Cambridge A Level Chemistry 9701 — 2016 May/June Paper 5 · Variant 1

READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. Electronic calculators may be used. You may lose marks if you do not show your working or if you do not use appropriate units. Use of a Data Booklet is unnecessary. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. CHEMISTRY 9701/51 Paper 5 Planning, Analysis and Evaluation May/June 2016 1 hour 15 minutes Candidates answer on the Question Paper. No Additional Materials are required. This document consists of 9 printed pages and 3 blank pages. [Turn over IB16 06_9701_51/4RP © UCLES 2016 *8628836894* Cambridge International Examinations Cambridge International Advanced Subsidiary and Advanced Level

2 9701/51/M/J/16 © UCLES 2016 1 Lithium is a soft alkali metal which may be cut with a knife. It is usually stored under oil because it reacts rapidly with moisture and oxygen in the air. Lithium is corrosive and may cause burns. Lithium is highly flammable and in large amounts reacts violently with water. 2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g) This reaction can be used to determine the relative atomic mass of lithium by measuring the volume of hydrogen produced from a small amount of lithium. (a) Draw the apparatus you could use to measure the volume of hydrogen produced, using standard laboratory equipment. Label the chemicals in your diagram and show how the reactants can be kept apart until the reaction is started. [3]

3 9701/51/M/J/16 © UCLES 2016 [Turn over (b) To successfully carry out this experiment a correct procedure must be followed. The lithium you will use is stored as large pieces under oil. (i) Beginning with a large piece of lithium being removed from the oil, state how you would prepare a small piece of lithium for use in this experiment. … … [1] (ii) By only observing the gas collecting apparatus, state how you would know the reaction had stopped. … … [1] (iii) Other than eye protection, state two precautions you would take to make sure that the experiment proceeds safely. 1. … … 2. … … [2] (iv) The relative atomic mass of lithium is known to be approximately 7. What approximate volume of hydrogen gas would a 0.1 g mass of lithium produce? (1 mol of gas occupies 24.0 dm3 at room temperature and pressure.) volume of H2(g) produced … [1] (v) What would be the capacity (volume) of the gas collecting apparatus you would use for the volume of hydrogen produced in (iv)? volume of gas collecting apparatus … [1]

4 9701/51/M/J/16 © UCLES 2016 (c) Another method that can be used to determine the relative atomic mass of lithium is by titration of the lithium hydroxide produced during its reaction with water. The following experimental procedure may be used. 1. Add 100.0 cm3 of distilled water to a clean beaker. 2. Add a known mass of lithium to the distilled water. 3. After the reaction is complete, transfer 25.0 cm3 of the solution of lithium hydroxide from the beaker to a clean conical flask. 4. Titrate this with an acid of known concentration. (i) State how you would accurately measure the total volume of distilled water in step 1. … [1] (ii) State how you would know that the reaction between lithium and distilled water was complete. … … [1] (iii) State how you would transfer 25.0 cm3 of the solution of lithium hydroxide into a clean conical flask in step 3. … [1] (iv) State how you would ensure that your titration result was reliable. … … [1] (d) To make sure that the beaker and the conical flask used in the experimental procedure in (c) are clean, a student decides to wash them out with some distilled water before starting the experiment. Some water remains in the beaker. State the effect, if any, this would have on the calculated relative atomic mass of lithium. Explain your reasoning. … … … Some water remains in the conical flask. State the effect, if any, this would have on the calculated relative atomic mass of lithium. Explain your reasoning. … … … [2] [Total: 15]

5 9701/51/M/J/16 © UCLES 2016 [Turn over 2 Activation energy, EA, is the minimum energy with which particles must collide so that a reaction occurs. The activation energy for the reaction of magnesium with aqueous hydrogen ions can be determined in the laboratory. Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g) A magnesium strip is placed in dilute hydrochloric acid and the time taken (t), in seconds, for the magnesium to disappear is measured. The initial rate of reaction is calculated as time 1 1 t ` j. If the experiment is repeated at several different temperatures then the following mathematical relationship can be used to calculate EA. log10 1 t ` j = E 0.0191 A × 1 T ` j T is the temperature measured in K. 1 t is the initial rate of reaction in s–1. A graph of log10 1 t ` j against 1 T can be plotted. Experimental procedure 1. 25 cm3 of dilute hydrochloric acid is added to a boiling tube. 2. The boiling tube is placed in a water bath until the dilute hydrochloric acid reaches a constant temperature. This temperature is recorded. 3. A magnesium strip of mass 0.10 g is added to the boiling tube, the mixture stirred and the time taken for the magnesium to disappear is recorded. 4. The temperature of the water bath is changed and the experiment is repeated.

