Marking Guidance Mark Comments

Transcription

1 Energetics Answers

2 Chemistry - AQA GCE Mark Scheme 200 June series Q Part Sub Part Marking Guidance Mark Comments a i M drawn curve starts at reactants and ends at products M2 curve peak is below the one drawn in the question (and may show one/two humps) 2 Tapered lines into the original curve gain credit for M Mark M and M2 independently a ii Exothermic (reaction) Ignore ΔH is negative a iii bond (enthalpy) reactants < bond (enthalpy) products The sum for H 2 and I 2 / reactants is less than / lower than / smaller than the sum for 2HI / products The sum for 2HI /products is more than / larger than / bigger than the sum for H 2 and I 2 / reactants Accept It the sum will be smaller or less a iv M p M2 (q p) p q q + p 2 M2 demands that the sign for an exothermic reaction is part of the outcome mathematically. Ignore case

3 Chemistry - AQA GCE Mark Scheme 200 June series b i Increase / speed up / faster (rate of attainment of equilibrium) Credit It took less time Increase / speed up / faster rate of both forward and reverse reaction Increase / speed up / faster rate of reaction b ii M Increase / speed up / faster (rate of attainment of equilibrium) M2 More particles / molecules in a given volume / space the particles / molecules are closer together an increase in concentration. 3 If M is blank, mark on and credit M in the text If M is given as decrease / no effect / no change then CE= 0 for clip M3 More / higher chance of successful / effective / productive collisions (between particles) more collisions / higher chance of collisions (of particles) with E>E Act In M, if increase both the forward and reverse reaction, but no mention of rate, penalise M but mark on. In M, if increase either forward rate or reverse rate only, then penalise M but mark on. Penalise M3 if an increase in the value of E Act / energy of particles is stated. Max for M2 and M3 if reference to atoms

4 Chemistry - AQA GCE Mark Scheme 200 June series 4 c i M (could be scored by a correct mathematical expression) 3 Full marks for correct answer. ΔH = ΔH products ΔH reactants Ignore units. a correct cycle of balanced equations M and M2 can be scored with correct moles as follows ΔH + 2( 46) = ( 286) Deduct one mark for an arithmetic error. ΔH 92 = 776 ΔH = M3 ΔH = 684 (kj mol ) (This is worth 3 marks) Award mark ONLY for c ii The value is quoted at a pressure of 00 kpa bar or 0 5 Pa Ignore atmosphere / 0 kpa Ignore constant pressure All reactants and products are in their standard states / their normal states at 00 kpa or bar

5 CHEM2 - AQA GCE Chemistry 200 Mark Scheme January series Q Part Sub Part Marking Guidance Mark Comments 2 a Heat (energy) change at constant pressure Ignore references to standard conditions, but credit specified pressure. 2 b The enthalpy change / heat (energy) change (at constant pressure) in a reaction is independent of the route / path taken (and depends only on the initial and final states) 2 c ΔH = ΔH = 507 (M) ΔH = (M and M2) ΔH = 470 (kj mol ) Award mark for Award full marks for correct answer Ignore units. Ignore numbers on the cycle M and M2 can score for an arithmetic error 5

6 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action June 20 Question Marking Guidance Mark Comments (a) M The activation energy is the minimum / least / lowest energy M2 (energy) for a reaction to occur / to go / to start (energy) for a successful / effective collision 2 Mark independently Ignore heat and ignore enthalpy Ignore breaking the bonds (b) M Catalysts provide an alternative route an alternative mechanism alternative / different path(way) 2 Mark independently Ignore reference to surface M2 Lowers the activation energy (c)(i) Stay(s) the same (c)(ii) Increases Credit increase or increased (c)(iii) Increases Credit increase or increased (c)(iv) Stay(s) the same 3

7 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action June 20 (d)(i) M yeast or zymase M2 ethanol 2 Ignore enzyme In M2, ignore alcohol and ignore any formula (d)(ii) M (Concentrated) H 3 PO 4 (Concentrated) H 2 SO 4 M2 butan-2-ol 2 Credit correct names Ignore hydrogenphosphate or hydrogensulfate Ignore dilute or aq Do not penalise absence of hyphens in name. In M2, ignore any formula 4

