AS Paper 1 Practical Questions

Transcription

1 AS Paper Practical Questions MARK SCHEME South Axholme School Amount of Substance M.(a) Stage : appreciation that the acid must be in excess and calculation of amount of solid that permits this Statement that there must be an excess of acid Moles of acid = / 000 = mol 2 mol of acid react with mol of calcium hydroxide therefore moles of solid weighed out must be less than half the moles of acid = = mol Mass of solid must be = Stage 2: Experimental method Measure out 50 cm 3 of acid using a pipette and add the weighed amount of solid in a conical flask Titrate against 0.00 (or 0.200) mol dm 3 NaOH added from a burette and record the volume (v) when an added indicator changes colour Stage 3: How to calculate M r from the experimental data Moles of calcium hydroxide = (v/2 conc NaOH) / 000 = z mol M r = mass of solid / z Extended response Maximum of 7 marks for answers which do not show a sustained line of reasoning which is coherent, relevant, substantiated and logically structured. Page

2 (b) Moles of calcium chloride = 3.56 /. = Moles of calcium sulfate = / 00 = Mass of calcium sulfate = = = 3.64 (g) Answer must be to 3 significant figures [] M2.(a) Space will fill during titration / titres or volumes added are too high Do not allow to improve accuracy without qualification. Do not allow incorrect end-point without qualification. Do not allow titres or volumes added are too low. Ignore titres or volumes added are different. (b) Less chance of losing liquid on swirling / liquid doesn t splash on swirling Do not accept easier to swirl on its own. (c) (i) Returns reagent on the sides of the flask to the reaction mixture (to ensure that all of the acid / alkali reacts) Do not allow to improve accuracy without qualification. Ignore reference to cleaning. (ii) This does not change the number of moles of reagents / water is not a reagent / water is one of the products Do not allow water does not affect the titration without qualification. Ignore water is neutral / has a ph of 7. Page 2

3 (d) Idea that a single titration could be flawed / anomalous Do not accept will improve reliability / reproducibility / accuracy without further qualification. Allow to obtain concordant results. South Axholme School [5] M3.Divides percentage by price Ratios are.668,.70 and.437 Dub-Lit Brick Cleaner is the best value Allow if divides price by percentage (ratios are 0.600, and 0.696). Lose mark if no working shown or contains an arithmetic error. [] M4.(a) (i) Uses sensible scales Lose this mark if the plotted points do not cover half of the paper. Lose this mark if the graph plot goes off the squared paper. Plots all of the points correctly ± one small square Draws a best-fit line Lose this mark if the student s line includes either of the points at 0.5 / 0.48 or 3.5 / 2.28 Lose this mark if the student s line is doubled or kinked. Lose this mark if the student s line does not pass within one small square of the origin, extending the line if necessary. (ii).38 to.47 Allow answer in this range only. Answer must correspond to value from the student s graph. (iii) M Page 3

4 Moles NaOH =.00 / 40 and Moles water = (part(b) ) / 8 Allow any correct method which uses the answer from part (b). South Axholme School M2 Ratio NaOH : H 2O is close to : Must compare experimental result with theoretical result to score M2. (b) (i) 2NaOH + 2Al + 6H 2O 2NaAl(OH) 4 + 3H 2 Ignore state symbols. (ii) Pressure build-up due to the production of hydrogen / H 2 / gas Ignore references to the flammability / explosive nature of hydrogen. (c) (Alkali is) corrosive / caustic Allow (alkali) burns skin. Ignore harmful, dangerous. Do not allow toxic or irritant. [9] M5.(a) To make sure all the solutions (from both the burette and pipette) react with each other / are in the flask Penalise solid or residue. Do not allow any suggestion of removal of species. (b) Water does not change the number of moles of either reagent / reactants Water is not a reagent / does not react with either reactant. Do not allow water is not involved in the reaction. Apply list principle. [2] Page 4

5 M6.(a) Initial mass must be the y-axis If axis unlabelled, use data to decide that Initial mass is on the y-axis. Sensible scale Do not award this mark if plotted points do not cover at least half of the grid. Do not award this mark if any plotted point is outside the grid. All points plotted correctly Allow ± one small square. Point at (0,0) is ringed (b) Best-fit straight line that goes through the origin ± ½ small square Mark consequentially to plotted points but the line must still go through the origin ± ½ small square. Lose this mark if the line is doubled or kinked. If the points are plotted correctly, lose this mark if the line deviates towards the anomalies. (c) Students 3 and 5 Allow masses of.5 and.53 or 2.82 and 3.58 Mark consequentially to plot. (d) Samples 3 or 5 have not lost all their water Allow reaction / decomposition incomplete. Sample not heated for enough time / larger masses will take a longer time to dehydrate / decompose Page 5

