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Barnard Castle School Chemistry Department iGCSE Chemistry Revision Questions MARK SCHEME 1. (a) He (b) carbon / silicon / germanium / tin / lead accept correctly written symbols C / Si / Ge / Sn / Pb (c) 1 copper 1 1 accept Cu (d) iodine 1 accept I or I2 [4] 2. (a) (b) (i) argon and potassium or tellurium and iodine or cobalt and nickel accept correct symbols allow argon and calcium 1 (ii) it would place them in incorrect groups (owtte) idea of not being placed with elements which have similar properties or converse argument accept would not have same number of electrons in outer shell allow it would put them in wrong period if linked to argon and potassium / calcium do not accept reasons based just on protons do not accept metals and non-metals mixed up 1 any two from: 2 each successive atom has one more electron atoms in the same group have the same number of electrons in their outer shells / energy levels number of electrons in outer shell = group number across a period an energy level / shell is being filled in the next period the next energy level / shell is being filled accept period number = shell number [4] 3. (a) 2 Calcium chloride Lithium sulphate Potassium carbonate Sodium nitrate Add barium chloride solution and dilute hydrochloric acid. A white precipitate formed. Do the flame test. Yellow flame produced. Add silver nitrate solution and dilute nitric acid. A white precipitate formed. Add hydrochloric acid. Carbon dioxide gas given off. all three correct = 2 one or two correct = 1 (b) blue 1 precipitate 1 solid (c) (i) any one from: 1 • mass spectrometry / spectroscopy • atomic (absorption) spectrometry • flame photometry • infra-red spectroscopy • ultra violet spectroscopy • chromatography (any valid method) (ii) any one from: 1 • rapid • sensitive • accurate • small amounts • easily automated • easily computerised (any valid) [6] 4. (a) pipette / burette 1 (b) named indicator eg methyl orange / phenolphthalein not universal accept litmus but not litmus paper 1 (c) 25 0.4 1000 1 2 for correct answer = 0.011 (d) 1KOH 1 HCl 1 0.01 moles HCl in 35 cm3 0.01 1000 = 0.29 35 2 for correct answer 0.3 = (1) (with correct working = (2)) 1 [6] 5. (a) 25 (cm3) 1 (b) strong alkali 1 (c) burette 1 plus any sensible reason: 1 eg ● more accurate ● smaller divisions ● tap allows better control etc [4] 6. (a) water (allow acid*) 3 (metal) hydroxide (solution)/alkali (allow metal salt / compound) hydrogen (allow hydrogen) for 1 mark each (*if named acid then matching salt) or but (b) oxygen / chlorine / halogen for 1 mark oxide / chloride / halide gains 1 mark named oxide(s) gains 2 marks (accept fully correct formulae throughout) idea that hydrogen and iodine would only react if heated to > 200°C / a high temperature (ignore reference to light / dark) reactivity decreases down the Group / iodine is lower in the Group than bromine / iodine is the lowest in the Group 2 (or converse N.B. credit this idea here even if given in (d)) for 1 mark each ½H2 + ½ Cl2 HCl (c) H2 + Cl2 2HCl or for 1 mark (d) idea that (or converse) in atoms lower down the Group the electrons in the outer shell / highest energy level (of electrons) are further from the nucleus or shielded by more (inner) shells / electrons [but not simply “more electrons”] or 1 2 are less strongly attracted to the nucleus so an electron is less easily gained for 1 mark each [8] 7. add aluminium/Devarda’s alloy 1 gas turns litmus blue 1 ammonia evolved 1 (b) any valid method for compounds -infra-red spectroscopy -ultra violet spectroscopy -mass spectroscopy -gas liquid chromatography -high pressure liquid chromatography -any other valid method do not accept abbreviations 1 (c) any valid answer -electronics -computers -fibre optics -magnetometry linked to some idea how it has helped ie, smaller, greater sensitivity, ease of operation/automation, greater versatility. 