Name _______________________________ Partner: ________________________________
Combustion Lab
Introduction
The burning, or combustion, of candle wax, C25H52, a hydrocarbon, can be summarized this way:
C25H52(s) + 38 O2(g)→ 25 CO2(g) + 26 H2O(g) + Energy (or “Heat of Combustion”)
Paraffin wax (alkane)
As you already know, a burning candle gives off energy—the reaction is exothermic. Thus, less energy must be stored in
the product molecules (in the carbon-to-oxygen bonds of carbon dioxide gas and the
hydrogen-to-oxygen bonds of water vapor) than was originally stored in the reactant molecules (in the carbon-tohydrogen bonds of wax and the oxygen-to-oxygen bonds of oxygen gas).
Fuels provide thermal energy as they burn. But how much energy is released? How can we measure the quantity of
released energy? In this investigation, you will measure the heat of combustion and molar heat of combustion of a candle
(paraffin wax) and compare these quantities with known values for other hydrocarbons. The heat of combustion is the
quantity of thermal energy that is released when a specific quantity of a material burns (in grams). The molar heat of
combustion is the quantity of thermal energy that is released when a specific quantity of a material burns (in moles).
In this laboratory investigation, you will record data before and after the candle is burned. The data table is on page 4.
Procedure
1. Determine the mass of the tea candle and index card. Record its value in the data table.
2. Measure the air room temperature with the thermometer to the nearest 0.1 °C, and record this value above the data
table on the last page.
3. Carefully pour about 100 mL of chilled water into a graduated cylinder. Record the volume to the 0.1 mL place in
the data table. Record the temperature of the chilled water to the nearest 0.1 °C in the data table. (The chilled
water, provided by your teacher, should be 10 to 15 °C colder than room temperature.) Pour the sample of chilled
water into an empty soft-drink can, being careful not to spill the water.
4. Place the candle and index card under the can of water, but do not light the candle yet! Adjust the can so that the
top of the candlewick is about 2 cm from the bottom of the can.
5. Light the candle and place it directly underneath the can. As the water heats, you can either carefully swirl the can,
or stir it gently, very gently, with the thermometer. As the candle burns and becomes shorter, you may need to lower
the can so the flame remains just below the bottom of the can. (Caution: Lower the can with great care, be careful not
to extinguish the flame.)
6. If the flame goes out, quickly re-light the candle and place it back under the can.
7. Continue heating the water until its temperature rises as far above room temperature as it was below room
temperature at the start. (For example, if the chilled water temperature was 15.0 °C before heating and room
temperature is 25.0 °C, you would heat the water to 35.0 °C, that is, 10.0 °C higher than room temperature.)
8. When the desired temperature is reached, extinguish the candle flame.
9. Continue stirring the water until its temperature stops rising. Record the highest temperature reached by the
warmed water.
10. Determine and record the mass of the cooled candle and the index card, including all wax drippings.
Data Analysis
A characteristic property of a material is the quantity of heat needed to raise the temperature of one gram of the material
by one degree Celsius. This value is called the specific heat capacity of the material. The specific heat capacity of
liquid water is about 4.184 J/(g · °C) (joules per gram per °C). This means that each 4.184 J of energy absorbed by one
gram of water will increase its temperature by one degree Celsius.
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Sample Problem: Suppose a 93.6-g water sample is heated from 25.3 °C to 34.7 °C, an increase of
9.4 °C. How much thermal energy must have been absorbed by the water?
The Law of Conservation of Energy states that energy is neither created nor destroyed. In a system such as the candle,
metal can, and water, we can assume that the energy (q) that is lost from the candle is transferred to the water. The
energy lost by the candle (q) is equal to the energy gained by the water:
q(lost by candle) =
q(gained by water)
The quantity of energy (q) gained by the water depends on:
1) the mass of the water (m) in grams
2) the specific heat of the water (c), and
3) the difference between the final and initial temperatures of the water (Tfinal – Tinitial = ∆T).