6 [8] M7.(a) (CO 2 from) burning (fossil) fuels (b) NaCl + CO 2 + NH 3 + H 2O NaHCO 3 + NH 4Cl Allow multiples, including fractions. Ignore state symbols. (c) CaO + 2NH 4Cl CaCl 2 + 2NH 3 + H 2O Allow multiples, including fractions. Allow ionic equations. Do not allow equations involving NH 4OH or NH 4 + on the right hand side. Ignore state symbols. (d) (i) = (06) 00 / (7 + 00(.)) Do not penalise precision but must be to minimum of two significant figures. = 48.8 This answer without working scores mark only. (ii) The percentage atom economy cannot be improved OR Sell the by-product / CaCl 2 (solution) Do not accept answers which refer to improving the efficiency of the process. (e) It is used up but then regenerated later in the cycle / No overall consumption of NH 3 Allow can act as a catalyst. Page 6

7 [7] M8.(a) Any three from: A method of weighing by difference / wash the solid from its weighing container into the beaker If the nature of any washing is imprecise penalise once only. Wash the (wet) rod into the flask / beaker after use Do not allow a method where the solution is made up directly in the flask. Wash the (wet) beaker into the flask after transfer Ignore any instructions that refer to rinsing equipment (before use) or use of deionised water. Wash the filter funnel (after transfer) into the flask Use a teat pipette to make up to the mark on the volumetric flask Ensure the bottom of the (liquid) meniscus is on the graduation mark Mix / shake the final solution in the flask / invert flask Max 3 (b) Do (a) further titration(s) Mark these points independently. To obtain concordant results Allow results with ± 0. [5] M9.(a) (Returns) reagent on the sides of the flask to the reaction mixture (to ensure that all of the acid / alkali reacts) Do not allow to get a better result without qualification. Do not allow to ensure that all of the acid / alkali reacts without qualification. Page 7

8 (b) Water is not a reagent / water one of the products / does not change the number of moles of reagents Do not allow water does not affect the titration without qualification. South Axholme School [2] M0.(a) (i) Mass loss would be too large / water would be lost when heating (so mass incorrect) Do not allow to improve accuracy without qualification. Do not allow water is a product of the reaction. Do not allow mass of crucible incorrect / too high. (ii) Prevents loss of solid / potassium carbonate / potassium hydrogencarbonate (from the crucible) Do not allow to improve accuracy without qualification. Do not allow stops anything escaping. (iii) Errors in weighing are too high / percentage errors in (obtaining) the mass are too high Do not allow hard to / can t weigh very small amounts without further qualification. (b) (i) M r of KHCO 3 is 00(.) Do not penalise precision. moles K 2CO 3 = / (M r 2) = If factor of 2 missing can only score first mark (M). Allow consequential answer on incorrect M r of KHCO 3 mass of K 2CO 3 = = 0.69(0) Correct mass without working scores one mark only (M3). (ii) Reaction / decomposition Page 8 incomplete

9 Do not allow a wet crucible was used. Ignore references to impurity and / or experimental errors. South Axholme School [7] M.(a) (i) Volume of crater-lake solution on x-axis Do not penalise missing axes labels. If axes unlabelled use data to decide. Lose this mark if axes mis-labelled. Sensible scales Lose this mark if plotted points do not cover at least half the paper or plot goes off the squared paper. All points plotted correctly +/ one square (ii) Draws appropriate line of best fit, omitting point at 20 cm 3 / 5 cm 3 Lose this mark if the line deviated towards the anomalous result. Lose this mark if the candidate s line is doubled or kinked. Candidate does not have to extrapolate to the origin. (iii) 6.5 cm 3 +/ 0.5 cm 3 Accept this answer only. Do not mark consequentially on candidate s graph. (iv) Value corresponding to 0 cm 3 crater-lake solution / 6.00 cm 3 Must have correct identity for explanation mark. Accept results aren t concordant. Greatest % error from use of burette Accept difficult to be accurate with small volumes (owtte). Page 9

