WJEC Chemistry for A2: Study and Rev Guide

Exam practice and techniques The second section covers the key skills needed for examination success and gives examples of responses to examination questions. We begin by giving you an insight into the examination itself, and how it is assessed, and then offering clues to success. A variety of questions are provided. Most of these are structured style questions with a few longer answer questions. This reflects the form of the examination. Each question presents you with two typical candidate answers with comments about their performance. Questions for Unit 3 tend to be focused on an individual topic whereas the questions for Unit 4 tend to cover a range of topics, as seen in the actual examinations. We advise you to seek out and learn from a number of sources and not to just rely on a particular book and the notes from your teachers. Try to make good use of the WJEC website www.wjec.co.uk, where you will find specimen papers and their mark schemes, and, as time goes on, the actual papers that were set in previous years. This new specification has more emphasis on the application of your knowledge and understanding and there is less dependence on recall questions than in the past. In addition, there will be more questions that require the use of mathematics. We hope that you find this book useful in your studies and that it helps you to gain the grades that will enable you to advance onto the next level of your career. David Ballard and Rhodri Thomas Exam practice and skills Aims Toencourage students to: ■ develop their interestandenthusiasm for Chemistryand in further studyand careers ■ developessentialknowledgeandunderstanding ofdifferentareasofChemistryandhow theyare related ■ appreciatehow societymakesdecisionsabout scientific issuesandhow science contributes to the successof theeconomyandof society in general. A2 Chemistry – a summary of assessment TheassessmentatA2 level comprises twowritten papers,eachof105minutes.Eachpaper carries 80marks.Bothareworth25%of the fullA level qualification. Inaddition there isapractical componentof thequalification (Unit5).Thisunit comprises two tasks–apracticalexamination carrying30marksanda shortwrittenPractical MethodsandAnalysis task thatalso carries30 marks.Unit5 contributes10%of the fullA level qualification.TheA2 level contributes60% in total to the fullA levelqualification. Unit3 coversPhysicaland InorganicChemistry. Unit4 coversOrganicChemistryandAnalysis. Eachpaper consistsofaSectionAwith short answerquestions for10marksandaSectionB that contains structuredandextendedanswer questions carryinga totalof70marks.TheQuality ofExtendedWriting (QER) isassessedonce ineach paperbyaquestion carrying6marks. Therearenomultiple choice stylequestions in thesepapers. Assessmentobjectives (AOs)andweightings Examinationquestionsarewritten to reflect theAOsdescribed in the specificationandyou shouldmeet the followingAOs in the contextof the subject contentwhich isgiven indetail in the specification. AO1 covers theknowledgeandunderstandingof allaspectsof the subject. Candidates shoulddemonstrateknowledge andunderstandingof scientific ideas, processes, techniquesandprocedures . AO2 is concernedwith theapplicationofacquired knowledgeandunderstanding. Candidates shouldapplyknowledgeand understandingof scientific ideas,processes, techniquesandprocedures: in theoretical andpractical contexts,bothwhenhandling qualitativeandquantitativedata. AO3 is concernedwith theanalysis, interpretation andevaluationofbothprovidedand learnt material. Candidates shouldanalyse, interpretand evaluate scientific information, ideasand evidenceand, in relation to issues,beable tomake judgementsand reach conclusions and tobeable todevelopand refinepractical designandprocedures. Candidates shouldbeawareof ‘How Science Works’ and reliably recordand communicate observationsandmeasurements. Theweighting for theseobjectives is the same for bothUnits3and4and is AO1–7.2%AO2–10.6AO3–7.2% Theweighting forUnit5 is AO1–2.0%AO2–5%AO3–3.0% Thesefiguresaddup to the full60% contribution towards the fullA levelqualification. A2Chemistry: Study and RevisionGuide 82 Tom’sanswer Seren’sanswer (i) orange green ✗  (ii) Theoxidation stateofCr inCr 2 O 7 2− is +12and in CrO 4 2− is 2 × +6 = +12. ✗ This isn’ta redox reaction because theoxidation statesdon’t change. ✓  (iii) The standard electrode potential is the voltage youwouldmeasure if you connecteda half-cell to the standard hydrogen electrode (S.H.E.) under standard conditions ( 1moldm –3 concentrations, 1atm pressure for gasesanda temperature of 298 ✓ ✗ ).TheS.H.E. hasa platinum electrode dipping ina solution containingH + (aq) ions,with hydrogen gas bubbledover it. ✓  (i) It turnsyellow. ✗  (ii) Chromium indichromate=+6; chromium in chromate=+6. ✓ This isnota redox reactionbecause theoxidation state stays the same. ✓  (iii) The standardelectrodepotential is thevaluemeasured on thehigh-resistancevoltmeterwhenahalf-cell is connected to the standardhydrogenelectrode.  