1 mark for technology, 1 mark for its effect. 2 (a) [6] ( 1 ) 2 D 3 B ( 4 5 8. (a) ) A ( 6 7 ) 4 C or outside boxes or in line in order 1 mark for each of 4 correct (b) the substance has disappeared/no bits/no lumps (the liquid) is clear (the liquid) is the same all the way through any 2 for 1 mark each 2 [6] 9. (a) (i) not fully ionised / dissociated or partially ionised accept weak acids produce fewer H+ if equal concentration or equal amounts of acid specified do not accept just weakly ionised 1 (ii) Test: Universal Indicator / pH indicator/ full range indicator do not accept other indicators 1 Result: e.g. stronger is a darker red or weaker is orange, yellow, paler red etc. or correct pH numbers < 7 accept redder accept pink for paler red do not accept green 1 or pH meter (1) result linked correct pH values (1) or Test: Mg / Zn / Fe / Sn or a carbonate/ hydrogencarbonate (1) Any suitable test but not titration Result: e.g. stronger is quicker / faster rate / more bubbles / more gas / more correctly named gas (1) (b) Quality of Written Communication The answer to this question requires ideas in good English in a sensible order with correct use of scientific terms. Quality of written communication should be considered in crediting points in the mark scheme. maximum 2 marks if ideas not well expressed acids produce hydrogen ions 1 acids are proton donors 1 Link: hydrogen ions (Arrhenius) and protons (Bronsted-Lowry) 1 [6] 10. (a) yellow / yellow orange orange 1 (b) (i) zinc carbonate 1 (ii) copper bromide 1 (iii) magnesium sulphate 1 (c) (white) precipitate / solid do not accept cloudy or milky do not accept residue green precipitate = 0 1 [5] 11. lower water heating costs 1 reduce soap used / no scum not no blocked showers 1 less maintenance / use of plumber / don’t need to buy new heater or shower / no descaling needed (pipes or showers) / no cleaning of blocked pipes 1 [3] 12. (a) light or caloric or beat (b) (i) any one from 1 max 1 lime magnesia barytes argilla silex accept chemical names or correct formula (ii) (c) no methods available at that time to split the compounds into elements accept unable to split up (the substance) any two from 1 max 2 (Mendeleev’s table.) contains only elements divides metals and non-metals contains far more elements which were discovered later groups elements according to properties puts elements in order of atomic number or atomic mass or table includes these numbers can use it to work out or is linked to electronic structures left gaps for missing elements which had not been discovered [5] 13. (a) (b) (i) calcium 1 (ii) oxygen 1 (iii) nitrate 1 Quality of written communication for correct linking of ideas 1 answers based on evaporation e.g. sun evaporates water 1 answers based on condensation to form water or rain e.g. clouds rise and cool starting point of description does not have to start with evaporation 1 (c) (i) nitrate at 34 mg/l on bar chart 1 (ii) (dissolve) carbon dioxide / CO2 1 pressure 1 [9] 14.(a) ideas that hydrogen is in a group / is with the halogens only seven groups / no group O / no noble gases / fewer elements halogens are in the first group / Group 1 other elements are in one group higher (or example) modern table only has two elements in the top row / period modern table not in order of atomic weight/mass metals and non-metals not at opposite ends 3 (NB allow converse answers throughout) any three for 1 mark each (b) ideas that all rows / periods are the same length / have seven elements all elements had to be in one of the groups he didn’t know about the noble gases / not all the elements had been discovered he didn’t know about atomic/proton number/electron structure he arranged elements in order of atomic weight/mass any one for 1 mark 1 (c) (i) ideas that tellurium and