max of 5 Acceptable Answers some reagent used to fill the jet (which does not react with the iodine solution) and so the titre is too high (1) and hence the percentage value would be too high (1) Allow only if the titre is said to be high If the titre is thought to be too low then allow percentage value too low for 2nd mark (1) 8 6CH0501 1006<br />

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