WJEC Chemistry for AS: Study and Rev Guide

Exam Practice and Technique The second section of the book covers the key skills for examination success and offers you examples based on real-life responses to examination questions. First, you will be guided into an understanding of how the examination system works, and then offered clues to success. A variety of structured and essay questions are provided in this section. Each essay includes the marking points expected followed by actual samples of candidates’ responses. A variety of structured questions is also provided, together with typical responses and comments. They offer a guide as to the standard that is required, and the commentary will explain why the responses gained the marks that they did. Most importantly, we advise that you should take responsibility for your own learning and not rely on your teachers to give you notes or tell you how to gain the grades that you require. You should look for additional notes to support your study of AS Chemistry. We advise that you look at the WJEC website www.wjec.co.uk. In particular, you need to be aware of the Specification. Look for specimen examination papers and mark schemes. You may find past papers useful as well. Good luck with your revision. Peter Blake, Elfed Charles and Kathryn Foster Halogenoalkanes Q & A ASChemistry: Study and RevisionGuide 146 29 (a) Methane reactswithgaseous chlorinegiving chloromethaneandhydrogen chloride. CH 4 (g) + Cl 2 (g) CH 3 Cl(g) + HCl(g) Ina reportof this reaction,a student cameacrossanumberof terms. Illustratingyouranswerwithanequation in each case, statewhat is meantby: (i) homolyticfission, [2] (ii) apropagation stage. [2] (b) Oneof theproductsof the reactionbetweenethaneand chlorine is 1,1,1-trichloroethane. Themanufactureanduseof1,1,1-trichloroethane isnow restricted becauseof itsadverseeffectson theozone layer.However, the correspondingfluorocompound1,1,1-trifluoroethanedoesnot cause environmentalproblems in theozone layer. (i) Explainwhyonly the chloro-compoundhas theseadverseeffects. [2] (ii) A sampleof1,1,1-trichloroethane is reactedwithanexcessof sodiumhydroxide solutionand thenacidified. I Oneof theproductsof this reaction is liquidRwhosemass spectrum showsamolecular ionatm/z60. The infrared spectrumofR shows characteristicabsorption frequenciesat1750 cm -1 and2500-3500 cm -1 . Use this information, showingyourworking, to suggesta structural formula for liquidR. [4] II Chloride ionsarealsoproducedwhen1,1,1-trichloroethane reacts withaqueous sodiumhydroxide.Theproductsof the reaction are thenacidifiedwithnitricacidand themixture tested for the presenceof chloride ions. State the reagent(s)usedand theobservationswhen themixture was tested for chloride ions. [2] 2.8 Instrumental analysis Mass spectra Amass spectrumgives theM r of the compound presentby thevalueofm/z for thepeakwith the highestm/z Peaksatotherm/zvaluesgive themassesof fragmentsandhence cluesabout the structureof the compound IR Spectra Theabsorptionpeaks in IR spectraarise fromenergy changes in thebondsof themolecule The sizeof theenergyabsorption, in cm –1 ,gives informationabout the functionalgrouppresent NMR spectra Energy changes, inppm, canbe seenon a spectrumusing 13 Cor 1 Hpresent in compounds Thenumberofpeaks ina 13 C spectrum gives thenumberof carbonenvironments whilst the chemical shiftofeachpeakgives the typeofenvironment Thenumberofpeaks ina 1 H spectrumgives thenumberofhydrogenenvironments whilst the chemical shiftofeachpeakgives the typeofenvironment.The ratioof the peakheightsgives the ratioof thenumber ofhydrogenatoms ineachenvironment 2.7 Alcohols and carboxylic acids Preparation of ethanol and biofuels Ethanol canbeproducedbyhydrationofetheneor by fermentationof sugars.Knowledgeof thegeneral conditionsused ineach case is required Bioethanolandbiodiesel canbothbeusedasalternatives to fossil fuelsbut therearebothadvantagesand disadvantages indoing so Classi cation and reactions of alcohols The functionalgroup inanalcohol is–OH Alcohols canbe classifiedasprimary, secondaryor tertiary according towhatelse isattached to the carbon towhich thealcoholOH isattached Mostalcohols canbedehydrated toproducealkenes–a varietyofdehydratingagents canbeused Primaryand secondaryalcohols canbeoxidised byoxidisingagents suchasacidifiedpotassium dichromate(VI).Primaryalcoholsareoxidised toaldehydes and then to carboxylicacidswhilst secondaryalcoholsare oxidised toketones Reactions of carboxylic acids Carboxylicacidsareweakacidswith –COOHas the functionalgroup Carboxylicacids reactwithbases toproduce saltandwater.They also reactwith carbonatesand hydrogencarbonates toproduce salt, carbondioxideandwater Carboxylicacids reactwithalcohols toproduceesters– this reaction is catalysedby concentrated sulfuricacid Practical work Esters canbemadebyheating, generallyunder reflux,a carboxylic acid,analcoholand concentrated sulfuricacid.This reactiongivesan equilibriummixture.Theester canbe separated from this reactionmixtureby distillation 110 ASChemistry: Study and RevisionGuide 5