RP 2

• Experimentally finding the enthalpy change of combustion

  1. Stage 1: preparing the chemicals

  2. Stage 2: carrying out the experiment

  3. Stage 3: interpreting the results
• Experimental values for enthalpy change calculated by students rarely match those quoted in data booklets
• Normally the experimental value is lower than that quoted

  Because of heat lost to the surroundings
• Adding more water to the calorimeter than accounted for


  Results in the calculated enthalpy change being larger than expected
• The percentage error for a thermometer that is precise to one decimal place is normally between 1 and 2% for this type of experiment

  Unnecessary to use a more precise thermometer
• Methods to minimise heat loss

  • Reduce the distance between the flame and the beaker / calorimeter
  • Put a protective sleeve around flame
  • Put a lid on the beaker / calorimeter
• Experimentally finding the enthalpy change of reaction accounting for heat loss

  1. Add a sample of solid potassium chloride to water and start the stop watch

  2. Record the temperature of the solution every subsequent minute for approximately 5 minutes

  3. Plot a graph of temperature (y-axis) versus time (x-axis)

  4. Extrapolate back from the time of mixing (T=0s) to establish the maximum temperature change

  5. Calculate ΔH in kJ mol-1 using the equation
• Experimentally finding the enthalpy change of a reaction via a graph

  1. Weigh out 1.00 g of anhydrous calcium chloride

  2. Measure out 25.0 cm3 of water and transfer to a plastic cup

  3. Record the temperature of the water every minute, stirring before reading

  4. Add the anhydrous calcium chloride at the fourth minute

  5. Continue recording temperature at minute intervals for five more minutes
• Use an appropriate graph to determine an accurate minimum temperature that is not influenced by heat loss. Then use this value to calculate ΔH for the reaction in kJmol-1.