iodine are in reverse order for 1 mark 1 (ii) elements are arranged in order of proton (atomic) number or based on electron structure/outer shell electrons (so tellurium is correctly placed before iodine) [tellurium = ‘dead mark’] each for 1 mark 2 [7] 15. (a) (i) liquid 1 allow l (ii) solid 1 allow s allow responses written in table (b) fluorine 1 accept F, reject fluoride (c) hydrogen bromide reject hydrogen bromine accept HBr/BrH 1 allow responses written in table [4] 16. (i) Bonds broken 4 × (C - H) 2 × (O = O) each for 1 mark 4 Bonds formed 2 × (C = O) 4 × (O - H) each for 1 mark (ii) Total energy change in breaking bonds (4 × 413) + (2 × 498) each gains 1 mark 4 Total energy change in forming bonds (2 × 805) + (4 × 464) but to break bonds = 2648 to form bonds = 3466 each gains 2 marks (iii) nett energy transfer = 818 (kj) this energy is released in the reaction/is an exothermic reaction (credit answers consistent with (ii) or derived from the initial information) each for 1 mark 2 [10] 17. (a) (b) acidic 1 reacts 1 hard 1 tastes 1 (i) scum / no lather / precipitate 1 (ii) rain water is soft / pure / nothing dissolved / no limestone not distilled 1 (iii) lake water is hard / not soft / contains calcium compounds / dissolved limestone not impurities 1 (iv) softened by boiling / loses hardness 1 temporary hardness removed / decomposition of calcium salts 1 [9] 18. (a) 2.8.2. magnesium or 2.8.8.2. calcium inner shell (2) outer shell (2) intermediate shell/s correct element named to match structure each for 1 mark (Structure correct for element name but not in group 2,award 2 marks) 4 (b) Similar hydrogen and/or an alkali/hydroxide produced for 1 mark 5 Reasons: chemical reaction involves loss of an electron Na + Li have the same number of electrons in the outer shell (do not allow same group of p.table) each for 1 mark Different rate of reaction faster for sodium* for 1 mark Reason: outer electron more easily lost from the sodium atom [* allow sodium hydroxide produced] for 1 mark [9] 19. any series of chemical tests that work should be given credit identifying all four substances unambiguously with no errors gains 6 marks eg flame test: yellow / orange means sodium ions so sodium sulfate ignore incorrect flame test colours for other compounds 1 add NaOH to remaining three samples 1 no (white) ppt / ammonia means ammonium ion so ammonium sulfate (damp red) litmus* goes blue means ammonium sulfate * or UI / pH indicator goes blue / purple 1 1 (white) ppt means magnesium ions / magnesium ions 1 add excess NaOH to the two samples which gave a (white) ppt: 1 ppt dissolves then aluminium sulfate ppt insoluble means magnesium sulfate or add NaOH: no ppt: ammonia produced means ammonium sulfate the other one with no ppt is sodium sulfate add excess NaOH to the two samples which gave the white ppt ppt dissolves then aluminium sulfate ppt insoluble then magnesium sulfate [6] 20. (a) sodium carbonate / sodium hydrogencarbonate / sodium bicarbonate Na2CO3 / NaHCO3 ie sodium / sodium ions (1 mark) carbonate / carbonate ions (1 mark) incorrect formula including Na and CO3 = 1 mark 2 (b) calcium chloride CaCl2 2 ie calcium / calcium ions (1 mark) chloride / chloride ions (1 mark) incorrect formula including Ca and Cl = 1 mark (c) iron or iron(II) ions Fe2+ ferrous ions ignore anions ignore nickel / chromium do not accept iron(III) or ferric ions5 1 [5] 21. (a) (i) to remove solids 1 (ii) to kill germs / bacteria / microbes etc 1 (i) any two from: 2 (ii) ● check for dissolved minerals ● check for microbes ● which could be harmful to health any two sensible answers such as: ● monitor continuously ● record results ● immediate graphical display ● take samples automatically 2 etc (c) cost of fuel / heat etc 1 (d) (i) calcium / magnesium accept symbols 1 (ii) produces scale (owtte) 1 reduces efficiency / blocks pipes