10 (b) (i) pv = nrt Accept any correct rearrangement. Ignore case. (ii) V = or n = ( ) / ( ) Mark consequentially on candidate s volume. n = (mol) Correct answer without working scores one mark only. Allow consequential mark using incorrect conversion. Incorrect units lose this mark. (iii) M r CaCO 3 = 00. (M) Accept 00 (can score this mark in calculation for M2 and M3). Moles CaCO 3 = ( ) = (M2) Do not penalise lack of units. Allow b(ii) 0 Allow Mass CaCO 3 = M M2 (= 3.27 g) Correct mass without working scores one mark only. Allow = 2.5 g (iv) (3.27 / 95) 00 Accept (b(iii) / 95) 00. Do not penalise precision. Page 0

11 3.44 g Do not penalise lack of units. Using 2.5 g gives 3.2 g Correct answer without working scores 2 marks. (v) Abundant / readily available Accept not caustic or alkaline. Non-corrosive Accept insoluble so safe to add in excess (owtte). [7] M2.Include washings or words to that effect / mix contents Accept use distilled / deionised water. Allow weigh directly into flask if washing included. [] M3.(a) 2-6 drops / cm 3 Accept a few drops (b) Incorrect volume recorded / space will fill during titration / produces larger titre value Do not accept to give an accurate result without further qualification Do not accept references to contamination [2] M4.(a) Mass of mineral on x-axis; If axes unlabelled use data to decide if mass of mineral is on the x-axis. Page

12 Sensible continuous scales; Lose this mark if the plotted points do not cover at least 9 squares by 7. Lose this mark if the graph plot goes off the squared paper. The graph does not have to start at the origin. Plots points correctly ± one square; Award this mark if the line is close to your line. Draws a best fit straight line Award this mark if best fit line is consistent with candidate s plotted points. Lose this mark if line is kinked or doubled. (b).48 or.49 or.50 or.5 (g); Accept these answers only Ignore precision of answer. Allow range.48.5 (c) (mol); Accept 0.02, Allow answer without working. (d) (.49 / 0.024) = ; Must divide answer to part (b) by answer to part (c) to score first mark. Allow consequential answer from part (b). Allow answer without working. Ignore precision of answer. (e) Answer to part (e) close to 20.3; Allow consequential answer from part (d). Allow correct calculation of x Page 2

13 (f) x must be a whole number; South Axholme School (g) Good / straight line so results good / reliable; Allow consequential answers from candidate s graph Do not allow so results are accurate. Anomaly at.34 g; Allow anomaly clearly indicated on the graph. (h) Ensure reaction / decomposition goes to completion; Do not allow to make fair test or improve reliability Accept to remove all carbon dioxide and water. (i) (i) Percentage errors too high / errors in weighing too high; Do not allow to make fair test or improve reliability Do not allow errors on its own. (ii) Incomplete decomposition or words to that effect; Do not allow to make fair test or improve reliability Do not allow takes too long or wastes chemicals Do not allow not all of the water removed. (j) / 8 = 2.70 and / 84.3 = 0.723; Allow M r of MgCO 3.H 2O = 38.3 MgCO 3.3H 2O; 54 / % MgCO 3.3H 2O without working scores mark. (k) Atom economy for Reaction is (40.3 / 84.3) x 00 = 47.8% Maximum mark if no working. Page 3

14 Ignore precision of answers. South Axholme School Atom economy for Reaction 2 is (40.3 / 58.3) x 00 = 69.% (l) No gas produced in stomach / won t cause wind; Do not allow gas produced on its own. [9] Bonding Currently no practical questions Energetics M.(a) Start a clock when KCl is added to water Record the temperature every subsequent minute for about 5 minutes Allow record the temperature at regular time intervals untilsome time after all the solid has dissolved for M2 Plot a graph of temperature vs time Extrapolate back to time of mixing = 0 and determine the temperature (b) Heat taken in = m c ΔT = = 28.6 J Max 2 if 4.6 C used as ΔT Moles of KCl = 5.00 / 74.6 = Page 4

