3.3 Combustion Reactions
Characteristics of Combustion Reactions with Hydrocarbons
Combustion reaction: the reaction of a substance with oxygen, producing one or more
oxides. Energy, in the form of heat and light, is released.
Hydrocarbon: a compound that is composed only of the elements carbon and
hydrogen
- The combustion of hydrocarbons can either be complete or incomplete.
Complete Combustion of Hydrocarbons
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Reactants: Oxygen, hydrocarbon
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Products: Carbon dioxide and water vapour + ENERGY
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Balanced Chemical Equation:
CxHy + O2 (g)
hydrocarbon + oxygen
CO2 (g) + H2O (g) + ENERGY
carbon dioxide + water + ENERGY
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Observations: A blue flame indicates that complete combustion is occurring.
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- Natural gas, which is mainly composed of methane, is used in laboratory bunsen
burners.
- It is important to adjust the air vents to allow the proper amount of oxygen to enter the
burner.
- If the flame is yellow = more oxygen is needed for complete combustion to occur!
-The blue flame that forms during complete combustion generates mostly heat and very
little light. In other words, complete combustion is a more efficient process for
generating heat.
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Examples: complete combustion of methane…
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (g) + ENERGY
Note: Don’t worry about what is in the box above^. The boxes with arrows are good explanations of what
is going on in the balanced chemical equation. Because combustion reactions are net exothermic
reactions, this means that the energy released when bonds are formed is GREATER than the
energy required to break bonds.
complete combustion cont’d….
- Reactions similar to combustion reactions can occur with other substances, such as fluorine,
taking the place of oxygen. (These reactions are called oxidation reactions and we will learn
them in Grade 12.)
- Many hydrocarbons are fuels. These hydrocarbons include: methane, ethane, propane,
butane, acetylene (ethyne) and benzene.
-These hydrocarbons, and many others, can undergo combustion reactions to produce carbon
dioxide and water
-When a hydrogen burns, the products of the reaction depend on the amount of oxygen that is
present
-Complete combustion occurs when oxygen is available in sufficient amounts.
-All the carbon atoms and hydrogen atoms from the hydrocarbon molecules combine
with oxygen atoms to form molecules of the oxides carbon dioxide and water vapour.
-Incomplete combustion occurs when there isn’t enough oxygen available.
Incomplete Combustion of Hydrocarbons
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Reactants: Hydrocarbon (𝐶𝑥𝐻𝑥) (𝑔𝑎𝑠)
+ Oxygen (gas)
Note that the reactants in incomplete combustion reactions are the same as those in
complete combustion reactions
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Products: Carbon (solid- in the form of soot), carbon monoxide (g), carbon dioxide (g),
and water vapour (g) + ENERGY
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As stated above, incomplete combustion occurs when there is not enough oxygen
available. Here is an illustration:
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Most of the energy given off by an incomplete combustion reaction is in the form of light.
It is not as efficient as complete combustion in producing heat.
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Balanced Chemical Equation: using the example of the incomplete combustion of
methane gas
methane + oxygen
2CH4(g) + 3O2(g)
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carbon monoxide + water + ENERGY
2CO(g) + 4H2O(l) + ENERGY
Observation: the solid, black, elemental carbon produced in an incomplete combustion
is known as soot (“fine particles consisting mostly of carbon, formed during the
incomplete combustion of a hydrocarbon.”)
The gaseous carbon monoxide produced in incomplete combustion reactions is very dangerous.
CO (g) is a toxic yet odourless/colourless gas whose molecules bind to blood vessels more
tightly than oxygen itself, stopping the body from transporting oxygen. It can lead to death by
suffocation. The picture below illustrates common symptoms of carbon monoxide poisoning.
To avoid carbon monoxide poisoning, you should have proper ventilation in areas where
combustion occurs, as well as put a procedure in place for detecting the gas (i.e., buying carbon
monoxide detectors).