etc 1 (iii) any sensible reason eg good for health 1 [11] 22. ideas that x = the energy required / taken in / used* to break the bonds of water / reactant [*not used up / formed] gains 1 mark but = the energy required taken in / used to break the bonds in water or activation energy gains 2 marks y = the energy released given out when bonds form gains 1 mark but = the energy released / given out when hydrogen / oxygen form gains 2 marks z = 1856 - 1370 or (+)486 kJ for 1 mark or difference between x and y or net energy transferred overall, energy is taken in / absorbed in the reaction or the reaction is endothermic or energy required to break existing bonds is > energy released when new bonds form for 1 mark [6] 23. (a) any four from max 4 sulphuric acid measured by pipette accept diagram potassium hydroxide in burette accept diagram if solutions reversed award 1 mark note initial reading use of indicator note final reading or amount used (b) 34 2 1000 = 0.068 2 award full marks for correct answer with no working (c) 1 0.5 moles H2SO4 reacts with 1 mole KOH 1 moles H2SO4 in 25.0 cm3 = 0.068 × 0.5 1 moles H2SO4 in 1 dm3 0.068 0.5 1000 25 1 = 1.36 mol dm–3 [9] 24. (a) calcium 1 allow formulae carbonate (b) (i) 1 soap 1 allow 2 marks for “contains CaSO4 / Ca(HCO3)2 / Ca2+ / Mg2+” (ii) (c) forms scum / no bubbles / no lather allow 1 mark for “contains Ca / Mg” do not allow “contains CaCO3” 1 taste / strengthen bones, teeth etc / health reason e.g. less heart disease / makes better beer 1 (shake with) soap; makes scum / no lather 1 boil (a fresh sample) 1 retest with soap 1 result/comparison 1 alternative answers: boil (not to dryness) cloudiness in water/some deposit formed if a comparison is made with a sample of a soft water, a further 2 marks would be possible (d) ion-exchange 1 [10] 25. (a) (i) e.g. moles NaOH = moles of acid or formula: 0.2 × 45 = 0.009 1000 15M1 = 0.2 × 45 1 rounding to 0.01 loses mark 1000 = 0.6(M) 15 M1 = 0.6(M) = 0.009 × 1 ecf for arithmetical error correct answer 2 marks (ii) 36 2 ecf – (a)(i) × 60 correct answer 2 marks 0.6 × 60 gets 1 mark relative formula mass of ethanoic acid = 60 for 1 mark 0.6 × incorrect molar mass gains second mark only (b) (i) (iii) (iv) A = hydrogen / H2 1 B = sodium hydroxide / NaOH or sodium oxide / Na2O 1 C = ethyl ethanoate (acetate) / CH3COOC2H5 / CH3CO2C2H5 1 D = (concentrated) sulphuric acid / H2SO4 do not accept dilute sulphuric acid E = sodium ethanoate (acetate) / CH3COONa / CH3CO2Na 1 1 [9] 26. (a) breaking of C-H bonds breaking of O-O bonds making of C-O bonds for 1 mark each 4 making of H-O bonds (b) X energy needed to break bonds has to be supplied/activation energy Y energy released when bonds form Z = Y-X overall, energy is released/reaction is exothermic each for 1 mark 5 [9] 27. (a) Mendeleev arranged known elements in order of mass or properties reject explanation in terms of electrons and / or atomic number 1 gaps in Periodic Table / group 1 1 (b) does not last long enough to experiment / very little of it allow it has a short half-life 1 (c) (i) (much) more violent accept more reactive 1 (ii) since outer electron / or shell further from nucleus do not credit lower down group larger / more shells neutral 1 therefore more easily lost accept screening by inner electrons 1 [6] 28. (a) Group 2 / Alkaline Earth Metals for 1 mark 1 (b) (i) MgCl2/Mg2+ (Cl–)2 (or equation with correct answer) for 1 mark 1 (ii) ionic / electrovalent 1 for 1 mark [3] 29. (a) evidence of Fe = 56, O = 16 gains 1 mark 3 but (56 × 2) + (16 × 3) gains 2 marks but 160 gains 3 marks (b) evidence of 112*/160 gains 1 mark 2 but 70* gains 2 marks * (credit answers consistent with part (a)) [5] 30. (a) positive 1 for 1 mark (b) any reference to loss of electrons for 1 mark 2 reference to charge being +2 (in (a)) or to loss of 2 electrons (in (b)) for 1 mark [3] 