Mark Scheme (Results) Summer 2012

Mark Scheme (Results)Summer 2012GCE Chemistry (6CH04) Paper 01General Principles of Chemistry IRates, Equilibria and Further OrganicChemistry(Including synoptic assessment)

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General Marking Guidance All candidates must receive the same treatment. Examiners mustmark the first candidate in exactly the same way as they mark thelast. Mark schemes should be applied positively. Candidates must berewarded for what they have shown they can do rather than penalisedfor omissions. Examiners should mark according to the mark scheme not accordingto their perception of where the grade boundaries may lie. There is no ceiling on achievement. All marks on the mark schemeshould be used appropriately. All the marks on the mark scheme are designed to be awarded.Examiners should always award full marks if deserved, i.e. if theanswer matches the mark scheme. Examiners should also beprepared to award zero marks if the candidate’s response is notworthy of credit according to the mark scheme. Where some judgement is required, mark schemes will provide theprinciples by which marks will be awarded and exemplification may belimited. When examiners are in doubt regarding the application of the markscheme to a candidate’s response, the team leader must beconsulted. Crossed out work should be marked UNLESS the candidate hasreplaced it with an alternative response. Mark schemes will indicate within the table where, and which strandsof QWC, are being assessed. Questions labelled with an asterix (*)are ones where the quality of your written communication will beassessed.

Using the Mark SchemeExaminers should look for qualities to reward rather than faults topenalise. This does NOT mean giving credit for incorrect or inadequateanswers, but it does mean allowing candidates to be rewarded foranswers showing correct application of principles and knowledge.Examiners should therefore read carefully and consider every response:even if it is not what is expected it may be worthy of credit.The mark scheme gives examiners: an idea of the types of response expected how individual marks are to be awarded the total mark for each question examples of responses that should NOT receive credit./ means that the responses are alternatives and either answer shouldreceivefullcredit.( ) means that a phrase/word is not essential for the award of the mark,but helps the examiner to get the sense of the expected answer.Phrases/words in bold indicate that the meaning of the phrase or theactual word is essential to the answer.ecf/TE/cq (error carried forward) means that a wrong answer given in anearlier part of a question is used correctly in answer to a later part of thesame question.Candidates must make their meaning clear to the examiner to gain themark. Make sure that the answer makes sense. Do not give credit forcorrect words/phrases which are put together in a meaningless manner.Answers must be in the correct context.Quality of Written CommunicationQuestions which involve the writing of continuous prose will expectcandidates to: write legibly, with accurate use of spelling, grammar and punctuation inorder to make the meaning clear select and use a form and style of writing appropriate to purpose and tocomplex subject matter organise information clearly and coherently, using specialist vocabularywhen appropriate.Full marks will be awarded if the candidate has demonstrated the aboveabilities.Questions where QWC is likely to be particularly important are indicated(QWC) in the mark scheme, but this does not preclude others.

Section A (multiple choice)QuestionNumber123a3b456a6b78a8b91011 a11 b12 a12 b12 c12 d13Correct AnswerRejectMarkTotal for Section A1111111111111111111120 marksDBCBCABCAACBDDAACDBB

Section BQuestionNumber14(a)(i)Acceptable AnswersRejectKa [CH3CO2 ] [H ]/[CH3CO2H]ORKa [CH3CO2 ] [H3O ]/[CH3CO2H]ORUse of [CH3COO ] instead of [CH3CO2 ]and [CH3COOH] instead of [CH3CO2H]Numerator as[H ]2Expressionsin terms ofHA aloneRound/curvedbrackets ‘()’Any othercarboxylicacidIGNORE state symbols even if wrongQuestion Acceptable AnswersNumber14(a)(ii) 1.7 x 10-5 [H ]2 / 0.5[H ] 1.7 x 10-5 x 0.5 / 2.915(476) x10 3(1) pH (-log[H ]) 2.53529OR 2.54OR 2.5(1)Reject4.77 or 4.8from usingpH -log Kaloses bothmarksMark1Mark2ALLOW TE for second mark from anyhydrogen ion concentration as long as pHless than 7Correct answer alone scores(2)ALLOW pH 2.53 if [H ] is rounded to2.92 x 10 3IGNORE sf except 1QuestionAcceptable AnswersNumber14(a)(iii) 20 (cm3) IGNORE unitsOR0.02 dm3RejectMark1

