OCR A AS Specimen Paper 1 Q21 (with explained solutions) |

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OCR A AS Specimen Paper 1 Q21

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14 21 Carbon monoxide can be made in the laboratory by heating a mixture of zinc metal and calcium carbonate. An equation for this reaction is shown below. Zn(s) + CaCO3(s) (cid:111) ZnO(s) + CaO(s) + CO(g) oxidation number change: fromto(a) (b) oxidation number change: fromtoThis reaction is a redox reaction. Deduce which element has been oxidised and which has been reduced, and state the change in oxidation number in each case. element oxidisedelement reduced . Carbon monoxide contains a triple bond, and includes a dative covalent bond. Construct a dot-and-cross diagram to show the outer electron pairs in a molecule of carbon monoxide. SPECIMEN [2] [2]OCR 2014 H032/01<br />
(c) A student carried out the reaction of zinc (Zn) and calcium carbonate (CaCO3) in a fume cupboard. The student measured the volume of gas produced. 15 A mixture containing 0.27 g of powdered zinc and 0.38 g of powdered CaCO3 was heated strongly for two minutes. The volume of gas collected in the 100 cm3 syringe was then measured. The experiment was then repeated. (i) Calculate the maximum volume of carbon monoxide, measured at room temperature and pressure, that could be produced by heating this mixture of Zn and CaCO3. Show all your working. SPECIMEN The student did not obtain the volume of gas predicted in (i) using this procedure. Apart from further repeats, suggest two improvements to the practical procedure that would allow the student to obtain a more accurate result. volume of carbon monoxide =cm3 [2][2] (ii)OCR 2014 H032/01 Turn over<br />
(d) The student repeated the experiment in (c) using different quantities of zinc and calcium 16 (i) Plot a graph from the data provided. carbonate. The student measured the total volume of gas collected over time. The students results are shown below. Time / s Total volume of gas collected / cm3 0 13 42 56 65 72 72 0 20 40 60 80 100 120 Include a line of best fit. SPECIMENOCR 2014 H032/01 [3]<br />
(ii) Using the graph, determine the rate of reaction, in cm3 s1, after 50 s. 17 Show your working on your graph. rate after 50 s =cm3 s1 [2] SPECIMENOCR 2014 H032/01 Turn over<br />

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June 20XX three shared electron pairs plus a lone pair on C and O (cid:57) one of the shared pairs shown as dativei.e. both with the same type of dot/cross as the other electrons around the O (cid:57)(cid:3) (c) (i) Determining limiting factor H032/01 Question 21 (a) (b) Element oxidised: Element reduced: Mark Scheme 0 to +2 (cid:57) Guidance Marks Answer zinc/Zn carbon/C +4 to +2 (cid:57)(cid:3) ALLOW 1 mark for all oxidation numbers correct, but oxidised and reduced the wrong way around max 1 mark if missing + or if given as charges e.g. 2+ mark can be awarded if either lone pair is missing, but there must be three shared pairs SPECIMEN evidence of 0.27/65.4 is required (or using the mass ratio to predict 0.116g of CO from 0.27g Zn) or use of the mass ratio to predict 0.106g CO from 0.38g CaCO3, and dividing by 28.0 to get 0.0038 mol CO ALLOW 2 sig figs up to calculator answer ALLOW second and third marks for correct final answer with no working ALLOW 2 marks for 99 cm3 from excess Zn mass n(Zn) 0.27/65.4 = 0.0041 mol AND n(CaCO3) = 0.38/100.1 = 0.0038 mol so Zn is in excess (cid:57) Determining volume of CO ratio 1:1, so n(CO) = 0.0038 (mol) vol. CO = 0.0038 x 24.0 = 0.091 dm3 = 91 (cm3) (cid:57) P M T<br />
(i) axes: labels correct, (ii) Tangent H032/01 Question (d) (ii) heat until syringe stops moving/no further gas produced June 20XX Mark Scheme Answer Guidance Marks ALLOW heat for longer than two minutes ALLOW heat a greater mass (cid:57) wait until the gas has cooled (to room temperature) before measuring the volume owtte (cid:57)(cid:3) AND units AND scales chosen so that the plotted points occupy at least half the graph grid in both the x and y directions (cid:57) ALL points plotted correctly (cid:57) Best curve drawn through points AND ignoring point at 20 s (cid:57) tangent drawn to curve at t = 50 s (cid:57) Calculation of rate from the gradient of tangent drawn e.g. rate = SPECIMEN Annotate tangent on graph Note: This mark can only be awarded from a tangent ALLOW ECF for tangent drawn at different time from 50 s ALLOW 10% of gradient of tangent drawn ALLOW 2 sig figs up to calculator value ALLOW trailing zeroes, e.g. 0.7 for 0.070 IGNOREsign for rate Note: if candidate calculates rate via ln 2 method, consult with TL 64 = 0.68 (cm3 s(cid:16)1) (cid:57) 94 Total 13 10 P M T<br />

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