OCR A Jan 2010 Paper 2 Q2 (with explained solutions) |

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OCR A Jan 2010 Paper 2 Q2

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2 Dilute aqueous hydrogen peroxide, H2O2(aq), is used to sterilise contact lenses. (a) Dilute H2O2(aq) slowly decomposes at room temperature to produce oxygen and water. The decomposition of H2O2(aq) can be made faster by:increasing the concentration of the H2O2(aq), adding a small amount of manganese(IV) oxide catalyst, heating the solution to 60 C. (i) Construct the equation for the decomposition of H2O2.[1] (ii) Explain why decomposition. increasing the concentration of H2O2(aq) increases the rate of[2] (iii) Explain how the catalyst can increase the rate of decomposition of H2O2(aq).[2]OCR 2010<br />
(iv) Explain why increasing decomposition. the temperature of H2O2(aq) increases the rate of As part of your answer, you should add a second curve and any necessary labels to the Boltzmann distribution of molecular kinetic energies shown below. number of molecules with a given energy energy[3]OCR 2010 Turn over<br />
(b) (i) In the past, hydrogen peroxide was manufactured by reacting barium peroxide, BaO2, with ice-cold dilute sulfuric acid. BaO2(s) + H2SO4(aq) BaSO4(s) + H2O2(aq) This method required the disposal of poisonous barium compounds. Calculate the atom economy for this manufacture of hydrogen peroxide from BaO2. Use the table of relative formula masses given below. compound relative formula mass BaO2 H2SO4 BaSO4 H2O2 169.3 98.1 233.4 34.0 atom economy =% [2]OCR 2010<br />
(ii) Nowadays, hydrogen peroxide is manufactured using hydrogen gas, oxygen from the air and a substance called anthraquinone. stage 1 stage 2 H2 + anthraquinone anthraquinol O2 + anthraquinol H2O2 + anthraquinone Compare the manufacture of H2O2 from hydrogen and oxygen with the manufacture from barium peroxide described in b(i). Explain the advantages of the manufacture of H2O2 from hydrogen and oxygen.[3] (c) Some reactions of H2O2 are exothermic. Use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic.[2] [Total: 15]OCR 2010 Turn over<br />

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$January 2010 Additional Guidance ALLOW any correct multiple including fractions IGNORE state symbols ALLOW particles are closer together DO NOT ALLOW area instead of volume IGNORE more concentrated particles ALLOW collisions more often OR increased rate of collision OR collisions are more likely OR there is a greater chance of collisions More collisions is not sufficient ALLOW catalyst changes reaction mechanism ALLOW an alternative approach using adsorption particles adsorbed onto surface (cid:57) so bonds weakened as a result of the adsorption (cid:57) F322 Question 2 (a) (i) 2H2O2 (cid:127)(cid:111) 2H2O + O2 (cid:57) Expected Answers Mark Scheme Marks (ii) More crowded particles OR more particles per (unit) volume (cid:57) more collisions per second OR more frequent collisions (cid:57) (iii) Any two from the following: Reaction takes alternative route (cid:57) Activation energy is lowered (cid:57) More molecules have energy above activation energy OR more molecules have enough energy to react (cid:57) 17<br /> F322 Mark Scheme January 2010 Question Expected Answers (iv) Correct curve for higher temperature (cid:57) Activation energy does not change OR clearly labelled on diagram, e.g. Ea OR E (cid:57) More molecules have energy above activation energy OR more molecules have enough energy to react (cid:57) (b) (i) 34.0100 267.4 (cid:57) 12.7% (cid:57) Marks Additional Guidance maximum of curve to right AND lower than maximum of original curve AND above dotted line at higher energy as shown in diagram below IGNORE minor point of inflexion of curve Note that the diagram above would score all 3 marks More successful collisions is not sufficient First mark for 267.4 OR (34.0 + 233.4) OR (169.3 + 98.1) at bottom of fraction with or without100 ALLOW from 2 sig figs up to calculator value ALLOW full marks for 13 OR 12.7 OR 12.72 OR 12.715 up to calculator value with no working out 12.71 scores one mark only NO ECF for this part from incorrect numbers in first expression 18<br /> F322 Question (c) (ii) Any three from the following: Expected Answers Oxygen comes from air (cid:57) No poisonous materials formed OR no poisonous materials involved (cid:57) No waste products formed OR atom economy is 100% (cid:57) Anthraquinone is regenerated OR recycled OR used again OR Anthraquinone acts as a catalyst (cid:57) Bond breaking absorbs energy AND bond making releases energy (cid:57) More energy released than absorbed (cid:57) Total 15 19 Mark Scheme January 2010 Marks Additional Guidance IGNORE hydrogen comes from the air IGNORE harmful ALLOW higher atom economy ALLOW bond breaking is endothermic AND bond making is exothermic ALLOW exothermic change transfers more energy than endothermic change OR bond making transfers more energy than bond breaking OR (the sum of the) bond enthalpies in the products is greater than the (sum of the) bond enthalpies in the reactants OR (the sum of the) bond enthalpies of the bonds made is greater than (the sum of) the bond enthalpies of the bonds broken IGNORE reference to strong and weak bonds IGNORE enthalpy of products is less than enthalpy of reactants<br />$

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