SOLUCIONARIO CINÉTICA QUÍMICA

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1 SOLUCIONARIO CINÉTICA QUÍMICA 2009/10/11 1. B 2. A 3. B 4. C 5. C 6. (a) k increases with increase in T / k decreases with decrease in T; 1 Do not allow answers giving just the Arrhenius equation or involving ln k relationships. (b) gradient = E a /R; (K) = E a /R; Allow value in range (K). 7. D E a =( =) J mol 1 /249 kj mol 1 ; 3 Allow value in range kj mol 1. Allow [3] for correct final answer. (c) = (mol dm 3 ); rate = (0.244 (0.180) 2 =) mol dm 3 s 1 ; 2 Award [2] for correct final answer. Award [1 max] for either mol dm 3 s 1 or mol dm 3 s (i) increase in concentration of product per unit time / decrease in concentration of reactant per unit time; 1 Accept change instead of increase/decrease and mass/amount/ volume instead of concentration. 9. (ii) frequency of collisions; kinetic energy/speed of reactant particles; collision geometry/orientation; 3 correctly labelled axes showing number of particles/frequency against (kinetic) energy; 1

2 10. D 11. C 12. A correctly shaped graph for T (curve must not touch or cross x axes); T curve to the right of T and with a peak lower than T; increasing the temperature increases the (kinetic) energy of the particles / more particles will possess the necessary activation energy; there will be more collisions per unit time / the frequency of collisions increases / there are more successful collisions; (i) the concentration (of nitrogen(ii) oxide); 1 Award [0] if reference made to equilibrium. 14. C 15. C 16. A (ii) mol 2 dm 6 s 1 / dm 6 mol 2 s 1 ; 1 Accept (mol 1 dm 3 ) 2 s (a) [I 2 ] does not affect rate / OWTTE; neither correct/both partially correct with explanation as to how; 2 (b) (c) more particles/molecules have sufficient energy to overcome activation energy / OWTTE; more frequent collisions; 2 (i) 18. C 19. B axes correctly labelled x = energy/velocity/speed, y = number/% of molecules/particles/probability; graph showing correct curve for Maxwell-Boltzmann distribution; If two curves are drawn, first and second mark can still be scored, but not third. Curve(s) must begin at origin and not go up at high energy. two activation energies shown with E cat shown lower; Award the mark for the final point if shown on an enthalpy level diagram. 3 (ii) catalyst provides an alternative pathway of lower energy / OWTTE; Accept catalyst lowers activation energy (of reaction) (a) to maintain a constant volume / OWTTE; 1 2

3 (b) (i) [H + ] order 1, [CH 3 COCH 3 ] order 1, [I 2 ] order 0; (rate =) k[h + ] [CH 3 COCH 3 ]; Award [2] for correct rate expression. Allow expressions including [I 2 ] 0. 2 (ii) neither were correct / Alex was right about propanone and wrong about iodine / Hannah was right about propanone and hydrogen ions but wrong about iodine / OWTTE; 1 (c) [CH 3 COCH 3 ] = mol dm 3 and [H + ] = mol dm 3 ; k = ( ) = ; mol 1 dm 3 s 1 ; Ignore calculation of [I 2 ]. No ECF here for incorrect units. 3 (d) (i) axes correctly labelled x = energy/velocity/speed, y = number/% of molecules/particles/probability; graph showing correct curve for Maxwell-Boltzmann distribution; If two curves are drawn, first and second marks can still be scored, but not third. Curve(s) must begin at origin and not go up at high energy. two activation energies shown with E cat shown lower; Award the mark for the final point if shown on an enthalpy level diagram. 3 (ii) catalyst provides an alternative pathway of lower energy / OWTTE; Accept catalyst lowers activation energy (of reaction) (a) (i) correct substitution of values and numbers of bonds broken / (1 945) + (3 436)/2253; correct substitution of values and numbers of bonds made / (6 391)/2346; H = (sum of energies of bonds broken) (sum of energies of bonds formed) = ( ) = 93 (kj); Ignore units. Award [3] for correct final answer. Award [2 max] for +93 or (ii) entropy of products = = 384; entropy of reactants = (3 131) = 586; S O (= sum of entropies of products) (sum of entropies of 3

