DHS Prelim P2_SuggestedSolutions

 DHS 2014 9647/02 2014 DHS Year 6 Preliminary Examination H2 Chemistry 9647/02 Suggested Solutions 1 Planning (P) A student was provided with a spirit burner containing a ‘fuel mixture’ which was prepared by mixing equimolar amounts of pentane and ethanol. The enthalpy change of combustion of this ‘fuel mixture’ is 11.8 kJ per mole of ‘fuel mixture’. He was told to use the enthalpy change of combustion of this ‘fuel mixture’ to find the heat capacity of a metal calorimeter using the apparatus shown below. Heat capacity is defined as the number of joules of heat needed to raise the temperature of the calorimeter by one Kelvin or one degree Celsius. Additional information: 1. Specific capacity of water is 4.2 J g1 K1. 2. The maximum capacity of the metal calorimeter is between 100 to 150 cm3. 3. A temperature rise of 5 C is considered to be significant for this experiment. (a) Construct a balanced equation for the complete combustion of the ‘fuel mixture’ with state symbols. [1] C5H12(l) + C2H5OH(l) + 11O2(g)  7CO2(g) + 9H2O(l) (b) Identify one possible source of error and suggest an improvement to overcome this error in the experiment. [2] Error: Heat loss to the surroundings by the calorimeter and water. Improvement: Conduct the experiment in a draught-free room. Also accept: 1. Provide lagging on the metal calorimeter. 2. Cover the metal calorimeter with a cover/lid. (c) Calculate the minimum mass of the fuel mixture required to bring about a 5 C temperature rise. Assuming no heat loss, Q = mc∆T = (100)(4.2)(5) = 2100 J ∆Hc = – 2100 / nfuel mixture  – 11.8 x 103 = – 2100 / nfuel mixture  nfuel mixture = 2100 / (11.8 x 103) = 0.1779 mol spirit burner containing the fuel mixture metal calorimeter

Y6 H2 Chemistry Preliminary Exam 2 Suggested Solutions  DHS 2014 9647/02  minimum mass of fuel mixture = 0.1779 x 59.0 = 10.5 g [2] (d) Write a plan to determine the heat capacity of the metal calorimeter using the apparatus provided. In your plan you should give details of the procedure (number your steps) and provide a table to record the readings to be taken, including the units. Details about the appropriate mass of water used and temperature rise should also be included. [4] 1) Weigh the spirit burner containing the ‘fuel mixture’ 2) Using a measuring cylinder, measure 100 cm 3 of water into the metal calorimeter. 3) Measure the initial temperature of the water using a thermometer after the water is allowed to stand for a few minutes. 4) Light the burner and allow it to heat the water in the calorimeter. 5) Monitor the temperature of water using the thermometer, and extinguish the flame when the te