OCR A Jun 2012 Paper 2 Q3 (with explained solutions) |

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OCR A Jun 2012 Paper 2 Q3

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3 Hydrogen has many industrial uses including making margarine and ammonia. Hydrogen can be made by the reaction between methane and steam. CH4(g) + H2O(g) CO(g) + 3H2(g) H = +210 kJ mol1 (a) The pressure of the equilibrium mixture is increased. Explain what happens to the position of the equilibrium.[2] (b) The temperature of the equilibrium mixture is increased. Explain what happens to the position of the equilibrium.[2] (c) The reaction is actually carried out in the presence of a nickel catalyst at a pressure of 30 atmospheres. (i) Suggest why the manufacturer uses a pressure of 30 atmospheres.[1]OCR 2012 Turn over<br />
10 (ii) The nickel catalyst increases the rate. Use a labelled diagram of the Boltzmann distribution of molecular energies to explain why.[3] (d) A chemical factory uses 200 tonnes of methane a day. The factory produces 68.4 tonnes of hydrogen per day by reacting methane with steam. CH4(g) + H2O(g) CO(g) + 3H2(g) Calculate the percentage yield of hydrogen. Give your answer to three significant figures. (1 tonne = 1106 g) percentage yield of hydrogen =% [3]OCR 2012<br />
(e) The carbon monoxide produced in the equation below can be reacted with hydrogen to make methanol. 11 CH4(g) + H2O(g) CO(g) + 3H2(g) (i) Construct the equation for the reaction of carbon monoxide with hydrogen to make methanol.[1] (ii) Suggest two reasons why it is important to use the carbon monoxide to make methanol.[2] (f) Describe how hydrogen can be used in the manufacture of margarine.[2] [Total: 16]OCR 2012 Turn over<br />

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$F322 Question 3 (a) (b) Answer (equilibrium position shifts) to the left (cid:57) (because there are) fewer moles (of gas) on the reactant side OR (there are) more moles (of gas) on product side (cid:57) This explanation mark is dependent on the correct shift of the equilibrium (equilibrium position shifts) to the right (cid:57) (because forward) reaction is endothermic OR reverse reaction is exothermic (cid:57) This explanation mark is dependent on the correct shift of the equilibrium (c) (i) Gives a high rate of reaction OR reaction is fast OR reasonable rate of reaction without shifting equilibrium too much to the left (cid:57) 13 Mark Scheme June 2012 Marks Guidance Note: ALLOW suitable alternatives for to left, eg: towards CH4 or H2O / towards reactants OR in backward direction OR in reverse direction OR decreases yield of CO or H2/products ALLOW favours the left, as alternative for shifts equilibrium to left ALLOW fewer molecules on reactant side OR smaller volume on the left hand side ALLOW ORA if specified IGNORE responses in terms of rate Note: ALLOW suitable alternatives for to right, eg: towards CO or H2 / towards products OR in forward direction OR increases yield of CO or H2/products OR decreases amount of CH4 or H2O/reactants ALLOW favours the right, as alternative for shifts equilibrium to right ALLOW reaction takes in heat ALLOW reverse reaction gives out heat ALLOW ORA if specified IGNORE responses in terms of rate ALLOW if greater pressure used it increases safety risk ALLOW if greater pressure used it is more expensive ALLOW higher pressure will shift equilibrium position even more to the left It is a compromise on its own is not sufficient but ALLOW compromise between rate and yield OR between rate and safety<br /> F322 (ii) Question 3 (c) Mark Scheme June 2012 Answer y-axis label is '(fraction of or number of) molecules' AND x- axis label is 'energy' AND correct curve (cid:57) Lowers activation energy (cid:57) More molecules with energy above activation energy with a catalyst OR more effective collisions OR more successful collisions (cid:57) Marks Guidance ANNOTATE ANSWER WITH TICKS AND CROSSES ETC Boltzmann distribution - must start at origin and must not end up at 0 on y-axis ie must not touch x-axis ALLOW particles OR moles as y-axis label IGNORE minor point of inflexion in the curve DO NOT ALLOW two curves DO NOT ALLOW atoms but credit atoms if used in a second marking point DO NOT ALLOW enthalpy for x-axis label ALLOW this mark from a labelled diagram more collisions per second is not sufficient (number of) molecules Ea cat Ea energy extra molecules with energy above Ea 14<br /> Mark Scheme June 2012 Marks Answer FIRST, CHECK THE ANSWER ON ANSWER LINE IF answer = 91.2 (%) award 3 marks theoretical amount of hydrogen = 3.75107 (mol) (cid:57) actual amount of hydrogen made = 3.42107 (mol) (cid:57) % = 91.2 (cid:57) Guidance IF there is an alternative answer, check to see if there is any ECF credit possible using working below IF ECF, ANNOTATE WITH TICKS AND CROSSES, etc Answer must have three significant figures ALLOW ECF from incorrect theoretical and actual amounts of hydrogen ALLOW answer that uses grams rather than tonnes where theoretical amount of hydrogen = 37.5 (mol) and actual amount of hydrogen = 34.2 (mol) ALLOW alternative approach based on the mass of hydrogen rather than the amount of hydrogen Theoretical amount of hydrogen = 3.75107 (mol) (cid:57) Theoretical mass of hydrogen made = 75 (tonnes) (cid:57) Percentage = 91.2 (cid:57) ALLOW correct multiples ALLOW CH4O IGNORE state symbols IGNORE harmful or dangerous ALLOW uses a waste product OR CO is then a desired product OR CO is no longer a waste product OR reduces amount of waste product ALLOW other uses of methanol eg petrol additive, solvent or organic feedstock F322 Question 3 (d) (e) (i) CO + 2H2 (cid:198) CH3OH (cid:57) (ii) Any two from: Carbon monoxide is toxic OR poisonous (cid:57) Increases atom economy of the process OR gives 100% atom economy (cid:57) Methanol is a fuel (cid:57) 15<br /> F322 Question 3 (f) Unsaturated (vegetable) oils Answer Mark Scheme June 2012 OR oils containing C=C bonds (cid:57) (reacted with hydrogen) in the presence of a nickel catalyst (cid:57) Total Marks Guidance ALLOW unsaturated fats OR unsaturated lipids OR unsaturated ester ALLOW oils become more saturated IGNORE unsaturated compound DO NOT ALLOW unsaturated hydrocarbon ALLOW Pt OR Pd 16 16<br />$

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