GCE 2004 June Series abc Mark Scheme Chemistry (Subject Code CHM5) Mark schemes are prepared by the Principal Examiner and considered, together with the relevant questions, by a panel of subject teachers. This mark scheme includes any amendments made at the standardisation meeting attended by all examiners and is the scheme which was used by them in this examination. The standardisation meeting ensures that the mark scheme covers the candidates responses to questions and that every examiner understands and applies it in the same correct way. As preparation for the standardisation meeting each examiner analyses a number of candidates scripts: alternative answers not already covered by the mark scheme are discussed at the meeting and legislated for. If, after this meeting, examiners encounter unusual answers which have not been discussed at the meeting they are required to refer these to the Principal Examiner. It must be stressed that a mark scheme is a working document, in many cases further developed and expanded on the basis of candidates reactions to a particular paper. Assumptions about future mark schemes on the basis of one year s document should be avoided; whilst the guiding principles of assessment remain constant, details will change, depending on the content of a particular examination paper.
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Mark Scheme Advanced Level - Chemistry CHM5 Thermodynamics and Further Inorganic Chemistry SECTION A Question (a) (i) H atomisation/sublimation of magnesium (ii) Bond/dissociation enthalpy of Cl-Cl OR 2 H atomisation of chlorine (iii) Second ionisation enthalpy of magnesium (iv) 2 electron affinity of chlorine (v) Lattice formation enthalpy of MgCl 2 (b) Equation 2MgCl(s) MgCl 2 (s) + Mg(s) State symbols not required but penalise if incorrect Calculation H reaction = Σ H f products Σ H f reactants = 653 (2 33) = 427 (kjmol ) Allow +427 to score () mark Other answers; award () for a correct H reaction expression (c) H soln MgCl 2 = H Lat.form. + H hyd.mg 2+ + 2 H hyd.cl or cycle = 2502 920 (2 364) = 46 (kjmol ) Allow + 46 to score () mark Other answers; award () for a correct H soln MgCl 2 expression/cycle Total 2 3
Chemistry Advanced Level Mark Scheme Question 2 Each section to be marked independently (a) (i) Ionic (ii) Sodium/Na (iii) Na 2 O + H 2 O 2NaOH Ignore state symbols (b) (i) Covalent (ii) Phosphorus/P (iii) H 3 PO 4 or other acid with P in oxidation state (V) or (III) (c) (i) Macromolecular/giant covalent/giant molecular (ii) Silicon/Si (iii) e.g. CaO + SiO 2 CaSiO 3 Base Balanced Total 0 4
Mark Scheme Advanced Level - Chemistry Question 3 (a) (i) Orange (ii) Red-violet/ruby/violet/ green (iii) Purple (b) Fe 2+ or Fe(II) (c) (i) 6 or (VI) (ii) 3 or (III) (d) (i) 2+ MnO 4 / Mn has a more positive E ο value than Cl 2 / Cl or data used and will oxidise Cl or change Cl to Cl 2 Allow converse answers (ii) NO3 / HNO 2 has a more positive E ο value then or data used Fe 3+ 2+ / Fe and will oxidise 2+ Fe 2+ or change Fe to 3+ Fe (e) (i) 0.5 (ii) 2 + + 2Mn + 8H 2O + 5S2O8 0SO4 + 2MnO4 + 6H Both SO4 and MnO 4 on right Balanced Total 3 5
