AQA Jun 2015 Paper 2 Q7 (with explained solutions) |

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AQA Jun 2015 Paper 2 Q7

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do not write outside the box a propanone can be formed when glucose comes into contact with bacteria in the absence of air a i balance the following equation for this reaction of glucose to form propanone carbon dioxide and water c h o ch coch co h o a ii deduce the role of the bacteria in this reaction mark mark b propanone is also formed by the oxidation of propan ol b i write an equation for this reaction using o to represent the oxidising agent mark b ii state the class of alcohols to which propan ol belongs mark wmp jun chem do not write outside the box c a student determined a value for the enthalpy change when a sample of propanone 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 propanone was burned in air use the student s results to calculate a value in kj mol for the enthalpy change when one mole of propanone is burned the specific heat capacity of water is j k g marks question continues on the next page turn over wmp jun chem d define the term standard enthalpy of combustion do not write outside the box marks e use the mean bond enthalpy data in table and the equation given below table to calculate a value for the standard enthalpy change when gaseous propanone is burned marks table c h c c c o o h c o o o mean bond enthalpy kj mol ch coch g o g co g h o g wmp jun chem do not write outside the box f suggest two reasons why the value obtained by the student in question c is different from the value calculated in question e marks reason reason turn over for the next question turn over wmp jun chem

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mark scheme a level chemistry chem june marking guidance mark comments c h o ch coch co h o to speed up the reaction or provide a catalyst or catalyses the reaction or biological catalyst or release contain provides an enzyme ch ch oh ch o ch coch h o or multiples ignore fermentation ignore to break down the glucose not simply enzyme on its own any correct representation for the two organic structures brackets not essential not sticks for the structures in this case secondary alcohol or o alcohol of question a i a ii b i b ii mark scheme a level chemistry chem june m q m c t or q m j or kj or kj also scores m m this mark is for dividing correctly the number of kj by the number of moles and arriving at a final answer in the range shown therefore h kj mol or to kj mol using mol range to gains marks but gains marks and gains marks and gains marks and gains marks award full marks for correct answer in m do not penalise incorrect cases in the formula penalise m only if correct numerical answer but sign is incorrect to gains marks penalise m for arithmetic error and mark on if t score q m c t only if c leads to penalise m only and mark on for m ignore incorrect units in m if units are given in m they must be either kj or kj mol in this case of c mark scheme a level chemistry chem june m the enthalpy change heat change at constant pressure when mol of a compound substance element m is burned combusts reacts completely in oxygen or burned combusted reacted in excess oxygen m with all reactants and products all substances in standard specified states or all reactants and products all substances in normal states under standard conditions kpa bar and specified t k if standard enthalpy of formation ce for m ignore reference to atmosphere of d m b reactants b products h or sum of bonds broken sum of bonds formed h or b c c b c o b c h b o o lhs b c o b o h rhs h m also scores m lhs rhs h or using only bonds broken and formed m h kj mol candidates may use a cycle and gain full marks e 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 addition error this would score marks m and m if no ae check for a correct method this requires either a correct cycle with o co and h o or a clear statement of m which could be in words and scores only m allow a maximum of one mark if the only scoring point is lhs or or rhs or award mark for of mark scheme a level chemistry chem june for the two marks m and m any two from heat loss or not all heat transferred to the apparatus or heat absorbed by the apparatus or specific heat capacity of the apparatus not considered incomplete combustion not completely burned reaction is not complete the idea that the water may end up in the gaseous state rather than liquid reactants and or products may not be in standard states mbe data refers to gaseous species but the enthalpy of combustion refers to liquids in their standard states liquid propanone and liquid water in standard states mbe do not refer to specific compounds or mbe values vary with different compounds molecules or are average mean values taken from a range of compounds molecules apply the list principle but ignore incomplete reasons that contain correct chemistry ignore evaporation ignore faulty equipment ignore human error not enough simply to state that mbe are mean average values of f

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