8 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action June 20 Question Marking Guidance Mark Comments 2(a) One from Ti is not produced TiC / carbide is produced titanium reacts with carbon Product is brittle Product is a poor engineering material Penalise titanium carbonate Ignore impure titanium Credit titanium is brittle 2(b) Heat (energy) change at constant pressure QoL 2(c) The enthalpy change in a reaction is independent of the route taken (and depends only on the initial and final states) Credit heat change at constant pressure as an alternative to enthalpy change 5

9 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action June 20 2(d) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / product 3 For M, credit correct reference to molecule/s or atom/s M2 is formed from its (constituent ) elements M3 with all reactants and products / all substances in standard states all reactants and products / all substances in normal states under standard conditions / 00 kpa / bar and any specified T (usually 298 K) Ignore reference to atmosphere 2(e)(i) Na / it is not in its standard state / normal state under standard conditions Standard state / normal state under standard conditions for Na is solid / (s) QoL Ignore sodium is a liquid or sodium is not a solid 6

10 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action June 20 2(e)(ii) M H r = H f (products) - H f (reactants) M2 H r = 4( 4) ( 720) 4(+3) = (This also scores M) M3 = 936 (kj mol - ) 3 Correct answer gains full marks Credit mark for (kj mol - ) Credit mark for 924 (kj mol - ) i.e. assuming value for Na(l) = 0 For other incorrect or incomplete answers, proceed as follows check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication; this would score 2 marks (M and M2) If no AE, check for a correct method; this requires either a correct cycle with 2Cl 2 and 4Na a clear complete statement of M which could be in words and scores only M 2(e)(iii) Reducing agent reductant reduces TiCl 4 electron donor Ignore reduces titanium 7

11 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry in Action June 202 Question Marking Guidance Mark Comments 2(a)(i) M (could be scored by a correct mathematical expression which must have all ΔH symbols and the or SUM) M ΔH r = ΔH f (products) - ΔH f (reactants) a correct cycle of balanced equations with C, 3H 2 and O 2 M2 ΔH r = 20 + ( 242) ( 394) ΔH r = ΔH r = (This also scores M) M3 = 49 (kj mol - ) (Award mark ONLY for + 49) 3 Correct answer gains full marks Credit mark ONLY for + 49 (kj mol - ) For other incorrect or incomplete answers, proceed as follows check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication; this would score 2 marks (M and M2) If no AE, check for a correct method; this requires either correct cycle of balanced equations with C, 3H 2 and O 2 a clear statement of M which could be in words and scores only M 2(a)(ii) It is an element / elemental By definition Ignore reference to standard state 5

12 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry in Action June 202 2(d)(iii) 3H 2 + ½O 2 3H 2 O 2H 2 + O 2 2H 2 O Ignore state symbols Accept multiples Extra species must be crossed through 2(e) M q = m c ΔT q =40 x 4.8 x 7.5 M2 = 4389 (J) (kj) 4.39 (kj) 4.4 (kj) (also scores M) M3 Using 0.00 mol therefore ΔH = 399 (kjmol - ) Award full marks for correct answer Ignore the case for each letter Penalise M3 ONLY if correct numerical answer but sign is incorrect; +399 gains 2 marks Penalise M2 for arithmetic error and mark on In M, do not penalise incorrect cases in the formula If ΔT = 280.5; score q = m c ΔT only If c = 4.8 (leads to ) penalise M2 ONLY and mark on for M3 = or +400 gains 2 marks Ignore incorrect units 8

