SCH 3U

Heat of Combustion

Purpose: To determine the heats of combustion of a) ethanol and b) paraffin wax.

Introduction: The heat of combustion of paraffin and ethanol could be determined by measuring the heat change using a simple calorimeter. The total heat of combustion will be the sum of the heat change of the beaker and the water.

Q_total = Q_water + Q_beaker

The specific heat capacity of glass is 0.84 J/g^oC

Procedure:

Part A: Combustion of Ethanol

• Weigh a clean dry 250-mL beaker. Add 100-mL of water using the graduated cylinder and record the temperature to the nearest 0.2^oC.



• Set up the retort stand and place the beaker so that it will be just above the evaporating dish (containing the ethanol).



• Measure 10-mL of ethanol into the evaporating dish (CAUTION: DO THIS AT THE FUME HOOD).



• Light the ethanol and allow it to burn. The water should be gently stirred as the ethanol burns.



• After the ethanol has completely burned, record the final temperature to the nearest 0.2^oC.



• Determine the mass of ethanol burned (Density of ethanol C₂H₅OH = 0.78 g /mL.



• Calculate the heat of combustion of ethanol in kJ/g and kJ/mol.

Part B: Combustion of Paraffin

• Weigh a clean dry 100-mL beaker. Add 20-mL of water using the graduated cylinder and record the temperature to the nearest 0.2^oC.



• Set up the retort stand and place the beaker so that it will be just above the flame from the candle.



• Weigh the candle.



• Place the candle beneath the beaker and light the wick. Allow the candle to burn until the temperature of water changes by 10^oC.



• Blow the candle out and allow it to cool. Record the highest temperature reached by the water. Re-weigh the candle and calculate the mass of paraffin that was burned.



• Calculate the heat of combustion of paraffin (kJ/g and kJ/mol). The molecular formula for candle wax is C₂₀H₄₂.

Observations:

Conclusion:

A concluding statement for your results (heats of combustion) and answer the following.

Questions:

• From your knowledge of combustion, and your observations from this experiment, write thermochemical equations for both reactions
  
• The value listed in reference books for the heat of complete combustion for the nearest hydrocarbon to paraffin, eiocosane, C₂₀H₄₂, is -13 360 kJ/mol. Most experimentally determined values vary by as much as 40 - 50% from this accepted value. Determine your percentage error.
  
• List as many possible sources of error (at least 4) in this exercise as you can. Indicate for each one whether the error would cause an increase or decrease in the value of the heat of combustion determined.



• How would you alter the experimental setup to obtain more accurate results?