31. (a) Idea that 5 the electrons do not belong to specific atoms/delocalised electrons [credit if done on appropriate diagram] metal atoms form positive ions the attraction which exists between particles with opposite charges, holds the metal together no specific bonds exist between adjacent atoms/ions atoms/ions can slide over each other so allowing metals to bend each for 1 mark (b) some electrons in the structure are delocalised/free to move for 1 mark 4 these free electrons carry the electric current for 1 mark from left to right across the period, atoms of elements have more free electrons gains 1 mark but from left to right across the period, atoms of elements have more free electrons because they have more electrons in the outer shells gains 2 marks [9] 32. (a) (i) are identical / the same or have the same number of protons / the same proton number / electrons not similar 1 (ii) any two of 2 low melting point low boiling point brittle (solid) do not credit just solid poor conductor of heat or heat insulator poor conductor of electricity or electrical insulator accept just poor / low conductor or just insulator once only dull surface or not shiny cannot be hammered / bent (into shape) or not malleable cannot be stretched (into shape) or not ductile does not make a clanging sound (when struck) or not sonorous acidic oxides low density (b) copper 1 iron 1 correct symbols (c) zinc 1 can be bent (easily) or malleable or can be joined easily do not credit just can be joined 2 not reactive or does not corrode or does not react (with hot water) accept does not rust (d) (i) chlorine do not credit chloride 1 (ii) one / 1 same number / amount 1 [10] 33. (i) electrons 1 for 1 mark (ii) covalent 1 for 1 mark (iii) made of small molecules: usually gas or liquid ) dependent on have low melting points ) having first have low boiling points ) point above forces between molecules are weak any 1 for 1 mark 3 [5] 34. (a) Quality of written communication: All scientific words used correctly (covalent, bonds, atoms) 1 any two from 2 large numbers of covalent bonds allow giant lattice / structure between atoms do not accept between molecules (covalent) bonds strong accept need much energy to break (b) W X C Y (c) Z each carbon has 4 electrons one shared pair four shared pairs 1 1 1 Cu2+ + 2e– Cu this answer only 1 [7] 35. (a) (b) (i) 40 + 12 + 48 1 = 100 1 (ii) carbon dioxide must be name 1 (i) suitable line - narrow neat single curve not dot to dot points all correct 2 marks one point incorrect 1 mark two points incorrect 0 marks (ii) reaction with X forms less gas must include X or Y do not penalise for H2 / O2 if (a)(ii) already penalised do not accept is finished in less time or slower/faster reaction or lower on graph (iii) concentration (of acid) decreases or less reacting particles or molecules not strength of acid decreases not acid/CaCO3 runs out/is used up 2 1 1 max 2 surface area of calcium carbonate decreases less collisions between reacting particles not smaller (amount of) CaCO3 [9] 36. (a) Mg + 2H+ Mg2+ + H2 * reactants correct in every detail * products correct in every detail if the spectator ions are sown then (1) mark should be credited but only if they are shown correctly on both sides e.g. Mg + 2H+ + 2CI- Mg2+ + 2CI- + H2 2 (b) 24 (parts) of magnesium 2 (parts) of hydrogen or equally clear working (so) 6 grams/g (are needed) unit required 1 1 (c) (i) 1 two (and no more) atoms shown to be sharing their single electrons examples do not credit if anything which contradicts the impression that these are hydrogen atoms H H H H H H H H H H Electron e e Proton (ii) (d) Electron e e Nucleus (single) covalent (bond) 2 (×100) 34 1 1 =6 (just 6 is worth (1) mark) 2 × 100 = 6 or similar is (0) 32 do not credit 5.8823529 and the like 1 [8] 37. (a) Zn + 2HC 1 ZnC12 + H2 1 (b) (i) 12.5 1 (ii) steeper curve same