3.2.1 Energetics. Enthalpy Change. 263 minutes. 259 marks. Page 1 of 33

Transcription

1 .. Energetics Enthalpy Change 6 minutes 59 marks Page of

2 M. (a) (Enthalpy change) when mol () of a compound is formed from its constituent elements () in their standard states () Allow energy or heat, Ignore evolved or absorbed Mark each point independently (b) (The enthalpy change for a reaction is) independent of the route () (c) ΔH R = H f products - H f reactants () = [( -86) + ( -94)] - (-48) () = -79 () (kj mol ) Deduct one mark for each error to zero [7] M. (a) Heat energy change () Not energy on its own measured at constant pressure () Mark separately, ignore constant temperature statements (b) (i) Enthalpy change when mol of a substance (or compound / product) () is formed from its constituent elements () in their standard states () under standard conditions () Mark separately (ii) Na(s) + S(s) + O (g) Na SO 4 (s) Balanced () State symbols (), but only if all species are correct Allow S 8 (s) 5 (c) Enthalpy change is independent of reaction route () Penalise incorrect additional statements Page of

3 (d) 56 + ( 85.8) + (4 9.5) + ΔH f C 4 H 4 O 4 = 0 ΔH f = kj mol If answer is incorrect: Score two marks Score ( ); ( ) and ( 4) for species - one mark If an incorrect negative answer given check for AE for loss of one mark [] M. (a) Enthalpy (Energy) to break a (covalent) bond () dissociation energy Varies between compounds so average value used () QL mark average of dissociation energies in a single molecule / e.g. CH 4 Do not allow mention of energy to form bonds but with this case can allow second mark otherwise nd mark consequential on first (b) (i) / N + / H NH () Ignore s s (ii) ΔH = (Σ)bonds broken (Σ)bonds formed () = / / () = 8 kj mol () Ignore no units, penalise wrong units Score / for -76 / for +8 Allow / for Page of

4 = -6 kj mol () (c) 4 (C H) + (C=C) + (H H) (6 (C H) + (C C)) = 6 () (C=C) + (H H) ((C C) + (C H)) = 6 (C H) = 86 () (C H) = 48 (kj mol ) () Note: allow () for 86 another () for 48 [9] M4. (a) They are elements () Ignore irrelevant comments (b) Enthalpy change () or heat energy change or heat change or ΔH or any named enthalpy change C.E. if change not mentioned Independent of route () depends on initial and final states Only give second mark if first mark awarded except allow if energy used instead of enthalpy (c) ΔH = ΣΔH f (products) - ΣΔH f (reactants) () (Or a cycle) = 4 + ½ 94 ( 65) () (also implies first mark) Ignore no units penalise wrong units +6 scores / [6] M5. (a) (i) enthalpy (or heat or heat energy) change when mol of a substance () (QL mark) is formed from its elements () all substances in their standard states () (or normal states at 98K, 00 kpa or std condits) not STP, NTP Page 4 of

5 (b) enthalpy change (or enthalpy of reaction) is independent of route () ΔH = ΣΔH f prods - ΣΔH f reactants (or cycle) () minimum correct cycle is: ΔH = ( ) () = 4 (kj mol ) () penalise this mark for wrong units +4 scores mark out of the last three 4 (c) ΔH = mct () (or mcδt) = = 670 J = 6.7J () mark is for 670 J or 6.7 kj moles HCl = conc = () = 0.5 () if error here mark on conseq. Therefore moles of MgO reacted = moles HCl/ () (mark is for/, CE if not/) = 0.5/ = Therefore ΔH = 6.7/0.075 () = 90 kj (mol ) kj must be given, allow 89 to 9 value () sign (); this mark can be given despite CE for / 8 Page 5 of

