AQA Jun 2014 Paper 2 Q9 (with explained solutions) |

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AQA Jun 2014 Paper 2 Q9

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ethanol is an important fuel do not write outside the box a a dilute aqueous solution of ethanol can be produced by the fermentation of an aqueous solution of glucose it is claimed that the ethanol obtained from this solution is a carbon neutral biofuel write an equation for this fermentation reaction give two other essential conditions for this reaction to produce a good yield of ethanol name a process used to produce a much more concentrated solution of ethanol from a dilute aqueous solution state the meaning of the term carbon neutral in the context of this biofuel marks extra space question continues on the next page turn over wmp jun chem do not write outside the box b a student carried out a laboratory experiment to determine the enthalpy change when a sample of ethanol was burned the heat produced was used to warm some water in a copper calorimeter the student found that the temperature of g of water increased by c when mol of pure ethanol was burned in air use the student s results to calculate a value in kj mol for the enthalpy change when one mole of ethanol is burned the specific heat capacity of water is j k g deduce two reasons why the student s value for the standard enthalpy of combustion of ethanol is different from a data book value of kj mol marks extra space wmp jun chem do not write outside the box c mean bond enthalpies can be used to calculate enthalpies of reaction c i give the meaning of the term mean bond enthalpy marks c ii consider the mean bond enthalpy data in table table c h c c c o o o c o o h mean bond enthalpy kj mol to be calculated use the data in table and the equation shown to calculate a value for the bond enthalpy for the o o double bond in an oxygen molecule marks ch ch oh g o g co g h o g h kj mol end of questions wmp jun chem

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$mark scheme a level chemistry chem june marking guidance mark comments m c h o ch ch oh co c h oh m and m any two conditions in any order for m and m from enzymes from yeast or zymase c t c or k t k anaerobic no oxygen no air or neutral ph m fractional distillation or glc m carbon neutral in this context means there is no net overall annual carbon dioxide co emission to the atmosphere or there is no change in the total amount level of carbon dioxide co present in the atmosphere penalise c h o for ethanol in m mark m and m independently a lack of oxygen can mean either without oxygen or not having enough oxygen and does not ensure no oxygen therefore only credit lack of oxygen if it is qualified penalise bacteria phosphoric acid high pressure using the list principle ignore reference to aqueous or water ie not part of the list principle for m must be about co and the atmosphere the idea that the carbon dioxide co given out equals the carbon dioxide co that was taken in from the atmosphere of question a b m q m c t this mark for correct mathematical formula m j or kj or kj or kj also scores m m using mol therefore h kj mol accept a range from to but do not penalise more than significant figures m and m in any order any two from incomplete combustion heat loss heat capacity of cu not included some ethanol lost by evaporation not all of the mol ethanol is burned reaction is incomplete mark scheme a level chemistry chem june full marks for m m and m for the correct answer in m do not penalise incorrect cases in the formula ignore incorrect units in m penalise m only if correct numerical answer but sign is incorrect therefore gains two marks if units are quoted in m they must be correct if t ce for the calculation and penalise m and m if c leads to penalise m only and mark on for m of mark scheme a level chemistry chem june m enthalpy heat energy change at constant pressure or enthalpy heat energy needed in breaking dissociating a covalent bond s m averaged for that type of bond over different a range of molecules compounds ignore bond making ignore reference to moles of c i c ii m b reactants b products h or sum of bonds broken sum of bonds formed h or b c c b c o b o h b c h b o o b c o b o h h m also scores m b o o or if o h cancelled h or if o h cancelled b o o kj mol m b o o kj mol award mark for students may use a cycle and gain full marks mark scheme a level chemistry chem june correct answer gains full marks credit mark for kj mol 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 co and h o or a clear statement of m which could be in words and scores only m credit a maximum of one mark if the only scoring point is bonds formed adds up to or or bonds broken includes the calculated value of or of$

Question 1 2 3 4 5 6 7 8 9

Topics to revise

AQA > Physical chemistry > Equilibrium AS >

1 Dynamic equilibrium and factors affecting equilibrium

AQA > Physical chemistry > Energetics and kinetics AS >

2 Calorimetry

AQA > Inorganic chemistry > Periodicity AS >

1 Periodicity

AQA > Organic Chemistry > Alcohols AS >

3 Forming ethanol

AQA > Physical chemistry > Thermodynamics >

2 Born-Haber cycle

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