CHEMISTRY LAB: HEAT OF COMBUSTION
(COMBUST.DOC)
Name: ________________________
Date: _________________________
1. Introduction: The heat released by the complete combustion of one mole of a
substance is the heat of combustion of the substance. In this experiment you will
determine the heat of combustion of candle wax, a mixture of hydrocarbons which has an
average formula of C25H52. The unit of heat used in this experiment is the calorie. The
calorie is the quantity of heat required to raise the temperature of one gram of water
through one Celsius degree. The SI unit of heat is the Joule; one calorie is 4.184 Joules.
2. Safety: Use caution to avoid burns from hot metal, hot water, or open flames. The
metal can chimney has sharp edges and points; use caution to avoid cuts. Handle the
thermometer gently and leave the triangular roll-guard on it to avoid it rolling into the
sink or onto the floor and breaking. Tie long hair back and watch loose clothing. Wear
goggles when candle is lighted.
3. Equipment and Supplies Needed:
a. From the prep area:
candle and safety matches
can lid or piece of index card to act as a drip catcher
metal can "chimney" (with pierced air holes at bottom)
thermometer and thermometer clamp (clamp optional)
ice cubes (carry to your lab station in your 250 ml beaker)
an empty soda-pop can with opening ring-tab still attached
b. From your lab station equipment drawer:
medium-sized support ring
c. From your lab station supply cupboard:
glass stirring rod
100 ml graduated cylinder
250 ml beaker
small plastic funnel
d. At your lab station:
ringstand
e. Laboratory apron for each member of your lab group.
4. Procedures:
a. Put on your apron. Light the candle and drip a little wax on the card or can lid
that will act as a drip catcher. Stand the candle in the hot wax and hold until cool to
"glue" the candle to the drip catcher. Blow out the candle. Do not let matches go down
the drain, but wet them to extinguish them and set aside for disposal in the trash later.
b. Weigh the candle, with drip catcher attached, to the nearest centigram. Record
this mass for comparison later.
c. Place the candle and drip catcher on the ringstand base. Place the chimney
over it with the air vents at the bottom.
d. Measure 100 ml of tap water and pour onto the ice in the beaker. Swirl to cool
the water to about five degrees Celsius. Pour out exactly 100 ml of cooled water back
into the graduated cylinder. Pour this into the empty soda can using a small funnel.
e. Slide the glass stirring rod through the opening tab of the soda can and hang it
by the glass rod through the support ring on the ringstand. Adjust the ring until the
bottom of the soda can hangs inside the chimney can about five centimeters above the
wick of the candle.
f. Using the thermometer clamp, clamp the thermometer so that the bulb is well
down in the water in the soda can, but not touching the bottom or the sides. This is how
everything has to be after the lighting of the candle. (Actually, the thermometer clamp is
optional; you can get good results with the thermometer just resting in the can.) When
the reading stops falling, record the starting temperature of the water to the nearest tenth
of a degree.
g. Now loosen clamps and move things so that you can light the candle.
Immediately replace all apparatus and let the heat from the candle warm the water in the
soda can. Watch the thermometer reading rise, swirling the water in the can now and
then to ensure an accurate average reading.
h. When the temperature of the water in the can reaches 35 degrees, extinguish
the candle and swirl the can to determine the final, highest temperature reading obtained.
Record this temperature to the nearest tenth of a degree.
i. Weigh the candle and drip catcher to the nearest centigram and record. Don't
lose any wax drips. Record this mass to the nearest centigram.
j. Return to your seat to do the calculations. If you’re way off, do another run or
two to determine the best answer.
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k. Return all materials and clean your area. Remember: no wax or matches
down the drain; use the trash can. Separate the candle from the drip catcher. Throw
away wax drippings. Fold and return your apron. Wash your hands before leaving.
5. Data:
mass of candle and base before burning
_____ g
volume of water in can
_____ ml
initial temperature of water in can
_____ C.
final temperature of water in can
_____ C.
mass of candle and base after burning
_____ g
6. Calculations:
a. Calculate the change in temperature of the water in the can.
T = _____ C.
b. Determine the mass of the water in the can. [The density of water is 1 g / ml.]
(mass) = (volume)  (density)
_____ g
c. Calculate the amount of heat absorbed by the water in the can. [In the formula
below, c = the specific heat capacity of water = 1 cal / gdeg.]
Q = (m)  (c)  (T)
_____ cal
d. Calculate the mass of candle wax burned.
_____ g
e. Assuming all the heat the candle gave off went to warming up the water in the
can, calculate the heat released by the burning of one gram of wax.
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_____ cal / g
f. Calculate the heat in Joules given off per gram of wax burned.
cal / g x 4.184 J / cal = _____ J / g
_____ J / g
g. Assuming the candle is made mostly of paraffin, a mixture of waxy
hydrocarbons whose average formula is C25H52, calculate the molar mass of candle wax.
[The atomic mass of carbon is 12.0111; of hydrogen, 1.00794.]
_____ g / mol
h. Calculate the heat given off in Joules per mole of wax burned.
(J / g) x (g / mol) = (J / mol)
_____ J / mol
i. Convert Joules per mole to kilojoules per mole. [1000 J = 1 kJ]
_____ kJ / mol
7. Analysis: Calculate the percent error by comparing your experimentally determined
value with the accepted value for the heat of combustion of candle wax when measured
this way, 10 895 kJ / mol.
percent error = {( | accepted - experimental | ) / (accepted)} x 100 %
_____ % error
8. Questions:
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a. Burning a peanut releases enough energy to raise the temperature of 500 grams
of water by 10 Celsius degrees. How many scientific calories are in a peanut? How
many food Calories are contained in a peanut? [1000 cal = 1 kcal = 1 Cal]
_____ cal = _____ Cal
b. What was the purpose of cooling the water below room temperature and then
allowing the water to heat to the same amount above room temperature?
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