6 Note various combinations of answers to part (c) score as follows: 89 to 9 kj (8) (or to 9000J) no units (7) +89 to +9 kj (7) (or to +9000J) no units (6) 44 to 46 kj (5) (or to J) no units (4) if units after 6.7 or 670 (5) +44 to +46 kj (4) (or to ) if no units and if no units after 6.7 or 670 () otherwise check, could be (4) [5] M6. (a) ΔH = Σ(bonds broken) Σ(bonds formed) (or cycle) = / (or ( /) = 0 (kj mol ) () (accept no units, wrong units loses a mark; +0 scores () only) (b) C(s) + H (g) CH 4 (g) equation () Correct state symbols () (c) (i) Macromolecular (accept giant molecule or carbon has many (4) bonds) (ii) ΔH = ΣΔH f (products) ΣΔH f (reactants) (or cycle) = ( 74.9) = 66 (kj mol ) (accept no units, wrong units loses one mark, allow 660 to 66, 66 scores one mark only) (iii) 66/4 = 45.5 (mark is for divide by four, allow if answer to (c)(ii) is wrong) [0] Page 6 of

7 M7. (a) enthalpy (or energy) to break (or dissociate) a bond; averaged over different molecules (environments); enthalpy (or heat energy) change when one mole of a compound; is formed from its elements; in their standard states; (b) enthalpy change = Σ(bonds broken) Σ(bonds formed) or cycle; = ( ); (or similar) = 789; (+ 789 scores only) (c) (i) zero; (ii) AH = Σ (enthalpies of formation of products) Σ (enthalpies of formation of reactants) = 4 4-(75 + ); = 777; (+ 777 scores one only) (d) mean bond enthalpies are not exact (or indication that actual values are different from real values) [] M8. (a) enthalpy change/ heat energy change when mol of a substance is completely burned in oxygen at 98K and 00 kpa or standard conditions (not atm) Page 7 of

8 (b) H = bonds broken bonds formed = (6 4) ( ) ((6 74) + (6 46)) = 57 kj mol (c) (d) by definition H f is formation from an element H c = H f products - H f reactants or cycle = ( 94) + ( 4) (+0) = 98 kj mol (e) bond enthalpies are mean/average values from a range of compounds [] M9. (a) Enthalpy change when mol of compound () Is formed from it s elements () All substances in their standard state () (b) ΔH = ΣΔH ο (reactants) c ΣΔHο (products) () c = (7x 94) + (4 x 86) ( 909) () = + 7 kjmol () (c) Heat change = m c ΔT () = = 6700J = 6.7kJ () Moles C 7 H 8 =.5 /9 = 0.07 () ΔH = 6.7 / 0.07 = 07 kj mol () (allow 00 to ) 4 Page 8 of

9 (d) Mass of water heated = = 75g Temp rise = = 8.5 C both for () mark Heat change = = 665 J =.665 kj () Moles HCl = 0.05 () ΔH =.665 / 0.05 = 5. kjmol () (allow 5 to 54) (e) Less heat loss () 4 [5] M0. (a) The enthalpy change when mol of a compound is completely burnt in oxygen under standard conditions, or 98K and 00kPA (b) (i) C H 6 + ½O CO + H O (ii) ΔH = ΔH f ο (CO ) + ΔH f ο (H O) ΔH f ο (C H 6 ) = ( 85) = 56 kj mol (c) moles methane = = kj evolved = = joules = (mc)δt ΔT = = 46.4 K [] Page 9 of

10 M. (a) (i) q = mc ΔT Ignore case except T (ii) = 6 (J) to 60.5() (J) Credit 0.6 provided it is clear that it is kj. Penalise wrong units (iii) = 09 (J) to 08.98(4) (J) Credit 0.09 provided it is clear that it is kj. Penalise wrong units. (iv) M Addition of (a)(ii) and (a)(iii) M Multiply by 0 and convert to kj (divide by 000) leading to an answer Consequential on (a)(ii) and (a)(iii) Penalise wrong units Ignore the sign Therefore ΔH = ( ).69 ( ).7(0) (kj mol ) Ignore greater numbers of significant figures (.69496) Subtraction in M is CE (b) One from: No account has been taken of the intermolecular forces initially in the two liquids each liquid has its own intermolecular forces in operation before mixing. The liquids may react or reference to reaction or reference to bonds broken or formed Any statement which shows that there are other intermolecular forces to consider. Ignore heat loss and ignore poor mixing. [6] Page 0 of

