CP Chemistry
Theodore Roosevelt High School
Lab #3-3
Heat of Combustion of a Candle Lab
Introduction
In the past several labs, we have used makeshift calorimeters with water to
measure the heat absorbed by or released from fusion, dissolution and
combustion. These setups have reflected temperature changes in the
correct direction (i.e., up or down) but have also had significant percent
error. In this lab, we will attempt to decrease our percent error by directly
measuring the difference between the heat released by burning a known
amount of paraffin wax and the heat absorbed by a known amount of water.
Purpose
Based on the introduction above, determine the purpose of this lab and include it in your lab write-up.
Prediction
Will the heat change for the combustion of paraffin have a positive or negative value? Explain.
Equipment
Erlenmeyer flask, 125 mL
graduated cylinder, 100 mL
ring stand
rubber stopper
Materials
matches
thermometer
tin can
utility clamp
paraffin candle [C25H52]
Safety Considerations
• Sometimes chemicals from previous labs still remain in glassware and on other lab equipment;
wash all lab equipment before and after performing this lab.
• Wash your hands thoroughly after completing this lab.
Procedure
1.
Obtain a candle; measure and record its mass.
2.
Measure out exactly 100.0 mL tap water into a 125-mL Erlenmeyer flask. Use a stopper with a
hole to seal the flask – make sure the number on the stopper matches the number on the flask!
3.
Using a ring stand, attach the flask to a utility clamp so that its bottom is 1-2 inches above the tin
can.
4.
Measure and record the starting temperature of the water.
5.
Away from the flask, use a match to light the candle and quickly slide it underneath the flask. DO
NOT PLACE USED MATCHES IN THE SINK.
6.
Allow the candle to burn for around 5 minutes or until the temperature of the water reaches 30°C,
whichever comes first.
7.
Gently blow out the candle and record the peak temperature of the water.
8.
Allow the candle to cool for several minutes; measure and record its mass.
9.
Repeat steps #2-8 for two additional trials, using new water each time. Record the starting
temperature, final temperature, and mass change for each trial.
CP Chemistry
Theodore Roosevelt High School
Lab #3-3
Additional Clean-up and Disposal
1.
Dispose of any weigh paper, paper towels or matches in the trash can.
Data – you should create a data table in your lab write-up that looks something like this:
Trial
Starting
Mass
Starting
Temp
Final
Mass
Final
Temp
ΔH*
q*
Trial #1
Trial #2
Calculations (Include these answers in your Conclusion!)
1.
Calculate the average theoretical heat change for the three trials due to the combustion of paraffin
(in kJ)
[ΔH = -41.5 kJ/g].
2.
Calculate the average heat change (q) of the water (in kJ) for the three trials.
3.
Calculate your percent error in this experiment, using the heat change of water as the
experimental value and the heat change due to combustion of paraffin as the accepted value.
Questions
1.
Why is the heat change for the combustion of paraffin a negative value? Why is the heat change
for the water a positive value?
2.
One of the products of the combustion of paraffin is water. What sort of experiment could you
design to show this is formed?
3.
When a candle burns, is it the wax or the wick that burns? Explain why.
Errors
Describe two possible errors you may have committed in this lab that may have somehow affected your
results. Explain the specific steps you will take to avoid each of these errors in the future.
Conclusion
Write two or more paragraphs summarizing your results, examining the validity of your prediction, and
explaining your calculations.
CP Chemistry
Theodore Roosevelt High School
Heat of Combustion of a Candle Lab workspace:
Lab #3-3