Chemistry Advanced Level Mark Scheme Question 4 (a) (i) An atom, ion or molecule which can donate a lone electron pair (ii) A central metal ion/species surrounded by co-ordinately bonded ligands or ion in which co-ordination number exceeds oxidation state (iii) The number of co-ordinate bonds formed to a central metal ion or number of electron pairs donated or donor atoms (b) (i) (ii) Allow the reverse of each substitution 2+ 2+ [ Co(H2 O) 6 ] + 6NH3 [Co(NH3) 6 ] + 6H2O Complex ions Balanced Allow partial substitution 2+ [ Co(H2 O) 6 ] + 4Cl CoCl4 + 6H2O Complex ions Balanced or H 2 O or NH 3 or C 2 O 2-4 by Cl - e.g. (iii) 2+ 4 [ Co(H2 O) 6 ] + 3C 2O4 [Co(C2O4 ) 3] + 6H2O Complex ions Balanced Allow all substitution except (i) NH 3 by H 2 O (ii) more than 2Cl - substituted for NH 3 or H 2 O e.g. (iv) 2+ 4 [ Co(H2 O) 6 ] + EDTA [Co(EDTA)] + 6H2O Complex ions Balanced or H 2 O or NH 3 by C 2O 4 and NH 3 orcl 4 by EDTA (c) (i) [Fe(H 2 O) 6 ] 2+ (ii) Fe(OH) 2 or Fe(OH) 2 (H 2 O)x where x = 0 to 4 (iii) Fe 2+ is oxidised to Fe 3+ or Fe(OH) 3 By oxygen in the air Total 5 6
Mark Scheme Advanced Level - Chemistry Question 5 (a) A catalyst in the same phase/phase as the reactants (b) (i) A reaction in which a product acts as a catalyst (ii) Mn 2+ or Mn 3+ Self-catalysing not allowed (c) (i) 2CO + 2NO 2CO 2 + N 2 or 4CO + 2NO 2 4CO 2 + N 2 C not allowed as a product Reducing agent CO (ii) Pt, Pd or Rh Deposited on a ceramic honeycomb or matrix or mesh or sponge To increase surface area of catalyst (d) (i) Reactants cannot move on surface or products not desorbed or Active sites blocked (ii) Reactants not brought together or No increase in reactant concentration on catalyst surface or Reactants not held long enough for a reaction to occur or Reactant bonds not weakened Total 0 7
Chemistry Advanced Level Mark Scheme Question 6 (a) FeCl 3 is a Lewis acid Accepts electron pairs (from water) [Fe(H 2 O) 6 ] 3+ is a Bronsted-Lowry acid Donated protons NB mark separately (b) (i) K a = [{ Fe(H 2 O) 5 (OH)} 2+ ][H + ]/ [{Fe(H 2 O) 6 } 3+ ] NB [ ] essential ph = log[h + ] or [H + ] = 3.02 0 2 Hence [H + ] = [{ Fe(H 2 O) 5 (OH)} 2+ ] K a = [H + ] 2 / [{Fe(H 2 O)} 3+ ] (= 3.02 0 2 ) 2 / 0.5) K a = 6.08 0 3 (Allow 6.0 to 6. 0 3 ) pk a = 2.22 (3 significant figures needed but ignore units) NB allow value of pk a consequentially to value of K a allow pk a = - logk a () if stated but no value of pk a calculated (ii) Mark consequentially to the value of K a obtained in (b)(i) New [{Fe(H 2 O)} 3+ ] = 0.250/4 (= 0.0625) K a = 6.08 0 3 = [H + ] 2 / 0.0625 [H + ] = (6.08 0 3 0.0625) (= 3.80 0 4 ) ph = log 0.0949 =.7 NB (i) Using the given value of K a = 4.50 0 3 and 0.0625 [H + ] = 0.068 and ph =.78 (Scores the full 3 marks) (ii) Penalise two marks if [{Fe(H 2 O)} 3+ ] = 0.250/n where n 4 Allow [H + ] = (6.08 0 3 0.250/n ) () (iii) Using K a = 4.50 0 3 and 0.0833 ph =.7 BEWARE of this answer it scores only mark (c) Fe 2+ ion has a smaller charge to size ratio or charge density Less polarising than Fe 3+ / less weakening effect on O - H bonds or Hydrolysis equilibrium displace more to the left Total 5 8
Mark Scheme Advanced Level - Chemistry SECTION B Question 7 (a) HOOC COOH \ / C = C / \ H H HOOC H \ / C = C / \ H COOH NB The bonds shown in the structure must be to correct Isomerism: Geometric or cis-trans If written answer is correct, ignore incorrect labelling of structures. If no written answer, allow correctly labelled structures. Both COOH groups must be on the same side/ close together/ cis. No rotation about C=C axis Structure H O \ // C - C O C - C / \\ H O Allow HC C O O HC C O 9
