AQA Jan 2011 Paper 2 Q9

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section b answer all questions in the spaces provided do not write outside the box a student devised an experiment to investigate the enthalpies of combustion of some alcohols the student chose the following series of primary alcohols name formula methanol ethanol propan ol butan ol pentan ol alcohol x heptan ol ch oh ch ch oh ch ch ch oh ch ch ch ch oh ch ch ch ch ch oh ch ch ch ch ch ch oh ch ch ch ch ch ch ch oh a i name alcohol x mark a ii state the general name of the type of series shown by these primary alcohols mark a iii draw the displayed formula of the position isomer of butan ol a iv using o to represent the oxidising agent write an equation for the oxidation of butan ol to form an aldehyde mark mark wmp jan chem a v draw the displayed formula of a functional group isomer of this aldehyde do not write outside the box b the student carried out a laboratory experiment to determine the enthalpy change when a sample of butan ol was burned the student found that the temperature of g of water increased by oc when x mol of pure butan ol was burned in air and the heat produced was used to warm the water mark use the student s results to calculate a value in kj mol for the enthalpy change when one mole of butan ol is burned the specific heat capacity of water is j k g marks extra space question continues on the next page turn over wmp jan chem c i give the meaning of the term standard enthalpy of combustion do not write outside the box marks extra space c ii use the standard enthalpy of formation data from the table and the equation for the combustion of butan ol to calculate a value for the standard enthalpy of combustion of butan ol ch ch ch ch oh i o g co g h o i h f kj mol ch ch ch ch oh i o g co g h o i marks extra space wmp jan chem do not write outside the box d the student repeated the experiment described in part b and obtained an experimental value for the enthalpy of combustion for each alcohol in this series these experimental values were then compared with calculated values from standard enthalpies of formation as shown in the graph below enthalpies of combustion values for a series of primary alcohols enthalpy of enthalpy of combustion combustion kj mol kj mol calculated experimental m of alcohol mr of alcohol d i in terms of bonds broken and bonds formed explain why the calculated values of enthalpies of combustion of these alcohols when plotted against mr follow a straight line marks extra space d ii give two reasons why the experimental values obtained by the student are lower than the calculated values using the enthalpy of formation data marks turn over wmp jan chem

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