Mark Scheme (Results) January 2008

GCE

GCE Chemistry (6242) Paper 1

Edexcel Limited. Registered in England and Wales No. 4496750 Registered Office: One90 High Holborn, London WC1V 7BH

General Marking Guidance

All candidates must receive the same treatment. Examiners must mark the first candidate in exactly the same way as they mark the last. Mark schemes should be applied positively. Candidates must be rewarded for what they have shown they can do rather than penalised for omissions. Examiners should mark according to the mark scheme not according to their perception of where the grade boundaries may lie. There is no ceiling on achievement. All marks on the mark scheme should 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 the answer matches the mark scheme. Examiners should also be prepared to award zero marks if the candidates response is not worthy of credit according to the mark scheme. Where some judgement is required, mark schemes will provide the principles by which marks will be awarded and exemplification may be limited. When examiners are in doubt regarding the application of the mark scheme to a candidates response, the team leader must be consulted. Crossed out work should be marked UNLESS the candidate has replaced it with an alternative response Using the mark scheme 1 2 3 4 5 / means that the responses are alternatives and either answer should receive full credit. ( ) 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. [ ] words inside square brackets are instructions or guidance for examiners. Phrases/words in bold indicate that the meaning of the phrase or the actual word is essential to the answer. ecf/TE/cq (error carried forward) means that a wrong answer given in an earlier part of a question is used correctly in answer to a later part of the same question. Quality of Written Communication Questions which involve the writing of continuous prose will expect candidates to: show clarity of expression construct and present coherent arguments demonstrate an effective use of grammar, punctuation and spelling.

Full marks will be awarded if the candidate has demonstrated the above abilities. Questions where QWC is likely to be particularly important are indicated QWC in the mark scheme BUT this does not preclude others.

Question Number 1.(a) Question Number 1.(b)

Correct Answer bauxite Correct Answer electrodes

Acceptable Answers

Reject

Mark 1

Acceptable Answers Anode / cathode

Reject A reducing agent Just to form carbon dioxide Reject Equilibrium

Mark 1

Question Number 1.(c)(i)

Correct Answer Al3+ + 3e() Al Ignore state symbols unless (aq) Correct Answer Oxidation (1) Stand alone loss of electrons (from O2- ions) (1) Conditional on first mark

Acceptable Answers Multiples Al3+ Al 3e(-) Acceptable Answers oxidisation

Mark 1

Question Number 1.(c)(ii)

Reject Oxidising / redox in terms of ox. no. Oxygen molecules or O2 or wrong formula for ion Reject

Mark 2

Question Number 1.(d)

Correct Answer 900 (0C)

Acceptable Answers 800 - 1000 (0C) any range or number within this range (inclusive) value in kelvin (1073 1273) provided unit given

Mark 1

Question Number 1.(e)

Correct Answer to dissolve the aluminium oxide/alumina/Al2O 3 Or As a solvent

Acceptable Answers

Reject To dissolve bauxite. Just lowers melting point (of aluminium oxide). Any reference to catalysts scores 0.

Mark 1

Question Number 1.(f)

Correct Answer (Generation of) electricity/or electrical energy. Ignore any reference to heat.

Acceptable Answers

Reject

Mark 1

Question Number 1.(g)

Correct Answer Any one correct use linked to appropriate property: Use Planes: Property low density high strength to weight ratio(1) low density (1) low density /does not corrode/oxidise (1) does not corrode/ easily extruded (1) do not corrode/ do not react with contents/ acids high strength to weight ratio. good heat conductor (1) good heat conductor / good reflector (1) high/good conductivity / low density (1)

Acceptable Answers

Reject

Mark 1

(Superstructure of) ships

light if used instead of low density Light and strong Light but strong Rust if used instead of corrode

bicycle frames/parts car bodies/engines

Light for its strength Protected by oxide layer

window/ greenhouse frame

protected by oxide layer

cans

Protected by oxide layer

Do not react with water Easy to recycle

pans (cooking) foil

power cables

Electric wiring

Question Number 2.(a)

Correct Answer N/N2 goes from 0 to 3 = reduction (1) H/H2 goes from 0 to (+)1 = oxidation (1)

Acceptable Answers If the oxidation number of N goes down hence reduced and the oxidation number of H goes up and hence oxidised (max 1) If all O.N. correct but fails to state which is oxidation and which is reduction scores 1.