11 M. (a) (i) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / product M M Is formed from its (constituent) elements 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 specified T / 98 K Ignore reference to atmosphere (ii) By definition Because they are elements (iii) M ΔH f = ΣΔH f (products) ΣΔH f (reactants) M = 669 ( 558) (This also scores M) M = (+) 5 (kj mol ) Correct answer gains full marks. Assume the value is positive unless specifically stated as negative. Credit mark if 5 (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 marks (M and M) If no AE, check for a correct method; this requires either a correct cycle with BaO a clear statement of M which could be in words and scores only M (b) (i) One from Aluminium is expensive (to extract due to electrolysis) High energy cost The cost of heating strongly This requires a clear statement about cost Page of

12 (ii) One from increase collision frequency more collisions more chance of colliding The answer MUST refer to more collisions. Ignore more available to collide (c) (i) Ba + H O Ba(OH) + H Ignore state symbols Allow multiples and correct ionic equations (ii) M Ba + + SO 4 BaSO 4 (or the ions together) M Allow crossed out Na + ions, but penalise if not crossed out White precipitate / white solid Ignore state symbols Ignore milky (iii) M Barium meal or ( internal ) X-ray or to block X-rays M BaSO 4 / barium sulfate is insoluble (and therefore not toxic) Accept a correct reference to M written in the explanation in M, unless contradictory. For M NOT barium ions NOT barium NOT barium meal and NOT It. Ignore radio-tracing. [4] M. (a) Heat (energy) change at constant pressure Ignore references to standard conditions, but credit specified pressure. (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) Page of

13 (c) ΔH + 96 = 75 4 ΔH + 96 = 507 (M) ΔH = (M and M) ΔH = 470 (kj mol ) Award mark for Award full marks for correct answer Ignore units. Ignore numbers on the cycle M and M can score for an arithmetic error [5] M4. (a) Three conditions in any order for M to M M yeast or zymase M 0 C T 4 C M anaerobic/no oxygen/no air neutral ph M4 C 6 H O 6 C H 5 OH + CO C 6 H O 6 4C H 5 OH + 4CO (b) M Carbon-neutral Mark independently Penalise bacteria and phosphoric acid using the list principle Ignore reference to aqueous or water (i.e. not part of the list principle) Or other multiples Ignore biofuel 4 M M 6 (mol/molecules) CO /carbon dioxide taken in/used/used up (to form glucose or in photosynthesis) 6 (mol/molecules) CO /carbon dioxide given out due to (mol/molecules) CO /carbon dioxide from fermentation/ Process and 4 (mol/molecules) CO /carbon dioxide from combustion/process It is NOT sufficient in M and M for equations alone without commentary or annotation or calculation Page of

14 (c) M (could be scored by a correct mathematical expression) (Sum of) bonds broken (Sum of) bonds made/formed = ΔH (Σ) B reactants (Σ) B products = ΔH (where B = bond enthalpy/bond energy) For M there must be a correct mathematical expression using ΔH or enthalpy change M Reactants = (+) 479 Products = ( ) 5750 M Overall = 0 (kj mol ) (This is worth marks) Award full marks for correct answer. Ignore units. M is for either value underlined M is NOT consequential on M Award mark ONLY for +0 Candidates may use a cycle and gain full marks. M4 Mean bond enthalpies are not specific for this reaction they are average values from many different compounds/molecules Do not forget to award this mark Page 4 of

15 (d) M q = m c ΔT (this mark for correct mathematical formula) M M M4 = 6688 (J) (kj) 6.69 (kj) 6.7 (kj) 0.46g is 0.0 mol therefore ΔH = 669 kj mol 670 kjmol kj mol Award M, M and M for correct answer to the calculation Penalise M ONLY if correct answer but sign is incorrect In M, do not penalise incorrect cases in the formula If m = 0.46 or m = if ΔT = 8, CE and penalise M and M If c = 4.8 (leads to 7696) penalise M ONLY and mark on for M = Ignore incorrect units in M Incomplete combustion Do not forget to award this mark. Mark independently 4 [5] M5. (a) The molecular ion is The molecule with one/an electron knocked off/lost Ignore the highest or biggest m/z peak The molecule with a (single) positive charge the ion with/it has the largest/highest/biggest m/z (value/ratio) Ignore the peak to the right the ion with/it has an m/z equal to the M r Ignore compound (b) (i) (4.0007) = A sum is needed to show this Page 5 of