This relationship can be represented by the following equation:
q = m x c x (∆T)
In the sample problem above, the amount of energy absorbed by the water is calculated by plugging in the data:
q = (93.6 grams) x (4.184 J/g x °C ) x (34.7°C – 25.3°C )
q = 3681 Joules = the energy absorbed by the water = the energy released by the combustion of the
candle
The quantity of thermal energy given off when a certain amount of a substance burns is called the heat of
combustion. See Table 3.9 (page 340 of your textbook). The heat of combustion can be expressed as the thermal
energy released when either one gram OR one mole of substance burns. If the amount of substance burned is one
mole, the quantity of thermal energy involved is called the molar heat of combustion.
Using your laboratory data, you can calculate the heat of combustion of paraffin wax.
1. Calculate, or determine, the mass of water heated. (Hint: The density of liquid water is 1.0 g/mL. Thus, each
milliliter (mL) of water has a mass of 1.0 g.)
Mass of water heated_____________________________
2. Calculate the total rise in the temperature of the water. Show your work.
Rise in temperature of water _______________________
3. Calculate how much thermal energy was absorbed by the water sample.
Step 1: Write the equation used in the above sample problem:
Step 2: Substitute your laboratory data into the equation and calculate:
Thermal energy (J) absorbed by the water________________
Step 3: Convert Joules to kilojoules, report your number to the tenths place:
Thermal energy (kJ) absorbed by the water ____________
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4. Determine the energy released by the burned candle.
Thermal energy (kJ) released by the candle____________
5. Calculate the total mass of the candle that burned. Show your work.
Mass of candle that burned (g) ________________________
6. Calculate the heat of combustion of the candle, expressed in units of kJ/g. Show your work.
Energy released by the candle
=
Mass of the burned candle
€
Heat of combustion (kJ/g)_____________________________
7. Calculate the molar heat of combustion of paraffin, expressed in units of kJ/mol:
Step 1: Calculate the molar mass of paraffin (the candle), C25H52:
Step 2: Calculate the moles of paraffin (candle) burned (hint: you will need the data from #5):
Step 3: Calculate the energy released per mole of paraffin (kJ/mol) (hint: calculation is similar to that in #6 above):
M olar Heat of combustion (kJ/mol)_____________________________
Questions
1. Write the calculated heat of combustion (in kJ/g) for paraffin wax (the candle), C25H52, and the accepted heat of
combustion for propane, C3H8 (see Table 3.9 (page 340 of your textbook):
My calculated heat of combustion (in kJ/g) for paraffin wax = __________________
Accepted heat of combustion (in kJ/g) for propane = __________________
Which alkane has the higher heat of combustion?______________________
2. Write the calculated molar heat of combustion (in kJ/mol) for paraffin and the accepted molar heat of combustion for
propane (pg. 340 of your textbook):
My calculated molar heat of combustion (in kJ/mol) for paraffin wax = __________________
Accepted molar heat of combustion (in kJ/mol) for propane = __________________
Which alkane has the higher molar heat of combustion?______________________
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3. Explain the differences in your answers to Questions 1 and 2 for paraffin and propane. (Hint: Compare the size of
one paraffin molecule to one propane molecule.)
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________
4. In your view, which hydrocarbon—paraffin or propane—is the better fuel? Explain your answer, citing your
answers to Questions 1 and 2.
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________________________________________
5. In calculating the heat of combustion of paraffin, you assumed that all thermal energy from the burning wax went to
heating the water.
a.
Was this a good assumption? Explain.
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________________
b.
What other laboratory conditions or assumptions might cause errors in your calculated values?
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________________________________________
_____________________________________________________________
DATA TABLE
Room T emperature (recorded to the nearest 0.1°C )
V olume of chilled water (recorded to the nearest 0.1 mL)
Before C andle
Burning
After C andle
Burning
M ass of T ea C andle
and index card (recorded
to the nearest 0.1 g)
W ater T emperature
recorded to the nearest
0.1°C
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