QuestionAcceptable AnswersNumber14(a)(iv) Moles of excess NaOH 10/1000 x 0.50 5 x 10-3(1)So [NaOH/OH-] 5 x 10-3 x 1000/50 (1)0.10 mol dm-3EITHERKw route:[H ] x 0.1 1 x 10-14(1)So pH - log 1 x 10-14 / 0.1 13(1)ORpOH route:pOH 1So pH (14 – 1) 13ALLOW TE throughoutCorrect final answer scores (4)(1)(1)RejectMark4

Question Acceptable AnswersNumber14(a)(v) Starting at pH 2-3ANDfinishing at pH between 12 and 13.7inclusiveVertical section at 20 cm3Reject3(1)(1)S-shaped curve, with gradual rise andvertical section within the pH range 5.5 and11.5 and of 3 to 5 units in length(1)These are stand alone marksMark

Question Acceptable AnswersNumber14(b)(i) EITHER[base] Ka [acid]/[H ]Or[H ] (10-pH4.70) 1.995 x 10-5Reject4(1)[base] 1.7 x 10-5 x 1/(1.995 x 10-5) 0.852(1)moles base 0.852 x 0.5 0.426 (mol)(1)mass base 0.426 x 82 34.9 g(1)IGNORE sf except 1Correct answer, with or without working (4)ORpH pKa – log[acid]/[base]4.70 4.8 – log [1/[base]]Log[1/[base]] 0.1(1)[base] 0.794(328) (mol dm-3)(1)So in 500 cm3Moles 0.794 x 0.5 0.397 mol(1)Mass 0.397 x 82 32.554/32.6 g(1)(ALLOW using pKa 4.77)Mark

QuestionNumber14(b)(ii)Acceptable AnswersReject MarkFirst markBuffer has large amount/ excess/ reservoir of(1)CH3COOH (and CH3COO )Second markOH ions added react with CH3COOHORCH3COOH OH CH3COO H2OOROH H H2O and CH3COOH CH3COO H OREquations described in words(1)Third markRatio / values of [CH3COOH] to [CH3COO ]remains (almost) unchanged(1)IGNORE concentration of hydrogen ionsremains constantALLOW answers in terms of HA and A 3

Question Acceptable AnswersNumber15(a)2,6-dimethylhept-5-enalEither part (1)(1)IGNORE missing/misplaced/misusedhyphens or commasALLOW ene for enALLOW methy or methly for methylQuestion Acceptable AnswersNumber15(b)(i) CH3C(CH3) HORCH3C(CH3) CHCH2CH2C (CH3) HCH2OH(1)RejectMarkC9H18O3ALLOW displayed or skeletal formulaeK2Cr2O7/Na2Cr2O7/name (oxidation statemust be correct if given (VI))(1)This is a stand alone markH2SO4/name (ignore any references toconcentration)(1)ALLOW H and Cr2O72-(2)‘Acidified dichromate’(1)QuestionAcceptable AnswersNumber15(b)(ii) (Steam) distil off melonal (as it forms)Allow add a limited amount of oxidizingagent/excess alcohol/excess X(1)To prevent further oxidation/To preventcarboxylic acid forming(1)Stand alone marksKMnO4 (0)for last 2marksHCl (0) for3rd markRejectMark2