16 (ii) Propane/C H 8 and carbon dioxide/co (and N O) or they or both the gases/molecules or all three gases/molecules have an (imprecise) M r of 44.0 ( 44) they have the same M r or molecular mass (to one d.p) This could be shown in a calculation of relative masses for propane and carbon dioxide (iii) By definition The standard/reference (value/isotope) Ignore element Ignore atom (c) (i) M (could be scored by a correct mathematical expression) ΔH = ΣΔH products ΣΔH reactants a correct cycle of balanced equations M and M can be scored with correct moles as follows ΔH + ( 46) = +8 + ( 86) ΔH 9 = 776 ΔH = M ΔH = 684 (kj mol ) (This is worth marks) Award mark ONLY for Full marks for correct answer. Ignore units. Deduct one mark for an arithmetic error. (ii) The value is quoted at a pressure of 00 kpa bar or 0 5 Pa All reactants and products are in their standard states/their normal states at 00 kpa or bar Ignore atmosphere/0 kpa Ignore constant pressure [8] Page 6 of

17 M6. (a) (i) M drawn curve starts at reactants and ends at products Tapered lines into the original curve gain credit for M M curve peak is below the one drawn in the question (and may show one/two humps) Mark M and M independently (ii) Exothermic (reaction) Ignore ΔH is negative (iii) Σ bond (enthalpy) reactants < Σ bond (enthalpy) products The sum for H and I /reactants is less than/lower than/smaller than the sum for HI/products The sum for HI/products is more than/larger than/bigger than the sum for H and I /reactants Accept It the sum will be smaller or less (iv) M p M (q p) p q q + p M demands that the sign for an exothermic reaction is part of the outcome mathematically. Ignore case (b) (i) Increase/speed up/faster (rate of attainment of equilibrium) Increase/speed up/faster rate of both forward and reverse reaction Increase/speed up/faster rate of reaction Credit It took less time Page 7 of

18 (ii) M Increase/speed up/faster (rate of attainment of equilibrium) M M More particles/molecules in a given volume/space the particles/molecules are closer together an increase in concentration. More/higher chance of successful/effective/productive collisions (between particles) more collisions/higher chance of collisions (of particles) with E>E Act 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 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 M if an increase in the value of E Act /energy of particles is stated. Max for M and M if reference to atoms [0] M7. (a) (i) Hexan--ol ONLY (ii) Homologous (series) ONLY (iii) Displayed formula for butan--ol All bonds must be drawn out including the O H bond Ignore bond angles (iv) CH CH CH CH OH + [O] CH CH CH CHO + H O Require this whole equation as written or formulae drawn out Penalise sticks Page 8 of

19 (v) Displayed formula for butanone (credit possible enols, ethers and cyclic structures for C 4 H 8 O) All bonds must be drawn out Ignore bond angles (b) M q = m c ΔT calculation M = 585 (J) 5.85 (kj) 5.9 (kj) (This also scores M) M mol, therefore ΔH = 70 (kj mol ) ΔH = 70.4 (kj mol ) ΔH = 00 (kj mol ) Award full marks for correct answer In M, do not penalise incorrect cases in the formula Ignore incorrect units in M Penalise M ONLY if correct answer but sign is incorrect value is in J mol If m = 5 0 if ΔT = 8, CE and only allow one mark for correct mathematical formula for M If c = 4.8 (leads to 674) penalise M ONLY and mark on for M = 50 ( 47) (c) (i) M The enthalpy change (or heat change at constant pressure) when mol of a compound/substance/alcohol M is burned completely in oxygen burned in excess oxygen M 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 a specified T/98 K For M Ignore reference to atmosphere Page 9 of

