Heat of Combustion of Candle Wax

Purpose: To observe a burning candle and calculate the heat associated with the combustion reaction.

Equipment & Materials:

  • balance
  • candle
  • matches
  • modeling clay
  • ruler
  • weigh boat, small

Procedure:

  1. Measure and record the length of a candle in centimeters.
  2. Stick the candle to the weigh boat using a small ball of modeling clay so that it stands upright.
  3. Measure the mass of the candle-clay-weigh boat system.
  4. Record the time (using the clock) as you light the candle.
  5. Let the candle burn for approximately 5 minutes.

(NOTE: WHILE YOU WAIT, start working on the Observations/Analysis section.)

  1. After about 5 minutes, carefully/gently blow out the candle and record the time.
  2. Measure the mass of the foil-candle system again.
  3. Remove the candle from its clay base. Measure and record the new length of your candle.
  4. Clean up all materials to your lab kit as directed by your instructor.

Data Table:Candle Measurements

# / Candle Measurements / Initial
(before lighting) / Final
(after burning)
1 / Time of day (hr:min)
2 / Length (cm)
3 / Mass of candle, clay and weigh boat (g)

Observations/Analysis: Using your experimental data, answer the following questions:

  1. Observe the candle as it burns and record what you see). Include both drawing(s) and a written description.

continued on back

  1. Examine the flame closely as it burns. Is it the wax or the wick that burns? How can you tell?
  1. If you said the wax burns, explain the function of the wick. If you said the wick burns, describe the function of the wax. (Essentially, you should explain how a candle works.)
  1. Using your data, how much length and mass did the candle lose? Are these data more consistent with the wax or the wick burning?

∆length = ∆mass =  was burning

(wax or wick)

Calculations:

  1. The formula for candle wax is C20H42. Write and balance an equation for the combustion of the candle wax.

(HINT!!! Think “combustion”. What must the other reactant be in a “combustion” reaction? Also, candle wax is a hydrocarbon. What are the 2 products of hydrocarbon combustion?)

  1. Calculate the number of moles of candle wax that burned in the experiment.
  1. Calculate the heat of combustion of candle wax in kJ/mol, given the following standard heats of formation:

∆HfC20H42(s)= -2230 kJ/mol;

∆HfCO2(g) = -394 kJ/mol

∆HfH2O(g) = -242 kJ/mol.