2815 04 Methods of Analysis and Detection 55

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OCR Oxford Cambridge and RSA Examinations is a unitary awarding body established by the. University of Cambridge Local Examinations Syndicate and the RSA Examinations Board in. January 1998 OCR provides a full range of GCSE A level GNVQ Key Skills and other. qualifications for schools and colleges in the United Kingdom including those previously. provided by MEG and OCEAC It is also responsible for developing new syllabuses to meet. national requirements and the needs of students and teachers . This mark scheme is published as an aid to teachers and students to indicate the requirements. of the examination It shows the basis on which marks were awarded by Examiners It does not. indicate the details of the discussions which took place at an Examiners meeting before marking. commenced , All Examiners are instructed that alternative correct answers and unexpected approaches in. candidates scripts must be given marks that fairly reflect the relevant knowledge and skills. demonstrated , Mark schemes should be read in conjunction with the published question papers and the Report. on the Examination , OCR will not enter into any discussion or correspondence in connection with this mark scheme . OCR 2008, Any enquiries about publications should be addressed to . OCR Publications,PO Box 5050,NOTTINGHAM,Telephone 0870 770 6622.
Facsimile 01223 552610,E mail publications ocr org uk. CONTENTS, Advanced GCE Chemistry 7882 , Advanced Subsidiary GCE Chemistry 3882 . MARK SCHEME FOR THE UNITS,Unit Content Page,2811 Foundation Chemistry 1. 2812 Chains and Rings 8, 2813 01 How Far How Fast Experimental Skills 1 Written Paper 21. 2813 03 How Far How Fast Experimental Skills 1 Practical Examination 27. 2814 Chains Rings and Spectroscopy 33,2815 01 Trends and Patterns 40.
2815 02 Biochemistry 45,2815 03 Environmental Chemistry 51. 2815 04 Methods of Analysis and Detection 55,2815 06 Transition Elements 59. 2816 01 Unifying Concepts in Chemistry Experimental Skills 2 Written Paper 67. 2816 03 Unifying Concepts in Chemistry Experimental Skills 2 Practical Examination 74. Grade Thresholds 80, 2811 Mark Scheme June 2008,2811 Foundation Chemistry. Question Expected Answers Marks Additional Guidance. 1 a 2, protons neutrons electrons, In 49 64 49 mark by row. In 49 66 49, In line correct 9, In line correct 9.
b A r 113 x 4 23 100 115 x 95 77 100 2 Allow one mark for A r 114 9154 with no working out. 114 9154 9 calculator value Allow two marks for A r 114 9 with no working out. 114 9 9 to 1 decimal place, If a candidate uses incorrect values in 1st line then the. 2nd mark can still be awarded if the calculated value is. from 113 1 to 114 9 expressed to one decimal place ie. if s are the wrong way round in 1st line then an, answer of 113 1 gets the 2nd mark . c 2, , , , with labels , scattering of labelled electrons between other species 9 1st mark is for any symbol that is labelled an electron. that is between something else , ie between ions atoms protons nuclei p circles . Allow e or e with no label, Do not allow with no label.
2nd mark for labelled ions positive ions cations, regular 2 D arrangement of labelled ions with some that can be touching and must be 2 D. attempt to show electrons 9 ie not just a row , Allow In or In with charge from 1 to 7 . NOT protons commonest mistake , 2811 Mark Scheme June 2008. Question Expected Answers Marks Additional Guidance. d i M r weighted mean average mass of a molecule 9 3 1st mark reference to molecule is essential. Allow just average mass of molecule or mean mass of. compared with carbon 12 9 molecule , 1 12th of mass of carbon 12 alternative allowable definitions . on a scale where carbon 12 is 12 9 but not 12 g mass of one mole of molecules 9. compared to 1 12th 9, the mass of one mole 12 g of carbon 12 9.
mass of one mole of molecules 9 , 1 12th 9 the mass of one mole 12 g of carbon 12 9. ii ratio In I 23 19 115 76 81 127 3 Allow use of 114 9 for In ie from answer to 1 b . If a candidate uses atomic numbers the ratio is still 1 3 . The 2nd and 3rd marks can still be awarded by error. carried forwards , Empirical formula InI 3 9, Molecular formula In 2 I 6 9 Although unlikely an correct answer of In 2 I 6 with no. working should be awarded all three marks , If candidate shows inverse for ratios . OR ie In I 115 23 19 127 76 81, then the candidate can be awarded the 2nd. mass In 23 19 x 992 100 OR 230 g AND mark only for In 3 I by error carried forwards . mass I 76 81 x 992 100 OR 762 g 9, moles In 230 115 OR 2 AND.