15 Enthalpy change per mole = / = J mol - = +6.8 (kj mol - ) Answer must be given to this precision (c) ΔH solution = ΔH lattice + ΔH(hydration of calcium ions) + 2 ΔH(hydration of chloride ions) ΔH lattice = ΔH solution ΔH(hydration of calcium ions) 2 ΔH(hydration of chloride ions) ΔH lattice = 82 9 ( ) = (kj mol ) (d) Magnesium ion is smaller than the calcium ion Therefore, it attracts the chloride ion more strongly / stronger ionic bonding [2] M2.(a) Temperature on y-axis If axes unlabelled use data to decide that temperature is on y-axis. Uses sensible scales Lose this mark if the plotted points do not cover half of the paper. Lose this mark if the temperature axis starts at 0 C. Plots all of the points correctly ± one square Lose this mark if the graph plot goes off the squared paper. Page 5

16 Draws two best-fit lines Candidate must draw two correct lines. Lose this mark if the candidate s line is doubled or kinked. South Axholme School Both extrapolations are correct to the 4 th minute Award this mark if the candidate s extrapolations are within one square of your extrapolations of the candidate s best-fit lines at the 4 th minute. (b) 9.5 ( C) Accept this answer only. (c) 26.5 ± 0.2 ( C) Do not penalise precision. (d) (c) (b) Only award this mark if temperature rise is recorded to d.p. (e) Uses mcδt equation Allow use of this equation with symbols or values for M even if the mass is wrong. Correct value using (d) 7.0 gives 732 J. Correct answer with no working scores one mark only. Do not penalise precision. Allow answer in J or kj. Ignore sign of enthalpy change. (f) 9.0() 0 3 Do not allow 0.0 Allow or in this case. Page 6

17 (g) If answer to (e) in J, then (e) / (000 (f)) or If answer to (e) in kj, then (e) / (f) 7.0 and gives 8.2 kj mol If answer to (e) is in J must convert to kj mol correctly to score mark. Enthalpy change has negative sign Award this mark independently, whatever the calculated value of the enthalpy change. (h) The idea that this ensures that all of the solution is at the same temperature Do not allow to get an accurate reading without qualification. (i) (i) Chlorine is toxic / poisonous / corrosive Do not allow harmful. (ii) Explosion risk / apparatus will fly apart / stopper will come out Ignore gas can t escape or gas can t enter the tube. [6] Kc and Equilbria M.(a) = (0.0745) (0.083) / (0.0424) (0.0525) = 2.72 Allow answer only without working if correct. Lose this mark if the wrong K c expression is used. Answer, whether or not correct, given to three significant figs Do not expect conversion from moles to concentration but allow if shown. Page 7

18 (b) Less acid is present (so less NaOH needed) South Axholme School Equilibrium would shift to right (side with more ester / less acid) [4] M2.(a) K c / K a / equilibrium constant / constant is temperature dependent Do not allow affects or shifts equilibrium. (b) Thermostat / water bath [2] Atomic Structure Currently no practical questions. Periodicity Currently no practical questions. Redox, Group 2 and Group 7 M.(a) (i) SrCl 2(aq) + Na 2SO 4(aq) SrSO 4(s) + 2NaCl(aq) Allow multiples, including fractions. Allow ionic equations. Lose this mark if any of the state symbols are missing or incorrect. (ii) Add nitric acid to the mixture (until in excess) Do not allow any suggestion that the solution is an emetic. Filter (to isolate strontium sulfate) (b) Insoluble barium sulfate is formed Allow removes barium ions as a precipitate. Page 8

19 (c) Add silver nitrate, then dilute ammonia (solution) M Do not allow answers which imply silver nitrate and ammonia are added at the same time. Allow add silver nitrate, then concentrated ammonia (solution). Can score M in the answer for M3 Cream precipitate M2 Allow off white precipitate. No visible change or precipitate dissolves slightly in dilute ammonia M3 Allow soluble / colourless solution / precipitate dissolves in concentrated ammonia. Allow 3 marks for: Add dilute ammonia (solution), then silver nitrate M No visible change M2 Cream / off white precipitate with silver nitrate M3 [7] M2.(a) (i) Do not penalise precision but must be to at least 2 significant figures. Do not accept 0 2 (ii) 5.4(0) 0 3 Allow (i) / 2 Do not penalise precision but must be to at least 2 significant figures. (iii) Lose this mark if answer not given to decimal place. Page 9