6 9701/51/M/J/16 © UCLES 2016 (a) The results of this experiment, carried out at different temperatures, are recorded in the table below. Process the results to calculate the reciprocal of temperature 1 T ` j and log10 1 t ` j. The first value of 1 T has been done for you. Record 1 T in standard form to three significant figures. Record log10 1 t ` j to two decimal places. You should expect log10 1 t ` j to be negative. temperature / °C time, t / s temperature, T / K 1 T / K–1 1 t / s–1 log10 1 t ` j 15 83 288 3.47 × 10–3 1.20 × 10–2 20 58 293 1.72 × 10–2 27 36 300 2.78 × 10–2 30 28 303 3.57 × 10–2 34 18 307 5.56 × 10–2 38 19 311 5.26 × 10–2 40 15 313 6.67 × 10–2 43 12 316 8.33 × 10–2 48 9 321 1.11 × 10–1 55 8 328 1.25 × 10–1 [3] (b) Plot a graph on the grid on page 7 to show the relationship between log10 1 t ` j and 1 T . Use a cross (×) to plot each data point. Draw a line of best fit. [2]

7 9701/51/M/J/16 © UCLES 2016 [Turn over −0.8 −0.9 −1.0 −1.1 −1.2 −1.3 −1.4 −1.5 −1.6 −1.7 −1.8 −1.9 −2.0 3.0 × 10–3 3.1 × 10–3 3.2 × 10–3 3.3 × 10–3 3.4 × 10–3 3.5 × 10–3 3.6 × 10–3 log10 ` j 1 t 1 T / K–1

8 9701/51/M/J/16 © UCLES 2016 (c) On your graph, circle the two points you consider to be the most anomalous. Label each one with a different letter. Explain what may have caused each of the anomalies you have identified, giving a different reason each time. Make it clear in your answer to which point you are referring. … … … … [2] (d) (i) Determine the gradient of your graph. State the co-ordinates of both points you used for your calculation. Record the value of the gradient to three significant figures. co-ordinates 1 … co-ordinates 2 … gradient = … [2] (ii) Use your gradient from (i) and the mathematical relationship on page 5 to calculate the activation energy, EA, in kJ mol–1. Include a sign in your answer. EA = … kJ mol–1 [2] (e) State whether you consider the results to be reliable. Explain your answer. … … … [1]

9 9701/51/M/J/16 © UCLES 2016 [Turn over (f) Student X commented that data collected at higher temperatures in the experiment may be less accurate than that collected at lower temperatures. State whether student X is correct. Explain why. … … … [1] (g) If the magnesium strip is not stirred it floats to the surface of the hydrochloric acid. State how this will affect the reaction time. Explain why. … … … [1] (h) The experiment in (a) is repeated using dilute ethanoic acid instead of dilute hydrochloric acid. The concentration of both acids is equal. The same temperatures are used as in (a). State the effect this change in acid will have on the initial rate values. Give a reason for this. … … … [1] [Total: 15]

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® IGCSE is the registered trademark of Cambridge International Examinations. This document consists of 6 printed pages. © UCLES 2016 [Turn over Cambridge International Examinations Cambridge International Advanced Subsidiary and Advanced Level CHEMISTRY 9701/51 Paper 5 Planning, Analysis and Evaluation May/June 2016 MARK SCHEME Maximum Mark: 30 Published This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge will not enter into discussions about these mark schemes. Cambridge is publishing the mark schemes for the May/June 2016 series for most Cambridge IGCSE®, Cambridge International A and AS Level components and some Cambridge O Level components.

