Comparing Fossil Fuel
and Biofuel Combustion
Short Introduction
Methane:
•
•
A non-renewable fossil fuel gas
pumped from deep below the
Earth’s surface from both coal
and oil deposits.
A major component of biogas - a
renewable fuel that is produced
from decaying plant and animal
manure.
Ethanol:
•
Renewable liquid biofuel
produced from several different
types of plant material.
Octane:
•
A non-renewable liquid fossil
fuel which is the major
component of gasoline refined
from crude oil.
A Quick Review
•
•
•
Carbon-based fuels such as
wood, coal, and gasoline supply
energy.
These fuels release exothermic
energy during a combustion
chemical reaction.
The energy has been stored
within the bonds of the
molecules.
•
•
The energy is released because
the chemical bond energy of the
products are less than the
chemical bond energy of the
reactants.
The products of most
combustion reactions include
water and carbon dioxide
Bond Energy between two atoms
Use the information in Table 1 to determine the energy in the bonds of each of the different
molecules.
TABLE 1: BOND ENERGY VALUES
Bond
Energy
(kJ/mole)
Bond
Energy
(kJ/mole)
Bond
Energy
(kJ/mole)
C-H
414.2
C-O
357.7
C=O
803.2
C-C
347.3
H-H
439.4
O=O
494.9
O-H
464.4
Calculating the Total Bond Energy of a
molecule.
TABLE 1: BOND ENERGY VALUES
Structural
Formula
Methane: CH4
Bond Type
Bond Energy
(kJ/mole)
# bonds in the
molecule
Total Bond
Energy
(kJ/mole)
Part A - Modeling Combustion Reactions
1. Use the model pieces to build each of the fuel
molecules below. Draw the structural formula for
each molecule. Hint: All bonds are single bonds.
METHANE
ETHANOL
Part A - Model the combustion of methane in the
presence of just enough oxygen.
1. Make the models of the reactants: one CH4 and two O2
molecules.
2. Next model the breaking of the reactant bonds by removing
all the white bonds from the atom of the three reactant
molecules.
3. Finally, model the formation of the products by
reassembling the six bonds and seven atoms from the
“broken” reactant molecules to make as many water (H2O)
and carbon dioxide (CO2) molecules as you can.
Part A - Model the combustion of methane in the
presence of just enough oxygen.
Count up the number of H2O and CO2 molecules you made and
complete the chemical equation below that shows the
combustion of methane.
CH4 + 2O2 → CO2 + 2H2O
Part A - Model the combustion of ethanol in the
presence of more than enough oxygen.
1. Make the models of the reactants: one C2H6O and five O2
molecules.
2. Next model the breaking of the reactant bonds of your C2H6O
and just enough (but not all) of your O2 molecules to make the
products water (H2O) and carbon dioxide (CO2) molecules.
Hint: If you break apart more O2 molecules than you need,
reassemble them.
Part A - Model the combustion of ethanol in the
presence of more than enough oxygen.
Determine the number of O2 molecules you needed to combust
the single molecule of ethanol and count the number of water
(H2O) and carbon dioxide (CO2) molecules you produced.
Use this data to complete the chemical equation that describes
the combustion of ethanol.
C2H6O + 3O2 → 2CO2 + 3H2O
Part A - Model the combustion of octane in the
presence of oxygen.
Octane, C8H18, has a structure very similar to methane. Draw
its structural formula.
Part A - Model the combustion of octane in the
presence of oxygen.
Below is the completed chemical equation that describes the
combustion of octane.
C8H18 + 12.5 O2 → 8CO2 + 9H2O
Part A - Model the combustion of octane in the
presence of oxygen.
1. Make the models of the reactants: one C8H18 and one O2
molecules.
2. Next model the breaking of the reactant bonds of your C8H18
and your O2 molecule to make the products 8 water (H2O) and
9 carbon dioxide (CO2) molecules.
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different reactant
molecules.
TABLE 1: BOND ENERGY VALUES
Bond
Energy
(kJ/mole)
Bond
Energy
(kJ/mole)
Bond
Energy
(kJ/mole)
C-H
414.2
C-O
357.7
C=O
803.3
C-C
347.3
H-H
436.0
O=O
497.9
O-H
464.4
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different REACTANT
molecules.
METHANE
Structural
Formula
bond type
H-C
bond energy
(kJ/mole)
414.2
# of bonds in
the molecule
4
Total bond
energy
(kJ/mole)
1656.8
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different REACTANT
molecules.
ETHANOL
Structural
Formula
bond type
H-C
C-C
C-O
O-H
bond energy
(kJ/mole)
414.2
347.3
357.7
464.4
# of bonds in
the molecule
5
1
1
1
Total bond
energy
(kJ/mole)
2070.0
347.3
357.7
464.4
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different REACTANT
molecules.
OCTANE
Structural Formula
bond type
H-C
C-C
bond
energy
(kJ/mole)
414.2
347.3
# of bonds in
the molecule
18
7
Total bond
energy
(kJ/mole)
7455.6
2431.1
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different REACTANT
molecules.