20 (ii) M (could be scored by a correct mathematical expression) M ΔH = ΣΔH f (products).σδh f (reactants) a correct cycle of balanced equations M = 4( 94) + 5( 86) ( 7) (This also scores M) M = 679 (kj mol ) 680 (kj mol ) Award mark ONLY for (+) 679 (+) 680 Correct answer to calculation gains full credit Credit mark if (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 marks (M and M) If no AE, check for correct method; this requires either a correct cycle with 4CO and 5H O a clear statement of M which could be in words and scores only M (d) (i) M This is about the change in formula up the series Each alcohol in the series (compared with the previous one) increases by/has an extra CH has one more C-C and two more C-H M This is about the reaction and bond breaking/making Combustion of each alcohol in the series breaks one more C-C and two more C-H compared with the previous one AND forms one more mol CO and one more mol H O A statement in which there is the idea that the extra additional difference in number of bonds broken and formed (as the series increases) is the same has the same difference in energy N.B. If the first statement here for M is given, both marks score Page 0 of

21 (ii) For the two marks M and M heat loss or heat absorbed by the apparatus incomplete combustion/not completely burned 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. [8] M8. (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 (b) (c) Heat (energy) change at constant pressure QoL 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 (d) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / product For M, credit correct reference to molecule/s or atom/s M is formed from its (constituent) elements M 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 98 K) Ignore reference to atmosphere Page of

22 (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 (ii) M H r = H f (products) - H f (reactants) M H r = 4( 4) ( 70) 4(+) = (This also scores M) M = 96 (kj mol ) Correct answer gains full marks Credit mark for + 96 (kj mol ) Credit mark for 94 (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 marks (M and M) If no AE, check for a correct method; this requires either a correct cycle with Cl and 4Na a clear complete statement of M which could be in words and scores only M (iii) Reducing agent Ignore reduces titanium reductant reduces TiCl 4 electron donor [7] M9. (a) (i) reduction reduced redox reduction oxidation Not oxidation alone Page of

23 (ii) Fe + + e Fe Ignore state symbols Do not penalise absence of charge on electron Credit Fe + Fe e Credit multiples (b) (i) Because (one of the following) CO is not the only product Reference to incomplete combustion to form CO does not answer the question (Some) complete combustion (also)occurs CO is (also) formed Further oxidation occurs (ii) (iii) 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) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / element For M, credit correct reference to molecule/s or atom/s M is burned completely / undergoes complete combustion in (excess) oxygen M with all reactants and products / all substances in standard states For M Ignore reference to atmosphere all reactants and products / all substances in normal / specified states under standard conditions / 00 kpa / bar and specified T / 98 K Page of

24 (c) M (could be scored by a correct mathematical expression which must have all H symbols and the ) Correct answer gains full marks Credit mark ONLY for (kj mol ) M H r = H f (products) H f (reactants) Credit mark ONLY for 7 (kj mol ) i.e. assuming value for Fe (l) = 0 correct cycle of balanced equations with Fe, C and O M H r = (+4) + ( 94) ( 8) ( ) = (This also scores M) M = (+) (kj mol ) (Award mark ONLY for ) (Award mark ONLY for 7) 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 marks (M and M) If no AE, check for a correct method; this requires either a correct cycle with Fe, C and O a clear statement of M which could be in words and scores only M (d) (i) C(s) + O (g) CO (g) State symbols essential Possible to include C(s, graphite) (ii) These two enthalpy changes are for the same reaction / same equation / same reactants and products Penalise reference to CO being produced by a different route They both make one mole of carbon dioxide only from carbon and oxygen (or this idea clearly implied) both form CO is not sufficient (since other products might occur e.g.co) The same number and same type of bonds are broken and formed [] Page 4 of

