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OCR A Jun 2016 Paper 2 Q4
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![10 This question is about equilibrium and catalysts. (a) The equilibrium between NO2 and N2O4 gases is set up in a gas syringe at room temperature. The two gases are different in appearance. 2NO2(g) brown N2O4(g) colourless H = 58 kJ mol1 Using le Chateliers principle, predict and explain how the following changes would affect the appearance of the equilibrium mixture. (i) The gas mixture is compressed by pushing in the plunger of the gas syringe.[2] (ii) The gas syringe is placed in a warm water bath.[2] (b) When potassium chromate(VI), K2CrO4, is dissolved in water an equilibrium is set up. The position of equilibrium is well to the left and the solution is a yellow colour. 2(aq) + 2H+(aq) 2CrO4 yellow Cr2O7 orange 2(aq) + H2O(l) The addition of aqueous acid turns the solution an orange colour. Aqueous alkali is then added and the solution turns a yellow colour. Explain these observations in terms of le Chateliers principle.[2]OCR 2016<br />
(c) Catalysts can be used to change the rate of some chemical reactions. 11 (i) Zinc and sulfuric acid react together to form a solution of zinc sulfate, ZnSO4, and hydrogen gas. The reaction is exothermic. The rate of the reaction increases when a catalyst is added.Complete the enthalpy profile diagram for this reaction using the formulae of the reactants and products. Label activation energies, Ea (without catalyst) and Ec (with catalyst). Label the enthalpy change of reaction, H. enthalpy progress of reaction [3]OCR 2016 Turn over<br />
12 (ii) Using a Boltzmann distribution, explain how a catalyst increases the rate of a chemical reaction. Include a labelled sketch of your Boltzmann distribution on the grid below. Label the axes and any other important features. Your answer needs to be clear and well organised using the correct terminology.[4]OCR 2016<br />
(d) The chemical industry uses catalysts for many of its reactions. 13 (i) State an example of a catalyst used by the chemical industry and write the equation for the reaction that is catalysed. catalystequation[1] (ii) State two ways that the use of catalysts helps chemical companies to make their processes more sustainable and less harmful to the environment.[2] (e) In the stratosphere, nitrogen oxides can catalyse the breakdown of ozone. (i) State two sources of nitrogen oxides in the stratosphere.[1] (ii) Write equations to show how nitrogen monoxide catalyses the breakdown of ozone.[2] [Total: 19]OCR 2016 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN16_question_4-0.jpg)
![F322 Question 4 (a) Answer (i) Equilibrium (position) shifts to right Mark scheme June 2016 (a) (ii) Equilibrium (position) shifts to left AND turns paler (brown) (cid:57) Right-hand side has fewer (gaseous) moles/molecules OR left-hand side has more (gaseous) moles/molecules (cid:57) AND turns darker/deeper (brown) (cid:57) (Forward) reaction is exothermic OR (forward) reaction gives out heat OR reverse reaction is endothermic OR reverse reaction takes in heat (cid:57) Marks Guidance ALLOW turns colourless IGNORE initially goes darker (brown) Note: ALLOW suitable alternatives for to right, e.g.: towards products OR towards N2O4 OR in forward direction OR favours the right IGNORE responses in terms of rate ALLOW turns brown Note: ALLOW suitable alternatives for to left, e.g.: towards reactants OR towards NO2 OR in reverse direction OR favours the left IGNORE comments about the exothermic side or endothermic side ALLOW equilibrium (position) shifts left AND in the endothermic direction for second marking point IGNORE responses in terms of rate 17<br />
June 2016 Guidance 2 2 / H2O ANNOTATE ANSWER WITH TICKS AND CROSSES IGNORE amount of acid increases (in question) ALLOW (added) acid reacts with CrO4 Note: ALLOW suitable alternatives for to right, e.g.: towards products OR towards Cr2O7 OR in forward direction OR favours the right ALLOW H+ + OH (cid:111) H2O ALLOW alkali reacts with (added) acid Note: ALLOW suitable alternatives for to left, e.g.: towards reactants OR towards CrO4 2 / H+ OR in reverse direction OR favours the left IGNORE just H+ concentration decreases (needs role of alkali) IGNORE concentration of water increases (needs role of alkali) Mark scheme Marks F322 Question (b) Answer Addition of acid [H+] OR H+ increases AND equilibrium (position) shifts to right (cid:57) Addition of alkali Alkali reacts with H+ OR alkali removes H+ AND equilibrium (position) shifts to left (cid:57) 18<br />