13 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 202 Question Marking Guidance Mark Comments 4(a)(i) reduction reduced redox reduction-oxidation Not oxidation alone 4(a)(ii) Fe e Fe Ignore state symbols Do not penalise absence of charge on electron Credit Fe 3+ Fe 3e Credit multiples 4(b)(i) Because (one of the following) CO is not the only product (Some) complete combustion (also)occurs CO 2 is (also) formed Further oxidation occurs Reference to incomplete combustion to form CO does not answer the question 4(b)(ii) The enthalpy change / heat (energy) change at constant pressure in a reaction is independent of the route / path taken (and depends only on the initial and final states) 7

14 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 202 4(b)(iii) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / element M2 is burned completely / undergoes complete combustion in (excess) oxygen M3 with all reactants and products / all substances in standard states all reactants and products / all substances in normal / specified states under standard conditions / 00 kpa / bar and specified T / 298 K 3 For M, credit correct reference to molecule/s or atom/s For M3 Ignore reference to atmosphere 4(c) M (could be scored by a correct mathematical expression which must have all ΔH symbols and the ) M ΔH r = ΔH f (products) - ΔH f (reactants) correct cycle of balanced equations with 2Fe, 3C and 3O 2 M2 ΔH r = 2(+4) + 3( 394) ( 822) 3( ) = (This also scores M) M3 = (+) (kj mol - ) (Award mark ONLY for ) (Award mark ONLY for 27) 3 Correct answer gains full marks Credit mark ONLY for (kj mol - ) Credit mark ONLY for 27 (kj mol - ) i.e. assuming value for Fe(l) = 0 For other incorrect or incomplete answers, proceed as follows check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication; this would score 2 marks (M and M2) If no AE, check for a correct method; this requires either a correct cycle with 2Fe, 3C and 3O 2 a clear statement of M which could be in words and scores only M 4(d)(i) C(s) + O 2 (g) CO 2 (g) State symbols essential Possible to include C(s, graphite) 8

15 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 202 4(d)(ii) These two enthalpy changes are for the same reaction / same equation / same reactants and products They both make one mole of carbon dioxide only from carbon and oxygen (or this idea clearly implied) The same number and same type of bonds are broken and formed Penalise reference to CO 2 being produced by a different route both form CO 2 is not sufficient (since other products might occur e.g.co) 9

16 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 203 Question Marking Guidance Mark Comments 3(a)(i) M (could be scored by a correct mathematical expression which must have all ΔH symbols and the or SUM) M ΔH = ΔH f (products) - ΔH f (reactants) a correct cycle of balanced equations M2 ΔH = 3( 394) 3( ) ( 97) (This also scores M) M3 = (+) 22 (kj mol - ) Award mark ONLY for 22 3 Correct answer gains full marks Credit mark ONLY if 22 (kj mol - ) For other incorrect or incomplete answers, proceed as follows check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication; this would score 2 marks (M and M2) If no AE, check for correct method; this requires either a correct cycle of balanced equations a clear statement of M which could be in words and scores M only 3(a)(ii) 3(b)(i) By definition Because it is an element / elemental TiO 2 + 2Cl 2 + 2C TiCl 4 + 2CO TiO 2 + 2Cl 2 + C TiCl 4 + CO 2 Ignore reference to standard state 2 Allow multiples Ignore state symbols M use of Cl 2 and C M2 a correct balanced equation 8

17 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 203 Question Marking Guidance Mark Comments 4(a) The enthalpy change / heat (energy) change (at constant pressure) in a reaction is independent of the route / path taken (and depends only on the initial and final states) Ignore the use of H for enthalpy 4(b) H exp + H 2 H = 0 Any correct mathematical statement that uses all three terms H exp + H 2 = H H = H exp + H 2 H exp = H H 2 H exp = H +( H 2 ) 4(c) H exp = H H 2 H exp = 56 2 = 68 (kj mol - ) Award the mark for the correct answer without any working Ignore units 0