Chemistry Advanced Level Mark Scheme (b) Br 2 / HBr / H 2 SO 4 / H + / Br + / NO 2 + (Mark M) HOOC COOH C = C () M2 H H H () M3 HOOC COOH M4() + C C H H H HOOC COOH Br C C H H H 4 Br :Br - NB If electrophile H + / Br + / NO + 2 allow M, M2 and M4 If the acid is incorrect, M2 and M3 can still be scored Allow M4 consequentially if a repeat error from part (a) (c) e.g. 2NaOH + HO 2 CCHCHCO 2 H NaO 2 CCHCHCO 2 Na +2H 2 O Both H replaced Balanced for atoms and charges NB Allow ionic equations and 2NaOH + C 4 H 4 O 4 C 4 H 2 O 4 Na 2 + 2H 2 O Allow one if structure incorrect but molecular formula correct Allow one for a correct equation showing one H replaced (d) M Two peaks M2 No splitting or singlets M3 (Two) non-equivalent protons or two proton environments M4 No adjacent protons M5 Same area under the two peaks or same relative intensity Max 3 NB Doublet could score M and M3 or M5 (Max 2) More than two peaks CE = 0 Apply the list principle to incorrect answers if more than 3 given Total 5 0
Mark Scheme Advanced Level - Chemistry Question 8 (a) M K p =( P Y) 3. ( P Z) 2 / ( P W) 2.( PX) NB [ ] wrong M2 temperature M3 increase M4 particles have more energy or greater velocity/speed M5 more collisions with E > Ea or more successful collisions M6 reaction exothermic or converse M7 equilibrium moves in the left Marks for other answers Increase in pressure or concentration allow M, M5, M6 Max 3 Addition of a catalyst; allow M, M5, M6 Max 3 Decrease in temperature; allow M, M2, M6 Max 3 Two or more changes made; allow M, M6 Max 2 (b) (i) Advantage; reaction goes to completion, not reversible or faster Disadvantage; reaction vigorous/dangerous (exothermic must be qualified) or HCl(g) evolved/toxic or CH 3 COCl expensive NB Allow converse answers Do not allow reactions with other reagents e.g. water or ease of separation (ii) S = ΣS products ΣS reactants S = (259 + 87) (20 + 6) S = 84 (JK mol ) (Ignore units) Allow 84 to score () mark G = H T S = 2.6 298 84/000 = 46.6 kj mol or 46 600 J mol Allow (2) for 46.6 without units (Mark G consequentially to incorrect S) (e.g. S = 84 gives G = +3.4 kj mol ) Total 5
Chemistry Advanced Level Mark Scheme Question 9 (a) Mg + 2HCl MgCl 2 + H 2 MgO + 2HCl MgCl 2 + H 2 O Allow ionic equations (b) Hydrogen collection Using a gas syringe or measuring cylinder/ graduated vessel over water Allow if shown in a diagram Measurements (i) P (ii) T (iii) V Use ideal gas equation to calculate mol hydrogen or mass/mr Mol H 2 = mol Mg (Mark consequentially to equation) (c) MgCl 2 + 2NaOH Mg(OH) 2 + 2NaCl Species Balanced Allow an ionic equation Mg(OH) 2 MgO + H 2 O 2
Mark Scheme Advanced Level - Chemistry (d) Allow 2 significant figures in these calculations and ignore additional figures. EITHER Mol MgO obtained stage 2 = mass MgO/MrMgO = 6.4/ 40.(3) = 0.59 Allow 0.6 Allow method mark if formula of magnesium oxide or M r incorrect. Moles of Mg = moles of H 2 hence Mol original MgO = mol MgO from stage 2 - mol H 2 = 0.59-0.0528 = 0.06 Allow 0. Mark consequentially to moles of magnesium oxide determined above OR Mass MgO formed from Mg = 0.0528 M r MgO {or 40.(3)} () = 2.3 g Allow 2. Allow method mark if formula of magnesium oxide or Mr incorrect. () Mass original MgO = total mass MgO - mass formed from Mg () = 6.4 2.3 = 4.28 g Allow 4.3 () Mark consequentially mass of magnesium oxide determined above NB As there is an error in part (d), the mass of sample should have been 6.25 NOT 2.65, award full marks to any candidate who has crossed out their correct first answer. Total 5 3