volume of gas evolved do not credit two intersects of straight lines accept a sharp bend 2 (iii) any two from: 2 stir it accept mix it better heat it accept warm it use a more finely divided catalyst accept use a better catalyst or more finely divided zinc do not credit use acid of a higher (c) (i) any one from 1 zinc is more reactive than copper accept zinc is above copper in the reactivity series zinc displaces copper accept it is higher than copper in the reactivity series (ii) zinc + copper sulphate copper + zinc sulphate ignore the presence of acid or water accept a balanced equation 1 [8] 37. (i) two 1 or 2 (ii) magnesium and chloride either order not positive / negative do not credit’chlorine’ 1 accept Mg++ and Cl do not credit just Mg and Cl– accept cation(s) and anion(s) (iii) 2 (iv) electrons 1 1 accept charges (v) any three from (is a) giant structure/lattice structure crystalline / hard accept just 'crystals(s)’ high melting point / solid high boiling point conductor (of electricity) when dissolved in water or conductor (of electricity) when ions are free to move conductor (of electricity) when molten soluble in water 3 [7] 38. (a) (b) proton + (1) both required neutron 1 both required electron (1) both required 2.8.3 3 1 accept words or diagram to this effect (c) (d) (i) 24 1 (ii) 52 1 any one of 1 gains one or more electrons accept gains an electron becomes an anion do not credit becomes an ion becomes a negative ion (e) sodium ions have a (single) positive charge and chloride ions have a 1 (single) negative charge do not credit ‘chlorine ions’ but allow this error to be carried forward ions with opposite charge are attracted (to each other) or the positive ions and the negative ions are attracted (to each other) or the sodium ions and the chloride ions are attracted (to each other) 1 (positive and negative) ions are arranged alternatively (in each direction or dimension) or ions with the same charge are repelled (by each other) no mark for just ionic bonds 1 [10] 39. (a) Mg S O4 24 + 32 + 16 ( 4) or 64 / evidence of all Ar’s gains 1 mark 2 but (Mr) = 120 gains 2 marks (b) evidence that 24(g) magnesium would produce 120(g) mapesiurn sulphate gains 1 mark 2 or correct scaling by 1/6 but 20(g) magnesium sulphate gains 2 marks [credit error carried forward from (a) with full marks in (b)] [4] 40. (a) (i) high temperature accept temperature given if 400°C ignore value if “high” stated, unless silly value endothermic or reaction takes in energy 1 (ii) or H is +ve independent marks 1 low pressure or up to and including 10 atmospheres 1 (low pressure) favours a reaction in which more molecules are formed 2 moles 4 moles (2 molecules 4 molecules) independent marks (b) 1 (iii) nickel and it is a transition / transitional element / metal (owtte) or nickel and variable oxidation state / number or it is similar to other named transition elements e.g. iron 1 (i) (bonds broken =) 2005 (kJ) 1 (bonds formed =) 2046 (kJ) 1 energy change = 2005 – 2046 = (–)41 for correct subtraction ignore sign 1 (ii) (exothermic) if in part (b)(i) answer is not 41 answer is consequential on endothermic or exothermic shown accept correct reasoning for incorrect answer from (b)(i) energy given out forming new bonds do not accept energy needed to form new bonds 1 greater than energy put in to break old bonds accept exothermic and more energy given out than taken in for 1 mark accept negative value for energy change or energy in products less than energy in reactants for 1 mark 1 [10] 41. (a) correct representation of 1 atom of hydrogen e.g. 2 H gains 1 mark but correct representation of 1 molecule of hydrogen e.g. or H-H H H gains 2 marks (b) idea that: hydrogen/metals form positive ions/lose electrons gains 1 mark 4 but hydrogen and the metals form positive ions/lose electrons gains 2 marks hydrogen/non-metals form