Mark scheme June 2016 F322 Question (i) (c) y p a h n E Marks Answer Ea Zn(s) + H2SO4(aq) Ec (cid:39)H Ea: without catalyst Ec: with catalyst ZnSO4(aq) + H2(g) Progress of reaction Zn and H2SO4 on LHS AND ZnSO4 + H2 on RHS(cid:57) H labelled with product below reactant AND arrow downwards (cid:57) Ea AND Ec correctly labelled with Ec below Ea (cid:57) Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES IGNORE state symbols. H: DO NOT ALLOW H ALLOW this arrow even if it has a small gap at the top and bottom i.e. does not quite reach reactant or product line ALLOW no arrowhead or arrowheads at both ends of activation energy line The Ea line must point to maximum (or near to the maximum) on the curve OR span approximately 80% of the distance between reactants and maximum regardless of position ALLOW AE or AE for Ea Ea: 19<br />
F322 Question (ii) (c) Answer Mark scheme Marks Correct drawing of a Boltzmann distribution curve (cid:57) Axes labelled y axis: (number of) molecules AND x axis: (kinetic) energy (cid:57) Catalyst lowers the activation energy (by providing an alternative route) (cid:57) QWC(With a catalyst a) greater proportion of molecules with energy greater than activation energy OR (With a catalyst a) greater proportion of molecules with energy equal to the activation energy OR (With a catalyst there is a) greater area under curve above the activation energy (cid:57) 20 June 2016 Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES Curve must start at origin. The limit of acceptability is that the curve must start within the first small square nearest the origin. Curve must not touch the x-axis at higher energy IGNORE a slight inflexion on the curve DO NOT ALLOW two curves DO NOT ALLOW a curve that bends up at the end by more than one small square ALLOW particles instead of molecules on y axis DO NOT ALLOW enthalpy for x-axis label DO NOT ALLOW atoms instead of particles or molecules ALLOW ECF for the subsequent use of atoms (instead of molecules or particles) ALLOW annotations on Boltzmann distribution diagram QWC requires more molecules have/exceed activation energy/Ea. IGNORE more molecules have enough energy to react for the QWC mark (as not linked to Ea) ORA if states the effect with no catalyst IGNORE (more) successful collisions<br />
F322 (d) Question Answer (i) Catalyst (name or correct formula) AND balanced equation for the reaction catalysed (cid:57) Mark scheme Marks (d) (ii) Any two from: lower temperatures/lower pressures (can be used) (cid:57) lower energy demand OR uses less fuel OR reduces CO2 emissions (cid:57) (different reactions can be used with) greater atom economy OR less waste OR can reduce use of toxic solvents OR can reduce use of toxic reactants (cid:57) (catalysts are often enzymes) generating specific products (cid:57) 21 June 2016 Guidance Many possible responses but in practice it is likely that examples will be few, e.g. Fe AND N2 + 3H2 (cid:111) 2NH3 V2O5/Pt AND 2SO2 + O2 (cid:111) 2SO3 H2SO4/H3PO4 AND C2H4 + H2O (cid:111) C2H5OH Hydrogenation of an alkene: e.g. Ni AND C2H4 + H2 (cid:111) C2H6 Esterification: e.g. H2SO4 AND CH3COOH + C2H5OH (cid:111) CH3COOC2H5 + H2O ALLOW multiples for equation Note: the reaction chosen must be a feasible industrial reaction. If you see an alternative from the list above please contact your TL IGNORE catalyst not used up in reaction IGNORE catalyst can be re-used IGNORE lower activation energy IGNORE cheaper IGNORE less greenhouse gases OR reduces global warming ALLOW increases atom economy ALLOW reduce use of hazardous/toxic/harmful/poisonous chemicals<br />
Mark scheme June 2016 Guidance IGNORE car engines ALLOW NO2 + O3 (cid:111) NO + 2O2 IGNORE dots IGNORE O + O3 (cid:111) 2O2 IGNORE 2O3 (cid:111) 3O2 F322 Question (i) (e) Thunderstorms/lightning AND aircraft (cid:57) Answer (e) (ii) NO + O3 (cid:111) NO2 + O2 (cid:57) NO2 + O (cid:111) NO + O2 (cid:57) Marks Total 19 22<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_2_QP_JUN16_answer_4-0.jpg)