4 reactants) / ( ) = 202 (J K 1 mol 1 ); Award [3] for correct final answer. Award [2 max] for +202 or 202. Ignore units. negative as more ordered/less disordered / four moles become two moles / fewer molecules of gas; C 23. D (b) (iii) ( G O = H O T S O = ( 0.202)) = 32.8 (kj mol 1 ); 1 (iv) reaction becomes less spontaneous; G becomes more positive/less negative /T S becomes larger; 2 macroscopic properties remain constant / concentrations remain constant / no change to copper solution seen; rate of reverse/backwards reaction = rate of forward reaction; 2 [NH 3 ] (c) (K c =) 3 [N 2 ][H 2 ] Do not award mark if [ ] missing or round brackets used. 1 (d) (i) [H 2 ] = 0.11 / 0.11 (mol dm 3 ); (e) [N 2 ] = 0.17 / 0.17 (mol dm 3 ); K c = 16; Ignore units. Allow ECF from incorrect equilibrium expression and incorrect concentrations for third mark. 3 (ii) decrease; heat is a product/reaction is exothermic so equilibrium moves to left / OWTTE; 2 yield increases / equilibrium moves to the right / more ammonia; 4 gas molecules 2 / decrease in volume / fewer gas molecules on right hand side; 2 (f) high pressure expensive / greater cost of operating at high pressure / reinforced pipes etc. needed; Do not accept high pressure is dangerous without further explanation. (g) lower temperature greater yield, but lowers rate; Do not award a mark just for the word compromise. 2 K c unaffected; position of equilibrium unaffected; rate of forward and reverse reactions are increased (equally); (i) volume rate = increase in = slope of graph; time initially/to begin with steeper slope / fastest rate / volume of gas/co 2 produced faster/quickly as concentration of HCl highest / OWTTE; as reaction progresses/with time, less steep slope / volume of gas production slows / rate decreases due to less frequent collisions as concentration (of HCl) decreases / OWTTE; curve flattens/becomes horizontal when HCl used up/consumed (as there are no more H + ions to collide with the CaCO 3 particles); 4

5 (ii) Each mark requires explanation. 3 max less steep curve; same maximum volume at later time; half/lower H + /acid concentration less frequent collisions/slower rate; same amount of HCl, same volume CO 2 produced; 4 (iii) mass loss/of CO 2 / mass of flask + content; OR OR 25. D 26. D 27. A 28. A 29. D 30. D Do not penalize for missing x-axis label or for missing units on y-axis. Accept if line meets time axis. 2 (iv) minimum energy (of colliding particles) for a reaction to occur / OWTTE; lower E a / greater surface area/contact between CaCO 3 and HCl / higher HCl concentration / (sufficient) particles/molecules have activation energy; (i) exothermic; Accept either of the following for the second mark. increasing temperature favours endothermic/reverse reaction; as yield decreases with increasing temperature; 2 max (ii) yield increases / equilibrium moves to the right / more ammonia; increase in pressure favours the reaction which has fewer moles of gaseous products; 2 (iii) (rate increases because) increase in frequency (of collisions); increase in energy (of collisions); more colliding molecules with E E a ; 2 max 5

6 32. C 33. B 34. (a) First and second structures should be mirror images. Tetrahedral arrangement around carbon must be shown. 2 (b) (i) order with respect to OH = 0; order with respect to X = 1; rate = k[x]; Award [3] for final correct answer D 36. D 37. C 38. C 39. C 40. C (ii) 0.2(0); min 1 ; 2 (iii) 2-bromo-2-methyl-propane; Do not penalize missing hyphens or added spaces. Accept 2-bromomethylpropane. tertiary (structure); 2 (iv) C 4 H 9 Br C 4 H Br / in equation with curly arrows and slow; C 4 H OH C 4 H 9 OH / in equation with curly arrows and fast; No penalty if primary structure is shown. No credit for S N 2 mechanism, except by ECF (a) (minimum) energy needed for a reaction to occur / (minimum) energy difference between reactants and transition state; 1 (b) (c) particles must collide; appropriate collision geometry/orientation; E E a ; Diagram showing: correct labelling of axes (enthalpy/h/(potential) energy for y-axis and time/progress/course of reaction/reaction coordinate for x-axis) and H (products) line shown below H (reactants) line; correct labelling of the two curves, catalysed and uncatalysed; correct position of E a shown with lines for a catalysed and uncatalysed reaction; the correct label H /change in enthalpy; Do not penalize if reactants and products are not labelled. 2 max 6