Reject If all O.N. correct but both reactions misclassified, scores zero. Any answer not referring to nitrogen or hydrogen scores zero.

Mark 2

Question Number 2.(b)(i)

Correct Answer Calculation of bonds broken 4633+944/ (=2252) (1) Calculation of bonds made 3886/ (=2328) (1) H = -76 (kJ mol-1) (1) mark consequential on numerical values calculated above

Acceptable Answers

Reject

Mark 3

Correct answer with some working scores 3 marks Correct answer alone scores 2 marks

Question Number 2.(b)(ii)

Correct Answer Average / mean bond enthalpy used for N-H bond / ammonia Correct Answer Thermodynamic: energy level of products lower than that of reactants OR energy released in bond formation > energy used to break bonds (1) kinetic: high activation energy (1) because strong NN (1) [confusion between thermodynamic and kinetic loses first 2 marks].

Acceptable Answers

Reject Just average bond enthalpies used

Mark 1

Question Number 2.(b)(iii)

Acceptable Answers H negative / reaction exothermic

Reject

Mark 3

because NN is 944/ total bond breaking energy is high/2252(kJmol-1 )

Question Number 2.(c)(i) Q W C

Correct Answer One way temperature increase therefore molecules have greater (average kinetic) energy (1) more molecules/collisions have E Eact (1) Therefore a greater proportion of/ more of the collisions are successful (1) Ignore greater frequency of collision Another way addition of (iron) catalyst (1) provides alternative route of lower activation energy (1) EITHER: A greater proportion of /more of the molecules/collisions have E Ecat/a greater proportion of collisions are successful OR provides (active) sites (where reactant molecules can bond / be adsorbed) (1) Ignore any answers referring to pressure or concentration. Do not penalise just more collisions are successful more than once

Acceptable Answers

Reject

Mark 6

moving faster E > Eact particles for molecules greater frequency of successful collisions/ more successful conditions per unit time platinum catalyst just more successful collisions

incorrect catalyst

just more successful collisions

Question Number 2.(c)(ii) Q W C

Correct Answer Decrease temperature(1) because (forward) reaction exothermic (1) increase pressure (1) because more moles (of gas) on left (1)

Acceptable Answers Low temperature H is negative High pressure Molecules for moles

Reject Answer based on endothermic reaction scores 0

Mark 4

Question Number 2.(d)(i) Question Number 2.(d)(ii)

Correct Answer (cool to) condense / liquefy OR cool to below critical temperature Correct Answer Recycle (the unreacted gases) OWTTE

Acceptable Answers

Reject Just cool

Mark 1 Mark 1

Acceptable Answers

Reject

Question Number 3.(a)(i)

Correct Answer 2-bromobutane the 2 must be in front of bromo Ignore punctuation and capitals Correct Answer CH3CHBrCH2CH3 + KOH CH3CHOHCH2CH3 + KBr OR CH3CHBrCH2CH3 + OH- CH3CHOHCH2CH3 + BrCorrect Answer water / H2O / aqueous ethanol

Acceptable Answers

Reject

Mark 1

Question Number 3.(a)(ii)

Acceptable Answers C2H5 instead of CH2CH3 Allow K+ as spectator ion

Reject eqns with NaOH

Mark 1

Question Number 3.(a)(iii)

Acceptable Answers C2H5OH (aq) / aqueous alcohol/KOH(aq)/aqueous Do not penalise use of NaOH(aq) again Acceptable Answers reasonable phonetic spelling Acceptable Answers

Reject just ethanol / ethanolic / alcoholic (KOH)

Mark 1

Question Number 3.(a)(iv) Question Number 3.(b)(i)

Correct Answer nucleophilic substitution (both needed) Correct Answer CH3CHBrCH2CH3 + OH- CH3CH=CHCH3 + H2O + BrOR CH3CHBrCH2CH3 + OH- CH2=CHCH2CH3 + H2O + BrDouble bond need not be shown Correct Answer Ethanol / C2H5OH / CH3CH2OH /

Reject

Mark 1

Reject

Mark 1

C2H5 instead of CH2CH3 Ignore spectator ions Acceptable Answers Alcohol OR Ethanolic/alcoholic KOH/NaOH C2H6O Any mention of water/aqueous Reject Mark 1

Question Number 3.(b)(ii)

