ENERGETICS

• The enthalpy of combustion determined experimentally is less exothermic than that calculated using enthalpies of formation.

  Give a possible reason for this.

  Heat loss

  Incomplete combustion

  Evaporation of fuel

  Experiment not completed under standard conditions
• Explain why the value given for the O=O bond enthalpy is not a mean value.

  Only substance with O=O is oxygen
• The student considered using a glass beaker on a tripod and gauze instead of the clamped copper calorimeter.

  Suggest two disadvantages of using a glass beaker on a tripod and gauze.

  Glass is poorer conductor than copper

  Tripod and gauze reduce heat transfer

  Tripod and gauze would have fixed height above flame
• Suggest one addition to this apparatus that would improve the accuracy of the enthalpy value obtained.

  Wind shield
• The student uses data to determine a value for the enthalpy change.

  Explain how the experimental method and use of apparatus can be improved to provide more accurate data.

  APPARATUS

  • burette/ pipette not measuring cylinder
  • polystyrene cup/ insulate
  • reweigh watchglass after adding solid
  • powdered solid

  MEASUREMENTS

  • measure/ record initial temperature of solution before addition
  • measure/ record temperature of solution after addition at regular intervals until trend observed

  DETERMINIATION

  • plot graph of temperature against time
  • extrapolate
  • determine enthalpy change at addition
• Suggest how, without changing the apparatus, the experiment in part (c) could be improved to reduce the percentage uncertainty in the temperature change.

  Increase concentration of solutions
• State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.

  Pressure not constant in bomb calorimeter (enthalpy change requires constant pressure)
• Give one reason why bond enthalpy that you calculated in part (c) is different from the mean bond enthalpy quoted in a data book.

  Data book value derived from a number of different compounds
• Suggest how the students' method, and the analysis of results, could be improved in order to determine a more accurate value for the enthalpy of reaction.

  Justify your suggestions.

  Do not refer to precision of measuring equipment/ amounts/ concentrations of chemicals.

  INSULATION

  • polystyrene cup + lid
  • to reduce heat loss

  TEMPERATURE RECORDING

  • record temperature at regular intervals
  • plot against time on a graph

  RESULTS ANALYSIS

  • extrapolate to point of addition
  • establish theoretical maximum temperature
• Suggest a reason why the value calculated in part (d) differs from the mean Xe-F bond enthalpy quoted in a data source.

  Mean bond enthalpy found by taking an average for Xe-F in a range of compounds
• A 50.0g sample of water is used in this experiment. Explain how you could measure out this mass of water without using a balance.

  Water has a known density

  Therefore a volume of 50 cm cubed could be measured out
• Define the term enthalpy change.

  Heat energy change at a constant pressure.