7 42. B 43. C 44. B If an endothermic reaction is shown, award [2 max] if all other parts are shown correctly. 3 max 45. (i) decrease in concentration/mass/amount/volume of reactant with time / increase in concentration/mass/amount/volume of product with time / change in concentration/mass/amount/volume of reactant/product with time; 1 (ii) MgCO 3 (s) + 2HCl(aq) MgCl 2 (aq) + CO 2 (g) + H 2 O(l); Ignore state symbols. 1 (iii) Plot starts at the origin and levels off. No mark awarded if axes are not labelled. 1 (iv) new curve reaches same height as original curve; new curve less steep than original curve; volume of gas produced is the same because the same amount of acid is used; reaction is slower because concentration is decreased; (i) (from experiments 1 and 2 at constant [H 2 ]), [NO] doubles, rate quadruples; hence, second order with respect to NO; (from experiments 2 and 3 at constant [NO]), [H 2 ] doubles, rate doubles; first order with respect to H 2 ; Allow alternative mathematical deductions also. 4 (ii) rate = k[no] 2 [H 2 ]; 1 7

8 (iii) k (= ( )/( ) 2 ( )) = ; Do not penalize if Experiments 1 or 2 are used to determine k. mol 2 dm 6 s 1 ; (i) step 1 / equation showing step 1; 1 (ii) O (atom) / oxygen atom; Do not allow oxygen or O (i) (minimum) energy needed for a reaction to occur / difference in energy between the reactants and transition state; 1 (ii) correct position of activation energy; correct position of H and H(CH 3 NC)/reactant line above H(CH 3 CN)/ product line; Accept E instead of H on diagram if y-axis is labelled as energy. Do not penalize if CH 3 NC and CH 3 CN are not labelled on diagram. correct position of transition state; Allow [2 max] if axes are not labelled on diagram B 50. A 51. (iii) as temperature/t increases rate constant/k increases (exponentially); 1 E (iv) from graph gradient m = a ; R measurement of gradient from chosen points on graph; Units of m are K. Do not penalize if not given, but do not award mark for incorrect units. Value of m is based on any two suitable points well separated on the plot. correct answer for E a ; correct units corresponding to answer; Note: A typical answer for E a = kj / kj mol 1. 4 labelled axes (including appropriate units); correctly drawn curve; correctly drawn tangent; rate equal to slope/gradient of tangent (at given time) / rate = x y at time t; [3 max] for straight line graph or graph showing product formation. 4 8

9 52. (a) (i) increases rate of reaction; molecules (of H 2 O 2 ) collide more frequently / more collisions per unit time; No ECF here. 2 (ii) no effect / (solution) remains unchanged; solid NaI is not reacting / aqueous solution of NaI is reacting / surface area of NaI is not relevant in preparing the solution / OWTTE; C 54. D (b) kinetic energy/speed of reacting molecules increases; frequency of collisions increases per unit time; greater proportion of molecules have energy greater than activation energy/e a ; Accept more energetic collisions. 3 max 55. (i) rate = k[no] 2 [Cl 2 ]; 1 (ii) rate of reaction will decrease by a factor of 4; no effect on the rate constant; 2 (iii) y axis labelled concentration/mol dm 3 and x axis is labelled time/s; gradient for [NO]; gradient for [NOCl] will be equal and opposite; equilibrium point identified / two curves level off at same time; Above 775 K: rate = k[no 2 ][CO]; Below 775 K: rate = k[no 2 ] 2 ; zero order reaction; all concentrations are 1.0 mol dm 3 ; slope = ( ) 10 (E a = ) 3 = ; 55.4 (kj mol 1 ); Accept in range Award [1] if (J) stated Award [2] for the correct final answer 2 9

10 59. B 60. A 61. C 62. (a) (order with respect to) H 2 = 1; (order with respect to) NO = 2; 2 (b) rate = k[h 2 ][NO] 2 ; 1 ECF from (a). (c) ( mol dm 3 s 1 = k(0.100 mol dm 3 )(0.100 mol dm 3 ) 2 ) k = ; mol 2 dm 6 s 1 ; 2 (d) (e) agrees/yes; ECF from (b). slow step depends on X and NO; (so) NO is involved twice and H 2 once; overall equation matches the stoichiometric equation / OWTTE; OR ECF for no, depending on answer for (b). agrees/yes; [X] and = constant; [H 2 ][NO] rate of slow step = k[x][no]; but X depends on H 2 and NO; rate of slow step = k[h 2 ][NO] 2 ; Award [1] each for any three of the four above. ECF for no, depending on answer for (b). 4 max reaction involves four molecules; statistically/geometrically unlikely; 2 (f) the rate of formation of H 2 O = 2 rate for N 2 ; because 2 moles H 2 O formed with 1 mole N 2 / OWTTE; 2 10