NJC_H2_CHEM_P2_Solution Prelim

NJC Preliminary Examination H2 Chemistry Paper 2 Solutions 1 Planning (P) A student was provided with a spirit burner . He was asked to determine the enthalpy change of combustion under laboratory conditions, Hc, for ethan ol using the following setup. Knowing that ther e could be significant heat loss arising from the above experimental setup, he decided to calibrate the calorimeter by burning 0.60 g of methanol (CH 3OH) to determine the calorimeter’s heat capacity, C calorimeter, which accounts for both the water and the copper can. Heat capacity is defined as the number of J oules of heat needed to raise the temperature of the calorimeter by one Kelvin or one degree Celsius. The temperature of the calorimeter rose from 25.0°C to 33.8°C. The same calorimeter was then used to measure the enthalpy of combustion of ethanol. (a) Given the enthalpy change of combustion of methanol is − 715 kJ mol –1, use the information above to calculate the heat capacity of the calorimeter, C calorimeter, stating its units. [2] Heat evolved from methanol = ( ) × 715000 = 13400 J Heat gained by calorimeter = CT = C × 8.8 Heat evolved from methanol = Heat gained by calorimeter 13400 = (8.8)C C = 1520 J K–1 calorimeter (copper can containing 50 cm3 of water) draught shield wick spirit burner containing selected alcohol stirrer lid thermometer

(b) Given the enthalpy change of combustion of ethanol is approximately −1370 kJ mol–1, calculate the minimum mass of ethanol required to give the same temperature change as that in the calibration. [2] For the combustion of ethanol to give the same temperature change as that in the calibration, it means: Heat evolved from ethanol = Heat gained by calorimeter ethanol × (1370 × 103) = 13400 ethanol = ( ) = 9.871 × 10–3 mol Minimum mass of ethanol = (9.871 × 10–3) × 46.0 = 0.450 g (c) Write a plan to determine the enthalpy change of combustion, Hc, of ethanol that the student will carry out. You may assume that you are provided with:  a thermometer with divisions of 0.2 °C division;  the apparatus normally found in a school or college laboratory. Your plan should include details of  the procedure to determine the enthalpy change of combustion of ethanol;  the readings recorded using appropriate table(s), including units;  precautions taken to ensure reliability of the experiment  an outline of how the results would be used to determine the enthalpy change of combustion of ethanol based on the plan that you have written using arbitrary values. [7] Procedure 1