moles I 762 127 OR 6 9, Molecular formula In 2 I 6 9. Total 12, 2811 Mark Scheme June 2008, Question Expected Answers Marks Additional Guidance. 2 a i Ca 9 1 Allow names throughout i vi , ii N9 1. iii Cl9 1, iv B9 1 Allow Al, v K9 1, vi C Si B9 1. b i cation shown with either 8 or 0 electrons 2 An ionic compound must be chosen and it must. AND anion shown with 8 electrons have correct formula to score at all. AND correct number of crosses and dots for example. chosen 9 For 1st mark if 8 electrons shown around cation then. extra electron s around anion must match symbol, Correct charges on both ions 9 chosen for electrons in cation .
e g Circles not required , 2 Ignore inner shell electrons. 2Na O Allow 2 Na 2 Na Na 2 brackets not required . Do not allow for Na 2 O Na 2 2 Na 2 2Na 2 Na , ii electron pair s in covalent bond shown correctly using 2 A covalent compound must be chosen and it must. dots and crosses in a molecule of a compound 9 have correct formula to score at all. correct number of outer shell electrons in example For dot and cross diagram accept different symbols for. chosen 9 electrons from each atom ie X and , e g H. If example chosen is molecule of an element then 2nd. mark can be awarded if candidate has used dots and. H crosses for all outer shell electrons around each atom . 2 x o between O and H for 1st mark, correct outer shell electrons for O and H for 2nd mark Circles not required. 2811 Mark Scheme June 2008, Question Expected Answers Marks Additional Guidance.
c 4 USE annotations with ticks crosses con ecf etc for this. across a period Ignore down a period across a group . atomic radius decreases If candidate responds with electrons are same distance. outer electrons closer to nucleus 9 from the nucleus anywhere is a CON . electrons are pulled in closer but ignore about the same distance . Ignore atomic number increases , nuclear charge increases protons increase 9. Ignore nucleus gets bigger , charge increases is not sufficient. Allow effective nuclear charge increases OR, shielded nuclear charge increases . A comparison must be included , greater attraction greater pull 9. ie greater pull more pull held more tightly , so pulled in closer would score the 1st marking.
point but not the 3rd marking point here, electrons added to the same shell OR. screening shielding remains the same or similar 9. Allow very small increase for similar , Total 14, 2811 Mark Scheme June 2008. Question Expected Answers Marks Additional Guidance. 3 a i moles 55 24 000 2 3 x 10 3 0 0023 mol 9 1 Allow calc 2 291666667 x 10 3. and correct rounding to a minimum of 2 sig fig ie 0 0023. ie rounding is being assessed here , i bleach 1000 x 2 3 x 10 3 3 0 77 mol dm 3 9 1 From a i allow use of calc value 0 763888888. For any rounded value of 2 291666667 x 10 3 down to a. minimum of 2 sig fig ie 0 0023 allow any value in range 0 76. to 0 77 mol dm 3, ie rounding has been assessed above . For ECF 1000 x ans to i 3, i moles HCl at start 1 0 x 6 0 1000 6 x 10 3 9 3 Marking screen shows parts i and iii .
i moles HCl that reacted 2 x 2 3 x 10 3 ECF ans to i x 2. 4 6 x 10 3 0 0046 mol 9, excess HCl 6 x 10 3 4 6 x 10 3 ECF moles HCl at start moles HCl that reacted. 1 4 x 10 3 mol 0 0014 mol 9 Common mistake If a candidate does not multiply ans to i . mark is for answer by 2 then ECF answer will be 0 00371 from 0 00229 . or 0 0037 from 0 0023 , Both answers would gain 2 marks for this part . b i iodine I 2 produced 9 2 I 2 as a product in an attempted equation would score 1st. correct balanced equation , Cl 2 2I I 2 2Cl Cl 2 2NaI I 2 2NaCl. i chlorine reacts with water forming Cl OR chloride 4 Allow Cl 2 H 2 O HClO HCl. i Cl 2 H 2 O ClO 2H Cl 9, AgCl s precipitate is silver chloride OR AgCl s 9 can be credited for this marking point in equation as AgCl s . chloride OR Cl reacts with silver nitrate OR Ag 9 can be credited for this marking point in equation as Cl . Ag Cl AgCl AgNO 3 HCl AgCl HNO 3 State symbols not required. 9 Ag Cl AgCl s would get last three marks , 2811 Mark Scheme June 2008.
Question Expected Answers Marks Additional Guidance. c i attraction of an atom nucleus for electrons 9 2 For 1st mark atom nucleus is essential. attraction for electrons in a covalent bond 9, Commonest correct answer . Attraction of an atom for the electrons in a covalent bond . i four bonds shown with at least 2 wedges 2 For bond into paper accept . i one in one out 9 Cl, Cl Cl Allow correct shape with no atom labels . Cl Cl Cl, Bond angle can just be stated as this is the only one bond. bond angle 109 5 9o angle that applies so no labelling required . Allow 109 110 , i 3 USE annotations with ticks crosses con ecf etc for this part . i Cl is more electronegative than H or C 9 Allow Cl is slightly negative. OR shown as dipole H Cl OR C Cl , Do not allow negative OR Cl OR chloride ion OR chlorine.