25 M0. (a) (i) M (could be scored by a correct mathematical expression which must have all Hsymbols and the or SUM) M ΔH r = ΣΔH f (products) - ΣΔH f (reactants) a correct cycle of balanced equations with C, H and O M ΔH r = 0 + ( 4) ( 94) ΔH r = ΔH r = (This also scores M) M = 49 (kj mol ) (Award mark ONLY for + 49) 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 marks (M and M) If no AE, check for a correct method; this requires either correct cycle of balanced equations with C, H and O a clear statement of M which could be in words and scores only M (ii) It is an element / elemental Ignore reference to standard state By definition (b) M (The yield) increases / goes up / gets more If M is given as decreases / no effect / no change then CE= 0 for clip, but mark on only M and M from a blank M M There are more moles / molecules (of gas) on the left / of reactants fewer moles / molecules (of gas) on the right / products there are 4 moles /molecules (of gas) on the left and moles / molecules on the right. (equilibrium) shifts / moves to the side with less moles / molecules Ignore volumes, particles atoms and species for M M: Can only score M if M is correct The (position of) equilibrium shifts / moves (from left to right) to oppose the increase in pressure For M, not simply to oppose the change For M credit the equilibrium shifts / moves (to right) to lower / decrease the pressure (There must be a specific reference to the change that is opposed) Page 5 of

26 (c) M Yield increases goes up M The (forward) reaction / to the right is endothermic takes in/ absorbs heat The reverse reaction / to the left is exothermic gives out / releases heat If M is given as decrease / no effect / no change then CE= 0 for clip, but mark on only M and M from a blank M Can only score M if M is correct M The (position of) equilibrium shifts / moves (from left to right) to oppose the increase in temperature (QoL) For M, not simply to oppose the change For M, credit the (position of) equilibrium shifts / moves (QoL) to absorb the heat to cool the reaction to lower the temperature (There must be a specific reference to the change that is opposed) (d) (i) An activity which has no net / overall (annual) carbon emissions to the atmosphere An activity which has no net / overall (annual) greenhouse gas emissions to the atmosphere. There is no change in the total amount / level of carbon dioxide /CO carbon /greenhouse gas present in the atmosphere. The idea that the carbon /CO given out equals the carbon /CO that was taken in from the atmosphere (ii) CH OH + ½ O CO + H O Ignore state symbols Accept multiples (iii) H + ½ O H O Ignore state symbols Accept multiples H + O H O Extra species must be crossed through Page 6 of

27 (e) M q = m c T Award full marks for correct answer Ignore the case for each letter q = M = 489 (J) 4.89 (kj) 4.9 (kj) 4.4 (kj)(also scores M) M Using 0.00 mol therefore H = 99 (kjmol ) 400 Penalise M ONLY if correct numerical answer but sign is incorrect; +99 gains marks Penalise M for arithmetic error and mark on In M, do not penalise incorrect cases in the formula If T = 80.5; score q = m c T only If c = 4.8 (leads to ) penalise M ONLY and mark on for M = or +400 gains marks Ignore incorrect units [6] M. (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 ΔH for enthalpy (b) ΔH exp + ΔH ΔH = 0 Any correct mathematical statement that uses all three terms ΔH exp + ΔH = ΔH ΔH = ΔH exp + ΔH ΔH exp = ΔH ΔH ΔH exp = ΔH +( ΔH ) (c) ΔH exp = ΔH ΔH ΔH exp = 56 = 68 (kj mol ) Ignore units Award the mark for the correct answer without any working Page 7 of

28 (d) (i) M q = m c ΔT calculation (5.0 x 4.8 x 4.0) Award full marks for correct answer M = 46J.46 kj (This also scores M) In M, do not penalise incorrect cases in the formula M must have both the correct value within the range specified and the minus sign Penalise M ONLY if correct numerical value but sign is incorrect; e.g to gains marks (ignore +70 after correct answer) For 0.00 mol, therefore ΔH = to 69.5 (kj mol - ) ΔH = 69.7 to 69.5 (kj mol ) Penalise M for arithmetic error but mark on Accept answers to sf or 4sf in the range 69.7 to 69.5 ΔT = 87, score q = m c ΔT only Ignore -70 after correct answer If c = 4.8 (leads to 684J ) penalise M ONLY and mark on for M = 80.7 (range 80.0 to 80.) Ignore incorrect units (ii) The idea of heat loss NOT impurity Incomplete reaction (of the copper sulfate) NOT incompetence Not all the copper sulfate has dissolved NOT incomplete combustion Page 8 of