Question Acceptable AnswersNumber15(c)Reject2Wavenumberrange / cm-1BondFunctionalgroup presentin melonal1740 – 1720OR2900 – 2820/2775 – 27001669 – 1645OR3095 - 3010C O(saturated)Aldehyde/CHO (1)JustcarbonylAlkeneALLOW‘carbon tocarbondouble bond’Just C Cin 3rdcolumnC-HC CC-HMark(1)ALLOW any single value or range within theranges aboveALLOW one mark if both wavenumberranges and bond columns are correct butneither bond identifiedQuestion Acceptable AnswersNumber15(d)C3H5O /CH3CHCHO (1)(1)C6H11 [ALLOW Structural, skeletal or displayedformulae]RejectMarkC4H9 C5H7O 2Penalise omission of charge once onlyALLOW any order of atoms if correct totals.Question Acceptable AnswersNumber15(e)(i)RejectCirclearoundany otheradditionalatomsMark1

Question Acceptable AnswersNumber15(e)(ii)RejectQuestion Acceptable AnswersNumber15(f)(i)RejectCirclearoundany otheradditionalatoms(1)Arrow from oxygen to H and from H-CN bondto CN(1)ALLOW arrow from O to H or to H2O1Mark3Arrow from anywhere on the cyanide ion tothe carbon of the carbonyl. Arrow to the Omust come from the carbonyl bond(1)Formula of intermediateMarkStartingfrom HCN/CN -Singleheadedarrows

Question Acceptable AnswersNumber15(f)(ii) These marks are stand aloneEITHERNoFirst mark:Reaction site/carbonyl/aldehyde/moleculeis planar(1)Second mark:Attack (equally likely) from both sidesORAttack (equally likely) from above andbelow(1)Third mark:(gives) racemic mixture/(gives) equalamounts of each isomer/(gives) equalamounts of each enantiomer(1)ORYesMelonal has a chiral carbon atom(1)Correct identification of chiral centre (1)This chiral centre unaffected by reaction(1)RejectMark3attack on a(planar)carbocationOR attackon ��Any/eitherdirection orany/eitherangle

Question Correct AnswerNumber16Sodium thiosulfate/Na2S2O3(a)(i)ALLOW S2O32- or thiosulfate ionsRejectQuestion Acceptable AnswersNumber16(a)(ii) Add (excess) sodium hydrogencarbonate/(1)NaHCO3RejectTo neutralize/remove/react with acid(catalyst)Justthiosulfate1Mark2(1)Cool in ice (water) with no reference toneutralization – allow 1 mark but ignore ifeither of first two marks awardedQuestion Acceptable AnswersNumber16(b)(i) Suitable graph and scaleNaOH/sodiumhydroxide/alkaliMarkjust coldwaterReject(1)Points plotted and line of best fit(1)0 order (with respect to iodine)(1)Mark3

QuestionAcceptable AnswersNumber16(b)(ii) Graph is a straight line/Gradient isconstantRejectMark2(1)Rate stays constant (as iodine used up)/Concentration has no effect on rate(1)Half life isconstantStand alone marksQuestion Acceptable AnswersNumber16(c)Colorimetry/use of pHmeter/conductivity/titrate withAgNO3/titrate with alkali (to monitorchange in [H ])RejectaC lorimetryUse ofstarch/Iodine clockreactionMark1

QuestionNumber17(a)(i)Correct AnswerRejectMethyl propanoateMark1ALLOW methy or methly for methylQuestion Acceptable AnswersNumber17(a)(ii) Toxic (steamy/misty) fumes/ toxicHCl(gas)/corrosive HCl(gas)/toxicpropanoyl chloride/lachrymatorypropanoyl chloride(1)So use in a fume cupboardRejectHCl(aq)/hydrochloric (b)Corrosive Propanoyl chloride is(1)So wear gloves when handling(1)Acceptable AnswersJustharmful/irritantRejectTableMark30.31, 0.16, 1.41all 3 scores 2, 2 out of 3 scores 1, 1or 0 out of 3 scores 0(2)Kc (0.21/V) x (1.41/V)(0.16/V) x (0.31/V)Kc 5.969758Kc 5.97IGNORE sf except 1IGNORE any units(1)ALLOW TE from incorrect values intable.TOTAL FOR SECTION B 52 MARKS