Page 2 Mark Scheme Syllabus Paper Cambridge International AS/A Level – May/June 2016 9701 51 © Cambridge International Examinations 2016 Question Expected Answer Mark 1 (a) lithium and water being labelled in an arrangement that shows them coming into contact at some time gas syringe OR collection over water, both using a leak-proof connection to the reaction vessel that would collect gas after the reagents have been mixed a valid separation of the two reagents [1] [1] [1] (b) (i) cut (a small piece) AND remove oil [1] (ii) no change / increase in volume OR bubbles (in collector) cease (over water) OR no more gas / hydrogen collected [1] (iii) safety precautions – in a Li / LiOH / H2 context Any two from • avoid skin contact / wear gloves / lab coat / use tongs in context of prevention of burns or corrosive contact only • keep piece of lithium / storage vessel / apparatus away from water • ensure unused lithium is all returned to storage vessel or stored under oil • keep away from naked flames / burner / sources of ignition [2] (iv) (0.1 / 7 × 1 / 2 × 24 000) = 171 cm3 correct unit MUST be present [1] (v) use 200 cm3 OR 250 cm3 OR 500 cm3 - the size must be reasonable and consistent with the volume in b(iv). correct unit MUST be present [1] (c) (i) use a burette twice or 50 cm3 pipette twice [1] (ii) lithium / solid has all reacted / disappears / dissolves [1]

Page 3 Mark Scheme Syllabus Paper Cambridge International AS/A Level – May/June 2016 9701 51 © Cambridge International Examinations 2016 Question Expected Answer Mark (iii) pipette / burette / syringe / graduated pipette [1] (iv) repeat the titration until results are within 0.1 cm3 [1] (d) beaker : Effect: (Ar) (appears) more / larger AND Reason: (LiOH) solution more dilute (than expected) Flask: Effect: (Ar) None AND Reason: moles of (LiOH) (put in conical flask) remains the same / volume (i.e. moles) of LiOH not altered [1] [1] [15]

Page 4 Mark Scheme Syllabus Paper Cambridge International AS/A Level – May/June 2016 9701 51 © Cambridge International Examinations 2016 Question Expected Answer Mark 2 (a) 1 / T / K–1 log10 (1 / t) 3.47 × 10–3 –1.92 3.41 × 10–3 –1.76 3.33 × 10–3 –1.56 3.30 × 10–3 –1.45 3.26 × 10–3 –1.25 or –1.26 3.22 × 10–3 –1.28 3.19 × 10–3 –1.18 3.16 × 10–3 –1.08 3.12 × 10–3 –0.95 3.05 × 10–3 –0.90 Column values for 1 / T correctly calculated Column values for log10(1 / t) correctly calculated 3sf in 1 / T AND 2 dp in log10(1 / t) [1] [1] [1] (b) candidate’s points plotted correctly from table in 2(a) line of best fit drawn [1] [1]

Page 5 Mark Scheme Syllabus Paper Cambridge International AS/A Level – May/June 2016 9701 51 © Cambridge International Examinations 2016 Question Expected Answer Mark (c) Two anomalies identified Reasons: Points to the left of the line: the time of disappearance was thought to be later OR the time was stopped too late (after reaction ended) OR the (hydrochloric acid) solution had not reached the temperature of the water bath OR the timer was started early OR magnesium folded up (reduced surface area) Points to the right of the line: the Mg may have been thought to have disappeared earlier than it did OR the timer was started late OR the timer was stopped too early (reaction still going) [1] (d) (i) two co-ordinates in correct x, y format gradient calculated correctly from candidate’s stated co-ordinates (the value MUST be negative unless the graph is mis-plotted) value MUST be to 3 significant figures Expected range –2500 to –3500 [1] [1] (ii) –EA = gradient × 0.0191 OR –EA = gradient × 0.0191 then divide by 1000 OR correct transformations correct calculation and sign from candidate’s gradient, gradient may be in calculation form, minimum 2 significant figures [1] [1]

Page 6 Mark Scheme Syllabus Paper Cambridge International AS/A Level – May/June 2016 9701 51 © Cambridge International Examinations 2016 Question Expected Answer Mark (e) valid answer dependent on candidate’s graph, e.g. reliable because most of the points on / close to the line OR unreliable as most points not on the line [1] (f) Student X is correct; reaction time less OR reaction is faster AND percentage error / uncertainty will be greater OR greater error / uncertainty in time / data / recordings [1] (g) reaction time is longer / rate slower AND some of the magnesium is not in contact (with the acid) OR less surface area for reaction (with HCl) OR only the bottom of the magnesium is reacting [1] (h) initial rate lower / slower. AND the concentration of H+ ions is lower / pH higher OR ethanoic acid less dissociated / weaker acid [1] [15]