20 (iv) mass = 5.4(0) =.44 g M Allow (ii) (iii). South Axholme School percentage = / 2.25 = 64.0 M2 Allow consequential answer from M Lose this mark if answer not given to 3 significant figures. Correct answer with no working scores M2 only. (v) Would give an incorrect / too large mass (of silver chloride) Do not allow to get an accurate result without qualification. 2 To remove soluble impurities / excess silver nitrate (solution) / strontium nitrate (solution) Do not allow to remove impurities. Do not allow to remove excess strontium chloride solution. (b) (i) Mg 2+ (aq) + 2OH - (aq) Mg(OH) 2(s) Allow Mg 2+ (aq) + 2OH - (aq) Mg 2+ (OH - ) 2(s) Allow multiples, including fractions. Lose mark if state symbols are missing or incorrect. Lose mark if incorrect charge on an ion. (ii) Does not produce CO 2 / gas which distends stomach / does not produce wind / does not increase pressure in stomach Allow prevents flatulence and prevents burping. Do not allow gas without qualification. (c) (CH 3COO) 2Ca CH 3COCH 3 + CaCO 3 Allow multiples. Allow propanone as C 3H 6O Allow (CH 3COO - ) 2Ca 2+ CH 3COCH 3 + Ca 2+ CO 3 2- Page 20

21 (d) Ca (salt) - no visible change with sodium chromate(vi) M Allow yellow solution formed or no ppt. forms. Allow M and M2 in any order. South Axholme School Sr and Ba (salts) give (yellow) precipitate with sodium chromate(vi) M2 Lose this mark if precipitate has an incorrect colour. Sr precipitate (chromate(vi)) dissolves in ethanoic acid / Ba precipitate (chromate(vi)) does not dissolve in ethanoic acid M3 If ethanoic acid is added first, allow access to M and M3. (e) C / 2, H 2.92 /, N 8.8 / 4, O / 6 and S 9.39 / 32. Accept any other correct method of working. If relative atomic mass has been divided by the percentage composition is used then CE = 0 / 2 C 2H 0N 2O 8S Correct answer with no working scores mark only. [5] M3.Mg 2+ and Cl Do not allow names. [] M4. (a) (i) M (yellow precipitate is) silver iodide OR AgI (which may be awarded from the equation) M2 Ag + + I AgI (Also scores M unless contradicted) M3 sodium chloride OR NaCl For M2 Accept multiples Ignore state symbols Page 2

22 Allow crossed out nitrate ions, but penalise if not crossed out South Axholme School 3 (ii) The silver nitrate is acidified to react with / remove ions that would interfere with the test prevent the formation of other silver precipitates / insoluble silver compounds that would interfere with the test remove (other) ions that react with the silver nitrate react with / remove carbonate / hydroxide / sulfite (ions) Ignore reference to false positive (iii) M and M2 in either order M Fluoride (ion) OR F M2 Silver fluoride / AgF is soluble / dissolves (in water) no precipitate would form / no visible /observable change Do not penalise the spelling fluoride, Penalise fluride once only Mark M and M2 independently 2 (b) M Ba 2+ + SO 4 2- BaSO4 (or the ions together) M2 white precipitate / white solid / white suspension M3 Barium meal or ( internal ) X-ray or to block X-rays M4 BaSO 4 / barium sulfate is insoluble (and therefore not toxic) For M, ignore state symbols Allow crossed out sodium ions, but penalise if not crossed out For M2, ignore milky If BaSO 3 OR BaS used in M and M4, penalise once only For M3 Ignore radio-tracing For M4 NOT barium ions NOT barium NOT barium meal NOT It unless clearly BaSO 4 4 Page 22

23 (c) M 2( ) + 4(.00794) = M2 Ethene and CO or they have an imprecise M r of 28.0 / 28 OR South Axholme School Ethene and CO or they have the same M r to one d.p. OR These may be shown by two clear, simple sums identifying both compounds M3 C 2H 4 + 2O 2 2CO + 2H 2O (H 2C=CH 2) M4 Displayed formula M5 Type of polymer = Addition (polymer) M must show working using 5 d.p.for hydrogen Penalise similar or close to, if this refers to the imprecise value in M2, since this does not mean the same For M3, accept CH 2=CH 2 OR CH 2CH 2 For M4, all bonds must be drawn out including those on either side of the unit. Penalise sticks Ignore brackets around correct repeating unit but penalise n Penalise additional 5 [5] M5.(a) Hydrochloric acid = C Page 23

