3.2.1 Energetics. Hess's Law. 183 minutes. 181 marks. Page 1 of 21

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1 .. Energetics Hess's Law 8 minutes 8 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

(d) 56 + ( 85.8) + (4 9.5) + ΔH f C 4 H 4 O 4 = 0 ΔH f = 789.6 kj mol If answer is incorrect: Score +789.

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) C H 6 O + 4O CO + H O () (or multiple) (b) (i) () = () allow 0.05 allow conseq on wrong M r.45/00, CE; C.E. Page 4 of

5 (ii) heat released = mcδt = () if.45 used in place of 00 CE = 0 = 400 J () (or 4.kJ) allow 400 to 400 ignore decimal places units tied to answer If use allow ½ for 4. with no units (iii) = 97 (kj mol ) () allow 968 to 97 allow +97 allow conseq allow no units penalise wrong units 5 (c) (i) Heat loss () or energy loss do not allow incomplete combustion (ii) Difference: more negative () (or more exothermic) QoL mark Explanation: heat (or energy) released when water vapour condenses () or heat/energy required to vaporise water or water molecules have more energy in the gaseous state (d) ΔH = ΣΔH reactants ΣΔH products () (or cycle ) = ( 94) + ( 86) + ( 97) ( 70) () = 77 () ignore units even if wrong Allow / for +77 [] M6. (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 5 of

6 (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 6 of

7 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] M7. (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 7 of

8 M8. (a) (i) enthalpy change when mol of a substance (or compound) (QL mark) is (completely) burned in oxygen (or reacted in excess oxygen) at 98 K and 00 kpa (or under standard conditions) (ii) heat produced = mass of water Sp heat capacity xδt (or mcδt) = (note if mass =. lose first marks then conseq) = 4000 J or = 40. kj (allow must have correct units) moles methanol = mass/m r =./ () = ΔH = 40./0.066 = 605 kj (mol ) (allow 60 to 608 or answer in J) (note allow conseq marking after all mistakes but note use of. g loses marks (b) (i) equilibrium shifts to left at high pressure because position of equilibrium moves to favour fewer moles (of gas) (ii) at high temperature reaction yield is low (or at low T yield is high) at low temperature reaction is slow (or at high T reaction is fast) therefore use a balance (or compromise) between rate and yield (c) ΔH = ΣΔH cο (reactants) ΣΔH c ο (products) (or correct cycle) ΔH c ο (CH OH) = ΔH cο (CO) + ΔH cο (H ) ΔH = ( 8) + ( 86) ( 9) (mark for previous equation or this) = 764 (kj mol ) ( units not essential but lose mark if units wrong) (note scores /) [5] Page 8 of

9 M9. (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) [] M0. (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 9 of

10 (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 [] M. (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 0 of

11 (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] 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) (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] Page of

12 M. (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 (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 Page of

13 (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] M4. (a) (i) reduction reduced redox reduction oxidation Not oxidation alone (ii) Fe + + e Fe Ignore state symbols Do not penalise absence of charge on electron Credit Fe + Fe e Credit multiples Page of

14 (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 4 of

15 (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 5 of

16 M5. (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 (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 Page 6 of

17 (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 (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] M6. (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 Page 7 of

18 (ii) By definition Ignore reference to standard state Because it is an element / elemental (b) (i) TiO + Cl + C TiCl 4 + CO Allow multiples 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 8Na CrO 4 + Fe O + 8CO Allow multiples Ignore state symbols (ii) Cr O + Al Al O + Cr Allow multiples Ignore state symbols [0] Page 8 of

19 M7. (a) M (could be scored by a correct mathematical expression Correct answer to the calculation gains all of M, M and M M ΔH = ΣΔH f (products) ΣΔH f (reactants) Credit mark for 0 (kj mol ) a correct cycle of balanced equations M = 669 ( 590) = (This also scores M) M = + 0 (kj mol ) Award mark ONLY for 0 M4 - Using powders Any one from 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 Sr and Al a clear statement of M which could be in words and scores only M To increase collision frequency / collisions in a given time / rate of collisions To increase the surface contact / contact between the solids / contact between (exposed) particles Ignore dividing final answer by Penalise M4 for reference to molecules. 5 M5 Major reason for expense of extraction Any one from Aluminium is extracted by electrolysis aluminium extraction uses (large amounts of) electricity Reaction / process / It / the mixture requires heat It is endothermic Page 9 of

20 (b) Calcium has a higher melting point than strontium, because Ignore general Group statements. Correct reference to size of cations / proximity of electrons M (For Ca) delocalised electrons closer to cations / positive ions / atoms / nucleus cations / positive ions / atoms are smaller cation / positive ion / atom or it has fewer (electron) shells / levels Penalise M if either of Ca or Sr is said to have more or less delocalised electrons the same nuclear charge. Ignore reference to shielding. Relative strength of metallic bonding M (Ca) has stronger attraction between the cations / positive ions / atoms / nucleus and the delocalised electrons stronger metallic bonding (assume argument refers to Ca but credit converse argument for Sr) CE= 0 for reference to molecules or Van der Waals forces or intermolecular forces or covalent bonds. (c) M Mg + O MgO M Mg + H O Mg(OH) + H Credit multiples of the equations. M Magnesium hydroxide is used as an antacid / relieve indigestion (heartburn) / neutralise (stomach) acidity / laxative Not simply milk of magnesia in M [0] Page 0 of

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AS Paper 1 and 2 Energetics MARK SCHEME

AS Paper 1 and 2 Energetics MARK SCHEME AS Paper and 2 Energetics MARK SCHEME M.D [] M2.D [] M3.A [] M4.D [] M5.B [] M6.C [] M7. (a) (Energy required) to break a given covalent bond () averaged over a range of compounds () Penalise first mark