covalent bonds/share electrons gains 1 mark but hydrogen and the non-metals form covalent bonds/share electrons gains 2 marks [6] 42. (a) electrons neutrons protons 3 for 1 mark each (b) mass number no. of neutrons 14 8 for 1 mark each 2 [5] 43. (a) covalent bonds for 1 mark 1 (b) any reference to shared electrons gains 1 mark 2 but idea that bond is shared pair of electrons gains 2 marks [3] 44. (a) the concentration of the (nitric) acid is decreasing accept the number of acid particles is decreasing or there are fewer collisions (the volume of carbon dioxide remains at 83 cm3) when the concentration of the (nitric) acid is zero accept no acid remains or all the acid is used up or no acid particles (b) (c) 1 1 line starts at origin is steeper and remains to the left of the original line 1 graph line levels off at 83 cm3 and before 12 minutes tolerance square 1 change the temperature accept increase or decrease the temperature accept change (increase or decrease) the concentration (of the nitric acid) ignore amounts of reactants or changes in pressure or stirring or use of catalyst 1 [5] 45. (a) sodium 1 (b) neutralisation 1 (c) increase/inc. number 1 (d) H+ 1 (e) OH– 1 (f) H+ + OH– H2O 1 [6] 46. (a) 2 Na + Cl2 2 NaCl allow 2 Na+ Cl– for 1 mark (allow Na + ½Cl2 Na Cl) 1 (b) (i) 1 idea that it has strong (attractive) forces/bonds between ions / charged particles for 1 mark (not ‘..it has a rigid structure’- this defines a solid or ‘...particles close together’ - they are in a liquid) (ii) ideas that 2 there is increased vibration of ions / particles on heating ions have sufficient energy to overcome attractive forces / to break out of the rigid structure / to move about (must be in terms of increased energy of particles lions) each for 1 mark (iii) (c) ions can go to electrodes / ions are free to move for 1 mark [do not credit ‘ions carry charges’] ideas that it has stronger attractive forces between atoms/particles (not ‘ions’) each carbon atom forms covalent bonds with neighbouring atoms each for 1 mark 1 2 [7] 47. (a) Mr (SiO2) = 60 if Mr incorrect ecf for max 2 1 60 g SiO2 28 g Si correct answer for 3 marks 1 2.14 g SiO2 1 g Si allow 2, 2.1, 2.14 (or anything rounding to 2.14), 2.16 or 2.2 a unit is not required but an incorrect unit loses the third mark 1 OR Mr (SiO2) = 60 moles if silicon needed = (1) 1 = 0.0357 28 mass of SiO2 needed = 0.0357 × 60 (1) = 2.14 g (1) allow 2, 2.1, 2.14 (or anything rounding to 2.14), 2.16 or 2.2 OR Mr (SiO2) = 60 (1) 60 mass SiO2 = 1 × 28 (1) = 2.14 g (1) allow 2, 2.1, 2.4 (or anything rounding to 2.14), 2.16 or 2.2 (b) (i) MgO(s) + 2HCl(aq) MgCl2(aq) + H2O(l) penalise incorrect symbols correctly balanced equation for 1 mark state symbols for 1 mark allow correct multiples / fractions 2 H (ii) H Si 1 H H or H H H H Si H or H H Si H or H H Si H H ignore inner shell electrons of silicon allow correct drawings without symbols must clearly indicate four shared pairs of electrons with one electron from each atom (iii) Si H 1.4 0.15 28 1 = 0.05 = 0.15 1 3 for whole number ratio can be implied Si H3 1 1 1 1 accept H3 Si or any correct formula with 1:3 ratio if in step 1 they get either of ratios incorrect they lose first 2 marks but can be ecf for 3rd and 4th mark evidence of mass / Ar 1 mark proportions of each whole number ratio correct formula 1 mark 1 mark 1 mark (iv) C 1 accept c (c) any four from: 4 giant structure / macromolecule / lattice / giant molecule allow giant molecular / giant atomic structure each silicon atom joined to four other atoms (or diagram) covalent bonds bonds are strong or large amount of energy needed to break bonds accept hard to break bonds large number of bonds to be broken mention of giant ionic structure or intermolecular forces or intermolecular bonds max 1 mark diamond or carbon discussion max 3 marks unless clearly linked to silicon [15]