24 Barium chloride = A (b) Barium sulfate is insoluble CuSO 4 + BaCl 2 BaSO 4 + CuCl 2 Accept multiples. Accept ionic equation. Do not penalise lack of state symbols, but if used they must be correct. (c) CO 2 / Carbon dioxide (d) Reagent silver nitrate (solution) Ignore lack of reference to acidifying prior to addition of silver nitrate solution. Observation White precipitate Reagent 2 (dilute) ammonia solution / aqueous ammonia Do not accept addition of ammonia only. Observation 2 (Colourless) solution Allow ppt dissolves. Do not allow goes colourless or goes clear. Chlorine and no visible change or solution does not become orange scores M3 and M4. Page 24

25 (e) Gloves / wash hands after use Ignore eye protection. Do not accept do not ingest the chemicals, wipe up spillages, use a fume cupboard, wear a lab coat (list principle). [0] M6.(a) (i) Blue to green Accept blue to yellow. (ii) Decrease / less acid needed Ignore references to rate (iii) Gloves or avoid skin contact Allow if reagent contacts skin wash off (immediately) or answers to that effect. Do not accept wash only. Ignore eye protection or lab coat or use of fume cupboard or don t ingest. (iv) Less chance of losing liquid on swirling / liquid doesn t splash on swirling Do not accept easier to swirl on its own. Do not accept easier to stir. (v) Idea that a single titration could be flawed / anomalous Allow an indication that the first titration is a rough titration. Do not allow to improve accuracy without qualification. Do not allow vague references to outliers. (b) (i) 2.3(3) 0 2 Do not penalise additional significant figures, but do not allow 0.02 Page 25

26 (ii) Dilution of acid needed / may react with carbon dioxide in air Accept poor end-point or no suitable indicator or a large volume (of calcium hydroxide) will be needed. Ignore references to low solubility or concentration too low. [7] M7.(a) Remove undissolved barium hydroxide / excess solid Do not accept remove impurities. (b) Filtration Do not accept decanting or sieving. Ignore references to heating or drying. (c) Remove (excess) sulfuric acid (d) Ba(OH) 2 + H 2SO 4 BaSO 4 + 2H 2O Accept multiples. Accept Ba 2+ + SO 4 2- BaSO 4 Ignore state symbols. (e) (i) Accept 233 (ii) 0.08(2) Do not penalise additional significant figures, but do not allow 0.02 Allow consequential answer from (i). (iii) 0.08(2) 7.3 = 3.2 Do not penalise precision. If 0.08 used, answer = 3.08 Page 26

27 0 = 3.2 Do not penalise precision. Allow this mark if 0.8(2) used directly. Correct answer without working scores one mark only. Allow consequential answer on (ii) (f) Barium sulfate / it is insoluble Do not accept answers based on small amount ingested. Do not accept barium. [9] M8.(a) sulfuric acid / H 2SO 4 (b) hydriodic acid / HI OR hydrobromic acid / HBr (c) add dilute ammonia solution Notes * do not allow concentrated ammonia or ammonia precipitate / ppt disappears / dissolves OR colourless solution forms (d) would react with the acid / no gas evolved in tests [5] M9.Add (hydrochloric) acid to the mixture; acid. Allow correct acid eg nitric Page 27

28 Filter to isolate strontium sulphate; Do not allow drain or decant [2] M0.(a) 2NaBr + 2H 2SO 4 Na 2SO 4 + Br 2 + SO 2 + 2H 2O Allow ionic equation 2Br + 2H 2SO 4 Br 2 + SO SO 2 + 2H 2O Br ions are bigger than Cl ions Therefore Br ions more easily oxidised / lose an electron more easily (than Cl ions) (b) This question is marked using levels of response. Refer to the Mark Scheme Instructions for Examiners for guidance on how to mark this question. Level 3 All stages are covered and the explanation of each stage is generally correct and virtually complete. Stages and 2 are supported by correct equations. Answer communicates the whole process coherently and shows a logical progression from stage to stage 2 and then stage 3. The steps in stage 3 are in a logical order. 5 6 marks Level 2 All stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies OR two stages are covered and the explanations are generally correct and virtually complete. Answer is mainly coherent and shows a progression through the stages. Some steps in each stage may be out of order and incomplete. 3 4 marks Page 28