18 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 203 4(d)(i) M q = m c ΔT calculation (25.0 x 4.8 x 4.0) M2 = 463 J.46 kj (This also scores M) M3 must have both the correct value within the range specified and the minus sign For mol, therefore ΔH = to (kj mol - ) ΔH = 69.7 to 69.5 (kj mol - ) Accept answers to 3sf or 4sf in the range 69.7 to 69.5 Ignore -70 after correct answer 3 Award full marks for correct answer In M, do not penalise incorrect cases in the formula Penalise M3 ONLY if correct numerical value but sign is incorrect; e.g to gains 2 marks (ignore +70 after correct answer) Penalise M2 for arithmetic error but mark on ΔT = 287, score q = m c ΔT only If c = 4.8 (leads to 684 J ) penalise M2 ONLY and mark on for M3 = 80.7 (range 80.0 to 80.2) Ignore incorrect units 4(d)(ii) The idea of heat loss Incomplete reaction (of the copper sulfate) Not all the copper sulfate has dissolved NOT impurity NOT incompetence NOT incomplete combustion

19 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 203 4(e) Impossible to add / react the exact / precise amount of water Very difficult to measure the temperature rise of a solid Difficult to prevent solid dissolving (Copper sulfate) solution will form Not just the reaction is incomplete 2

20 Mark Scheme General Certificate of Education (A-level) Chemistry Unit 2: Chemistry In Action January 203 Question Marking Guidance Mark Comments 6(a) 3N 2 H 4 4NH 3 + N 2 Or multiples Ignore state symbols 6(b) M enthalpy / heat (energy) change / required / needed to break / dissociate a covalent bond (or a specified covalent bond) 2 Ignore bond making Ignore standard conditions M2 requires an attempt at M M2 average / mean over different molecules / compounds / substances 6(c) M (bonds broken) (bonds formed) = ΔH Sum of bonds broken Sum of bonds formed = ΔH M2 (also scores M) 4(+388) (46) + 4(463) 944 8(463) = ΔH broken (2007) formed 4648 (2796) M3 ΔH = 789 (kj mol ) Award mark for Students may use a cycle and gain full marks 3 M could stand alone Award full marks for correct answer Ignore units Two marks can score with an arithmetic error in the working Credit one mark only for calculating either the sum of the bonds broken or the sum of the bonds formed provided this is the only mark that is to be awarded 7

21 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 204 3(c)(i) M (could be scored by a correct mathematical expression) M ΔH r = ΔH f (products) - ΔH f (reactants) 3 Correct answer gains full marks. Credit mark for +04 (kj mol ). a correct cycle of balanced equations / correct numbers of moles M2 = 2(+20) + 3( 394) ( 705) 3( ) = = 42 ( 038) (This also scores M) M3 = 04 (kj mol ) For other incorrect or incomplete answers, proceed as follows: check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication; this would score 2 marks. If no AE, check for a correct method; this requires either a correct cycle with 3CO, 2Sb and 3CO 2 a clear statement of M which could be in words and scores only M. (Award mark ONLY for + 04) 3(c)(ii) It / Sb is not in its standard state Standard state (for Sb) is solid / (s) (Sb) liquid is not its standard state Credit a correct definition of standard state as an alternative to the words standard state. QoL 3(c)(iii) Reduction reduced redox 8 of 36

22 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 204 3(d) Low-grade ore extraction / it uses (cheap) scrap / waste iron / steel is a single-step process uses / requires less / low(er) energy Ignore references to temperature / heat or labour or technology. 9 of 36

23 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 204 9(b) M q = m c ΔT (this mark for correct mathematical formula) 5 Full marks for M, M2 and M3 for the correct answer. M2 = ( ) 724 (J).724 (kj).72 (kj).7 (kj) (also scores M) In M, do not penalise incorrect cases in the formula. Ignore incorrect units in M2 M3 Using mol therefore ΔH = 78 (kj mol ) (Accept a range from 708 to 79 but do not penalise more than 3 significant figures) M4 and M5 in any order Any two from incomplete combustion heat loss heat capacity of Cu not included some ethanol lost by evaporation not all of the ( mol) ethanol is burned / reaction is incomplete Penalise M3 ONLY if correct numerical answer but sign is incorrect. Therefore +78 gains two marks. If units are quoted in M3 they must be correct If ΔT = 278.5, CE for the calculation and penalise M2 and M3 If c = 4.8 (leads to 984) penalise M2 ONLY and mark on for M3 = of 36