CCl 4 is symmetrical 9 Allow CCl 4 is tetrahedral. In CCl 4 dipoles cancel 9, Total 18, 2811 Mark Scheme June 2008. Question Expected Answers Marks Additional Guidance. 4 a A CaO 9 6, B CO 2 9, C Ca OH 2 9 Brackets essential. D CaCl 2 9, E H2O 9, F Ca HCO 3 2 CaH 2 C 2 O 6 9 Allow any order of atoms in a correct formula. b 2Ca s O 2 g 2CaO s 4 USE annotations with ticks crosses con ecf etc for this part . Ca s O 2 g CaO s Allow multiples ie 4Ca s 2O 2 g 4CaO s . state symbols for Ca O 2 and CaO 9 Allow balanced equation with a species on both sides . correct balanced equation 9 ie Ca s O 2 g CaO s O 2 g . Oxidation is loss of electrons Must be in terms of electrons. AND reduction is gain of electrons 9 Ignore any reference to oxidation number. Ca loses 2 electrons AND O gains 2 electrons OR Allow equations accept e without sign . Ca loses 2 electrons AND O 2 gains 4 electrons9 Ca Ca2 2e Ca 2e Ca2 . O 2 4e 2O2 O 2e O2 , reactivity increases down the group 9 5 USE annotations with ticks crosses con ecf etc for this part . down the group not required, atomic radii increases there are more shells 9.
there is more shielding more screening 9 more is essential. allow more electron repulsion from inner shells , Increased shielding and distance outweigh the. increased nuclear charge Allow nuclear pull , the nuclear attraction decreases 9 ignore any reference to effective nuclear charge . easier to remove outer electrons , ionisation energy decreases 9. QWC At least two sentences that show legible 1 QWC mark must be indicated with a tick or cross through the. text with accurate spelling punctuation and Quality of Written Communication prompt at the bottom of page 9 . grammar so that the meaning is clear 9 Then scroll up to start of b counting ticks . Total 16, 2812 Mark Scheme June 2008,2812 Chains and Rings. Question Expected Answers Marks Additional Guidance. 1a i C 6 H 14 9 1 there is no other acceptable response. ii C 3 H 6 Br 9 1 there is no other acceptable response. iii hexan 2 ol 9 1 Allow hexanol 2, do not allow e g 2 hexanol hex 2 ol hexa 2 ol.
iv HBr 9 1 Allow NaBr H 2 SO 4 HBr H 2 SO 4, Do not allow dil H 2 SO 4 H 2 SO 4 aq . 2812 Mark Scheme June 2008,b i 4 curly arrow must have full arrow head. H H H H, if half arrow heads used penalize once only . C C CH3 CH2 3 C C H, ignore on C C double bond, 1 . CH3 CH2 3 H 3 Br, Br , ignore on Br Br bond, 2 Br curly arrows must be precise .
curly arrow 1 must start at the C C double bond not. the C and must go to the Br or just above the Br . 1 curly arrow 2 must start from the Br Br bond and go to. curly arrow from bond to Br 9 the Br or just past the Br . correct dipoles curly arrow from bond to Br 9, 3 intermediate either primary secondary carbonium bromonium curly arrow 3 must go from Br to the C . 4 Br must have charge and lone pair Curly arrow from anywhere allow primary carbonium ion or bromonium ion as an. on Br to C 9 alternative to the carbonium ion, If HBr is used instead of Br 2 candidate loses marking. If ethene or other alkene appears as the, intermediate candidate loses marking point 3 . If intermediate has C candidate loses marking point. If intermediate drawn as, CH 3 CH 2 3 C C H, candidate loses marking point 3 . 2812 Mark Scheme June 2008, Question Expected Answers Marks Additional Guidance.
ii electrophilic addition 9 1 nucleophilic addition loses the mark. ignore bromination , iii decolourises red orange brown yellow to colourless 9 1 not goes clear. not discolours, c i 2 Ignore bond linkage to CH 2 3 CH 3 unless the bond. 2815 01 Trends and Patterns 40 2815 02 Biochemistry 45 2815 03 Environmental Chemistry 51 2815 04 Methods of Analysis and Detection 55 2815 06 Transition Elements 59 2816 01 Unifying Concepts in Chemistry Experimental Skills 2 Written Paper 67 2816 03 Unifying Concepts in Chemistry Experimental Skills 2 Practical Examination 74 Grade Thresholds 80

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