OXYGEN
Structural Formula
O=O
bond type
O=O
bond
energy
(kJ/mole)
497.9
# of bonds in
the molecule
1
Total bond
energy
(kJ/mole)
497.9
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different PRODUCT
molecules.
CARBON DIOXIDE
Structural Formula
O=C=O
bond type
C=O
bond
energy
(kJ/mole)
803.3
# of bonds in
the molecule
2
Total bond
energy
(kJ/mole)
1606.6
Part B - Calculating the Energy Released
During Combustion Reactions
Use the information in Table 1 to determine the energy
in the bonds of each of the different PRODUCT
molecules.
WATER
Structural Formula
H-O-H
bond type
H-O
bond
energy
(kJ/mole)
497.9
# of bonds in
the molecule
2
Total bond
energy
(kJ/mole)
928.8
Part B - Calculating the Energy Released
During METHANE Combustion
Energy released = (total bond energy of products) - (total bond energy of all reactants)
R
E
A
C
T
A
N
T
S
P
R
O
D
U
C
T
S
CH4 + 2O2 → CO2 + 2H2O
Molecule
Bond energy per
molecule (kJ/mole)
# of molecules
Total bond energy
(kJ/mole)
CH4
1656.8
1
1656.8
O2
497.9
2
989.8
Total Bond Energy in reactants = 2646.6 kJ/mole
Molecule
Bond energy per
molecule (kJ/mole)
# of molecules
Total bond energy
(kJ/mole)
CO2
1606.6
1
1606.6
H2O
928.8
2
1857.6
Total Bond Energy in products = 3464.2 kJ/mole
Part B - Calculating the Energy Released
During METHANE Combustion
Energy released = (total bond energy of products) - (total bond energy of all reactants)
CH4 + 2O2 → CO2 + 2H2O
Products - Reactants =
3464.2
combusted
-
Energy Released
2646.6 =
817.6 kJ/mole of methane
Part B - Calculating the Energy Released
During ETHANOL Combustion
Energy released = (total bond energy of products) - (total bond energy of all reactants)
R
E
A
C
T
A
N
T
S
P
R
O
D
U
C
T
S
C2H6O + 3O2 → 2CO2 + 3H2O
Molecule
Bond energy per
molecule (kJ/mole)
# of molecules
Total bond energy
(kJ/mole)
C2H6O
3240.4
1
3240.4
O2
949.9
3
1484.7
Total Bond Energy in reactants = 4725.1kJ/mole
Molecule
Bond energy per
molecule (kJ/mole)
# of molecules
Total bond energy
(kJ/mole)
CO2
1606.6
2
3213.2
H2O
928.8
3
2786.4
Total Bond Energy in products = 5999.6 kJ/mole
Part B - Calculating the Energy Released
During ETHANOL Combustion
Energy released = (total bond energy of products) - (total bond energy of all reactants)
C2H6O + 3O2 → 2CO2 + 3H2O
Products - Reactants =
5999.6
combusted
-
Energy Released
4725.1 =
1274.5 kJ/mole of ethanol
Part B - Calculating the Energy Released
During OCTANE Combustion
Energy released = (total bond energy of products) - (total bond energy of all reactants)
R
E
A
C
T
A
N
T
S
P
R
O
D
U
C
T
S
C8H18 + 12.5O2 → 8CO2 + 9H2O
Molecule
Bond energy per
molecule (kJ/mole)
# of molecules
Total bond energy
(kJ/mole)
C8H18
9886.7
1
9886.7
O2
494.9
12.5
6186.25
Total Bond Energy in reactants = 16,072.95kJ/mole
Molecule
Bond energy per
molecule (kJ/mole)
# of molecules
Total bond energy
(kJ/mole)
CO2
1606.6
8
12852.8
H2O
928.8
9
8359.0
Total Bond Energy in products = 21,212.0 kJ/mole
Part B - Calculating the Energy Released
During OCTANE Combustion
Energy released = (total bond energy of products) - (total bond energy of all reactants)
C8H18 + 12.5O2 → 8CO2 + 9H2O
Products - Reactants =
21,212.0
combusted
-
Energy Released
16,072.95 = 5139.0 kJ/mole of octane
Analysis
The three fuels from most to least energy
released:
The three fuels from most to least CO2
produced:
MOST = octane (5139 kJ/mole)
MOST = octane (8 = coefficient number = moles)
ethanol (1274.5 kJ/mole)
ethanol (2 = coefficient number = moles)
LEAST = methane (817.6 kJ/mole)
LEAST = methane (1 = coefficient number = moles)
Ratio of energy released to CO2
methane = 817.6/1 = 817.6 kJ/mole
ethanol = 1274.5/2 = 637.2 kJ/mole
octane = 5139.0/8 = 642.4 kJ/mole
Of all the three fuels analyzed in this activity,
which do you think is the best? Explain
METHANE - highest energy to CO2 ratio