24 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 204 9(c)(i) M enthalpy / heat / energy change (at constant pressure) or enthalpy / heat / energy needed in breaking / dissociating (a) covalent bond(s) M2 averaged for that type of bond over different / a range of molecules / compounds 2 Ignore bond making. Ignore reference to moles. 28 of 36

25 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 204 9(c)(ii) M B(reactants) B(products) = ΔH Sum of bonds broken Sum of bonds formed = ΔH B(C-C) + B(C-O) + B(O-H) + 5B(C-H) + 3B(O=O) 4B(C=O) 6B(O H) = ΔH = 279 M2 (also scores M) (42)+ 3B(O=O) (323) (or 2768 if O H cancelled) 4(805) 6(463) = ΔH = 279 (5998) (or 5535 if O H cancelled) 3B(O=O) = 488 (kj mol ) M3 B(O=O) = 496 (kj mol ) 3 Correct answer gains full marks. Credit mark for 496 (kj mol ) For other incorrect or incomplete answers, proceed as follows check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication; this would score 2 marks (M and M2). If no AE, check for a correct method; this requires either a correct cycle with 2CO 2 and 3H 2 O a clear statement of M which could be in words and scores only M. Credit a maximum of one mark if the only scoring point is bonds formed adds up to 5998 (or 5535) bonds broken includes the calculated value of 323 (or 2768). Award mark for 496 Students may use a cycle and gain full marks 29 of 36

26 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 205 7(c) M q = m c ΔT q = M2 M3 = (±) 506 (J) 5.06 (kj) 5.02 (kj) (also scores M) This mark is for dividing correctly the number of kj by the number of moles and arriving at a final answer in the range shown. Using mol therefore ΔH = 5 (kj mol - ) 4.6 to 20 (kj mol - ) Range (+)4.6 to (+)20 gains 2 marks BUT 0 gains 3 marks and +0 gains 2 marks AND 00 gains 3 marks and +00 gains 2 marks 3 Award full marks for correct answer In M, do not penalise incorrect cases in the formula Penalise M3 ONLY if correct numerical answer but sign is incorrect; (+)4.6 to (+)20 gains 2 marks Penalise M2 for arithmetic error and mark on If ΔT = 28; score q = m c ΔT only If c = 4.8 (leads to 5772) penalise M2 ONLY and mark on for M3 = 283 Ignore incorrect units in M2 If units are given in M3 they must be either kj or kj mol in this case 20 of 36

27 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 205 7(d) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / element 3 If standard enthalpy of formation CE=0 M2 is burned / combusts / reacts completely in oxygen burned / combusted / reacted in excess oxygen M3 with (all) reactants and products / (all) substances in standard / specified states (all) reactants and products / (all) substances in normal states under standard conditions / 00 kpa / bar and specified T / 298 K For M3 Ignore reference to atmosphere 2 of 36

28 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 205 7(e) M B(reactants) B(products) = ΔH Sum of bonds broken Sum of bonds formed = ΔH 2B(C-C) + B(C=O) + 6B(C-H) + 4B(O=O) (LHS) 6B(C=O) 6B(O H) (RHS) = ΔH M2 (also scores M) 2(348)+805+6(42)+4(496) [LHS = 5957] (696) (2472) (984) 6(805) 6(463) [RHS = ( )7608] = ΔH (4830) (2778) using only bonds broken and formed ( ) M3 ΔH = 65 (kj mol ) Candidates may use a cycle and gain full marks. 3 Correct answer gains full marks Credit mark for (+) 65 (kj mol - ) For other incorrect or incomplete answers, proceed as follows check for an arithmetic error (AE), which is either a transposition error or an incorrect multiplication / addition error; this would score 2 marks (M and M2) If no AE, check for a correct method; this requires either a correct cycle with 4O 2, 3CO 2 and 3H 2 O a clear statement of M which could be in words and scores only M Allow a maximum of one mark if the only scoring point is LHS = 5957 (or 552) RHS = 7608 (or 6803) Award mark for of 36