H H H C C OH H H

Question Number 3.(b)(iii)

Correct Answer elimination ignore nucleophilic

Acceptable Answers

Reject electrophilic elimination

Mark 1

Question Number 3.(c)(i)

Correct Answer

Acceptable Answers

Reject

Mark 1

CH3 C C H

CH3 H

CH3 C C H

H CH3

bond to H of CH3 on left carbon structure with 90o bond angles Acceptable Answers limited rotation on the carbon Reject

Question Number 3.(c)(ii)

Correct Answer no / restricted rotation around double bond / C=C / bond (1) has two different groups joined to each C (of double bond) OR each (carbon of C=C) has a CH3 and a H (1)

Mark 2

Question Number 3.(d)(i)

Correct Answer nickel / Ni OR platinum / Pt OR palladium / Pd Correct Answer butane / CH3CH2CH2CH3 Correct Answer CH3CHBrCH2CH3 + 2NH3 CH3CHNH2CH2CH3 + NH4Br OR CH3CHBrCH2CH3 + NH3 CH3CHNH3+CH2CH3 + BrCorrect Answer excess / concentrated / ethanolic ammonia Correct Answer 74.4: 14.7: 10.9 (1)(= 6.2: 14.7: 0.779) 12 1 14 6.2 : 14.7 : 0.779 = 8:19:1 0.779 0.779 0.779 so C8H19N (1)

Acceptable Answers

Reject

Mark 1

Question Number 3.(d)(ii) Question Number 3.(e)(i)

Acceptable Answers C2H5 for CH3CH2 Acceptable Answers CH3CHBrCH2CH3 + NH3 CH3CHNH2CH2CH3 + HBr

Reject JUST C4H10 Reject

Mark 1 Mark 1

Question Number 3.(e)(ii)

Acceptable Answers heat in sealed tube

Reject Just heat Just sealed tube Reject dividing by atomic number scores zero

Mark 1

Question Number 3.(e)(iii)

Acceptable Answers Correct answer alone scores (2)

Mark 2

Question Number 4.(a)(i)

Correct Answer Hf (products) - Hf (reactants) / [(-394) + (2 x 242)] (-75) (1) = -803 (kJ mol-1) (1)

Acceptable Answers correct answer without working scores (2) -561 (kJ mol-1) scores (1) Acceptable Answers Reverse argument Water is not in its standard state Acceptable Answers converse argument

Reject any positive value scores zero

Mark 2

Question Number 4.(a)(ii)

Correct Answer (under standard conditions) water condenses / is a liquid (more heat evolved)

Reject Any answer in terms of average bond energies Just conditions are not standard Reject

Mark 1

Question Number 4.(a)(iii) Q W C

Correct Answer Any 4 of: H2 better because: cheaper per kJ (1) more energy per gram / less weight/mass to carry for same energy (1) no CO2 /only H2O produced (at point of use) (1) H2 worse because: gas storage needs pressurised/large containers (1) which are heavy (1) needs to be cooled to very low temperature to be liquefied (1) Ignore problems with refuelling

Mark 4

Just cheaper Just more energy Just hard to store

Hydrogen is flammable/ dangerous/explosive

Question Number 4.(b)(i)

Correct Answer

Acceptable Answers

Reject

Mark 2

H H C C H

H O
(1)

H H C C H

O O H
(1)

-OH for O H

Question Number 4.(b)(ii)

Correct Answer structural formula of any tertiary alcohol (1) and its name (1) - must not contradict the formula and conditional on tertiary alcohol Correct Answer (concentrated) sulphuric acid / H2SO4 OR phosphoric acid / H3PO4 OR aluminium oxide/Al2O3 Correct Answer -CH2CH2- (1) Correct Answer bags / bottles / packaging / (food) containers / buckets / bowls

Acceptable Answers 2nd mark can be awarded if minor slip in formula or no formula given

Reject

Mark 2

Question Number 4.(c)

Acceptable Answers

Reject Dilute H2SO4 Or H2SO4(aq) Or Dilute H3PO4

Mark 1

pumice 50% H2SO4 Acceptable Answers -(-CH2 - CH2 -)nAcceptable Answers Electrical insulation /cling film/water pipes Reject - CH2 Reject Clothing, light fittings, ropes Mark 1 Mark 1

Question Number 4.(d)(i) Question Number 4.(d)(ii)