Q & A 14 Reductionandoxidation Acidi edpotassiumdichromate isa common oxidisingagent,butadding sodiumhydroxide solution causes the reactionbelow: Cr 2 O 7 2− + 2OH − → 2CrO 4 2− +H 2 O (i) Give the colour change seenduring this reaction. [1] (ii) Use theoxidation states of chromium to show that this isnot a redox reaction. [2] (iii) The standard electrodepotential for acidi edpotassiumdichromatewhenused as anoxidising agent is+1.33V.Explainwhat ismeantby the term standard electrode potential anddescribehow this valuewouldbemeasured. [6] Tom’sanswer Seren’sanswer (i) orange → green ✓  (ii) Theoxidation stateofCr inCr 2 O 7 2− is +12 andCrO 4 2− is 2 × +6 = +12. ✗ This isn’ta redox reaction because theoxidation statesdon’t change. ✓  (iii) The standard electrode potential is the voltage youwouldmeasure if you connecteda half-cell to the stn ard hydrogen el ctrode (S.H.E.) under stnd rd conditions ( 1moldm –3 concentrations, 1atm pressure for gases anda temperatureof 298 ✓ ✗ ).TheS.H.E. hasa platinum electrodedipping ina solution containingH + (aq) ions,with hydrogen gas bubbledover it. ✓  (i) It turnsyellow. ✗  (ii) Chromium indichromate=+6; chromium in chromate=+6. ✓ This isnota redox reactionbecause theoxidation state stays the same. ✓  (iii) The standardelectrodepotential is thevaluemeasuredon the high resistancevoltmeterwhenahalf-cell is connetd to the sta dardhydrogenelectrode.  Exam iner commenta ry  The colou r changegiveno ccurswhendichromate is reduced,andnot in this reaction.  Hehasdoubled theoxidation state,buthasgained amark forhis reasoning.  Thisanswer isnotprecise.He receivesmarks for giving the subs tances in theS.H.E.andanother for giving two standard conditions.He loses a second mark for conditions bym issing theK from298K,and loses severalmarksbynotdiscussing the second half-cellorhow theyare connected. Tomachieves4marksoutof9. Examiner commentary  Nomark as the initial colour of the solution ismissing.  Serengainsbothmarkshere.  The labelleddiagramprovides threemarks for the substances ineach cell, theplatinumelectrodes.Shealsogains amark for thehigh resistancevoltmeterand saltbridge. She failed togive standard conditions sodoesn’tgain these marks. Serenachieves 6marksoutof9. V platinum electrodes saltbridge H 2 gas in H + (aq) high-resistance voltmeter amixtureofCr 3+ (aq) andCr 2 0 7 2 – (aq) V platinum electrodes saltbridge 1 atmH 2 gas 1moldm –3 H + (aq) high-resistance voltmeter 1moldm –3 of Fe 2+ (aq) / 1moldm –3 of Fe 3+ T=298K Al Al Cl Cl Cl Cl Cl Cl Al Al Cl Cl Cl Cl Cl Cl ✓✓✓✓ 104 A2Chemistry: StudyandRevisionGuide Examiner commentary  The colour changegiven occurswhendichromate is reduced, andnot in this reaction.  Hehasdoubled the oxidation state,buthasgained a mark forhis reasoning.  This answer isnotprecise.He receivesmarks for giving the subs tances in the S.H.E. and another for giving two standard conditions.He losesa secondmark for conditionsbymi ss ing theK from298K, and loses severalmarksbynotdiscussing the second half-cell orhow they are connected. Tomachieves 3marksoutof9. Examiner commentary  Nomarkas the initial colour of the solution is missing.  Serengainsbothmarkshere.  The labelleddiagr mprovides threemarks for the substances in each cell, theplatinumelectrodes.She alsogains amark for thehigh-resistance voltmeter and saltbridge. She failed togive standard conditions sodoesn’tgain thesemarks. Serenachieves6marksoutof 9. Unit 3 Physical and inorganic chemistry Q & A A2Chemistry: Study and RevisionGuide 86 Reduction and oxidation 1 Acidi edpotassiumdichromate isa commonoxidisingagent,butadding sodium hydroxide solution causes the reactionbelow: Cr 2 O 7 2− +2OH – 2CrO 4 2− +H 2 O (i) Give the colour change seenduring this reaction. [1] (ii) Use theoxi ation satesof chromium to show that this isnota redox reaction. [2] (iii) The standardelectrodepotential foracidi edpotassiumdichromate whenusedasanoxidisingagent is+1.33V.Explainwhat ismeantby the term standardelectrodepotentialanddescribehow thisvaluewouldbe measured. [6] 5