29 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 205 7(f) For the two marks M and M2, any two from heat loss or not all heat transferred to the apparatus or heat absorbed by the apparatus or (specific) heat capacity of the apparatus not considered incomplete combustion / not completely burned / reaction is not complete The idea that the water may end up in the gaseous state (rather than liquid) reactants and/or products may not be in standard states. MBE data refers to gaseous species but the enthalpy of combustion refers to liquids in their standard states / liquid propanone and liquid water in standard states MBE do not refer to specific compounds MBE values vary with different compounds / molecules are average / mean values taken from a range of compounds / molecules 2 Apply the list principle but ignore incomplete reasons that contain correct chemistry Ignore evaporation Ignore faulty equipment Ignore human error Not enough simply to state that MBE are mean / average values 23 of 36

30 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 205 Question Marking Guidance Mark Comments 2(a) The enthalpy (change) to break mol of H O / bonds Allow heat energy Averaged over a range of compounds / molecules Penalise energy but mark on ignore states CE=0 for ionic bonds 2(b) H O 2 H 2 O H = (H H) + (O=O) 2(H O) / sum of(bonds broken) - sum of (bonds 2 formed) = / = 244 (kj mol ) Allow mark only for +244 and 488 Units not essential but penalise incorrect units 2(c)(i) same reaction / same equation / same number / same reactants and same products / same number and type of bonds broken and formed Do not allow similar 6 of 22

31 MARK SCHEME A-LEVEL CHEMISTRY CHEM5 JUNE 205 2(c)(ii) There must be a slight difference between the actual bond enthalpy (in water) and mean bond enthalpies for the O H bond (in other molecules) Allow bond enthalpy value for enthalpy of formation may not be under standard conditions. Allow reference to bond energy rather than bond enthalpy Do not allow heat loss or experimental error Do not allow mean bond enthalpies are not accurate 7 of 22

32 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 206 Question Answers Mark Additional Comments/Guidance 2a) If enthalpy of formation definition given CE=O 2b) The enthalpy / heat energy change when mol (of a substance) is burned/reacts completely in oxygen with all reactants and products in their standard states With all reactants and products in their normal states at 298K/given temp & 00kPa ΔH = ΣΔH c (reactants) - ΣΔH c (products) correctly and fully balanced cycle NOT just energy ALLOW alternatives for substance e.g. molecule/compound/element ALLOW reacts in excess oxygen ALLOW everything for reactants and products Penalise incorrect conditions if given ALLOW normal states under standard conditions Correct answer scores 3 ΔH = [3(-394) + 4(-286)] (-200) ΔH = ΔH = -36 (kj mol - ) +36 scores mark only M2 also scores M IGNE units Check for AE in working can award M3 as ecf (error carried forward) from M2 if M2 not given due to AE 4 of 22

33 MARK SCHEME A-LEVEL CHEMISTRY CHEM2 JUNE 206 2c) ΔH/-893 = ΣB(reactants) - ΣB(products) ΔH/-893 = ΣBonds broken - ΣBonds formed ΔH/-893 = 2B(C-C) + 7B(C-H) + B(C-O) + B(O-H) + 4½ B(O=O) 6B(C=O) 8B(O-H) Correct answer gains 3 marks -893= 2B(C-C) + 7(42) ½(496) 6(805) 8(463) -893= 2B(C-C) = 2B(C-C) B(C-C) = 702 M2 also scores M May see no 463 in bonds broken and 7x463 in made (gives ) B(C-C) = (+)35(kJ mol - ) If NOT 35 check for AE. This would lose M2, but could gain M and M3 (+)234 scores (due to 3(C-C)) Total 9 NOT M3 from incorrect M2 unless incorrect M2 is due to AE IGNE Units If no other mark awarded then ALLOW if 5939 or 5476 or 8534 or 807 seen 5 of 22