29 Level South Axholme School Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies, OR only one stage is covered but the explanation is generally correct and virtually complete. Answer includes some isolated statements, but these are not presented in a logical order or show confused reasoning. 2 marks Level 0 Insufficient correct chemistry to warrant a mark. Indicative chemistry content Stage : formation of precipitates Add silver nitrate to form precipitates of AgCl and AgBr AgNO 3 + NaCl AgCl + NaNO 3 AgNO 3 + NaBr AgBr + NaNO 3 0 marks Stage 2: selective dissolving of AgCl Add excess of dilute ammonia to the mixture of precipitates the silver chloride precipitate dissolves AgCl + 2NH 3 Ag(NH 3) Cl Stage 3: separation and purification of AgBr Filter off the remaining silver bromide precipitate Wash to remove soluble compounds Dry to remove water 6 (c) Cl 2 + 2HO OCl + Cl + H 2O OCl is + Cl is Both required for the mark [] M. (a) (i) M iodine OR I 2 OR I 3 Page 29

30 Ignore state symbols Credit M for iodine solution M2 Cl 2 + 2I 2Cl + I 2 OR ½ Cl 2 + I Cl + ½ I 2 Penalise multiples in M2 except those shown M2 accept correct use of I 3 M3 redox or reduction-oxidation or displacement 3 (ii) M (the white precipitate is) silver chloride M must be named and for this mark ignore incorrect formula M2 Ag + + Cl AgCl For M2 ignore state symbols Penalise multiples M3 OR (white) precipitate / it dissolves colourless solution Ignore references to clear alone 3 (b) (i) M H 2SO 4 + 2Cl 2HCl + SO 4 2 For M ignore state symbols OR H 2SO 4 + Cl HCl + HSO 4 Penalise multiples for equations and apply the list principle OR H + + Cl HCl M2 hydrogen chloride OR HCl OR hydrochloric acid 2 (ii) M and M2 in either order For M and M2, ignore state symbols and credit multiples M 2I I 2 + 2e OR 8I 4I 2 + 8e Do not penalise absence of charge on the electron Credit electrons shown correctly on the other side of each equation M2 H 2SO 4 + 8H + + 8e H 2S + 4H 2O Page 30

31 OR South Axholme School SO H+ + 8e H 2S + 4H 2O Additional equations should not contradict M3 oxidising agent / oxidises the iodide (ions) OR electron acceptor M4 sulfur OR S OR S 2 OR S 8 OR sulphur 4 (iii) M The NaOH / OH / (sodium) hydroxide reacts with / neutralises the H + / acid / HBr (lowering its concentration) OR a correct neutralisation equation for H + or HBr with NaOH or with hydroxide ion Ignore reference to NaOH reacting with bromide ions Ignore reference to NaOH reacting with HBrO alone M2 Requires a correct statement for M The (position of) equilibrium moves / shifts(from L to R) to replace the H + / acid / HBr that has been removed / lost OR to increase the H + / acid / HBr concentration OR to make more H + / acid / HBr / product(s) OR to oppose the loss of H + / loss of product(s) OR to oppose the decrease in concentration of product(s) In M2, answers must refer to the (position of) equilibrium shifts / moves and is not enough to state simply that it / the system / the reaction shifts to oppose the change. M3 The (health) benefit outweighs the risk or wtte OR a clear statement that once it has done its job, little of it remains OR used in (very) dilute concentrations / small amounts / low doses 3 [5] Page 3

32 M2. (a) NaBr ONLY Penalise incorrect case or additional formulae. Ignore names South Axholme School (b) NaF ONLY Penalise incorrect case or additional formulae. Ignore names (c) NaF ONLY one from either OR NaCl Penalise incorrect case or additional formulae. Ignore names (d) NaI ONLY Penalise incorrect case or additional formulae. Ignore names [4] M3.(a) Increase Van der Waal s forces between molecules Increase with size (or M r or surface area etc) More energy needed to break (overcome) these forces (Note max 2 from last three marks if no mention of molecules or molecular ) (b) (i) Brown solution (or yellow or orange) Page 32

33 Cl 2 + 2Br 2C + Br 2 (ii) cream precipitate Br + Ag + AgBr Precipitate dissolves (iii) orange (brown) fumes (gas), White fumes (or misty fumes), choking gas (any 2) 2 (c) 2H + + H 2SO 4 + 2Br SO 2 + Br 2 + 2H 2O (SO 2 and Br 2 (), equation ()) 2 [3] Page 33