29 (e) Impossible to add / react the exact / precise amount of water Not just the reaction is incomplete Very difficult to measure the temperature rise of a solid Difficult to prevent solid dissolving (Copper sulfate) solution will form [8] M. (a) (i) M (could be scored by a correct mathematical expression which must have all ΔH symbols and the or SUM) Correct answer gains full marks Credit mark ONLY if (kj mol ) M ΔH = ΔH f (products) - ΔH f (reactants) a correct cycle of balanced equations M ΔH = ( 94) ( ) ( 97) (This also scores M) M = (+) (kj mol ) Award mark ONLY for 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 marks (M and M) 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 (ii) By definition Ignore reference to standard state Because it is an element / elemental Page 9 of

30 (b) (i) TiO + Cl + C Allow multiples TiCl 4 + CO TiO + Cl + C TiCl 4 CO Ignore state symbols M use of Cl and C M a correct balanced equation (ii) TiCl 4 + 4Na Allow multiples Ti + 4NaCl TiCl 4 + Mg Ti + MgCl Ignore state symbols M use of Na Mg M a correct balanced equation (c) (i) 4 FeCr O Na CO + 7O Allow multiples Ignore state symbols 8Na CrO 4 + Fe O + 8CO (ii) Cr O + Al Allow multiples Ignore state symbols Al O + Cr [0] M. (a) (i) M c(oncentrated) phosphoric acid / c(onc.) H PO 4 c(oncentrated) sulfuric acid / c(onc.) H SO 4 In M, the acid must be concentrated. Ignore an incorrect attempt at the correct formula that is written in addition to the correct name. M Re-circulate / re-cycle the (unreacted) ethene (and steam) / the reactants pass the gases over the catalyst several / many times In M, ignore remove the ethanol. Credit re-use. Page 0 of

31 (ii) M (By Le Chatelier s principle) the equilibrium is driven / shifts / moves to the right / L to R / forwards / in the forward direction M depends on a correct statement of M The equilibrium moves / shifts to oppose the addition of / increased concentration of / increased moles / increased amount of water / steam to decrease the amount of steam / water Mark M independently M Yield of product / conversion increase ethanol increases / goes up / gets more (iii) M Poly(ethene) / polyethene / polythene / HDPE / LDPE M At higher pressures More / higher cost of electrical energy to pump / pumping cost Cost of higher pressure equipment / valves / gaskets / piping etc. expensive equipment Credit all converse arguments for M (b) M for balanced equation M for state symbols in a correctly balanced equation C(s / graphite) + H (g) + ½O (g) CH CH OH(l) (C H 5 OH) Not multiples but credit correct state symbols in a correctly balanced equation. Penalise C H 6 O but credit correct state symbols in a correctly balanced equation. (c) (i) M The enthalpy change / heat change at constant pressure when mol of a compound / substance / element If standard enthalpy of formation CE=0 M is burned / combusts / reacts completely in oxygen burned / combusted / reacted in excess oxygen M 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 / 98 K For M Ignore reference to atmosphere Page of

32 (ii) M Correct answer gains full marks ΣB(reactants) ΣB(products) = ΔH Credit mark for (+) 79 (kj mol ) Sum of bonds broken Sum of bonds formed = ΔH B(C-C) + B(C-O) + B(O-H) + 5B(C-H) + B(O=O) (LHS) 4B(C=O) 6B(O H) (RHS) = ΔH M (also scores M) (4)+(496) [LHS = 479] (060) (488) 4(805) 6(46) [RHS = 5998] = ΔH (0) (778) using only bonds broken and formed ( ) 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 marks (M and M) If no AE, check for a correct method; this requires either a correct cycle with C and 6H and 7O a clear statement of M which could be in words and scores only M M ΔH= 79 (kj mol ) Allow a maximum of one mark if the only scoring point is LHS = 479 RHS = 5998 Award mark for +79 Candidates may use a cycle and gain full marks (d) (i) Reducing agent reductant electron donor to reduce the copper oxide Not reduction. Not oxidation. Not electron pair donor. (ii) CH COOH [7] Page of

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