Unit 7: The Combustion of Alkanes
Understanding how alkanes are used as fuels and the environmental impact of their combustion.
7.17 Alkanes as Fuels
Alkanes, the primary components of natural gas and petrol, are excellent fuels. A fuel is a substance that releases a large amount of energy when it undergoes a chemical or nuclear change. For alkanes, this change is combustion, a rapid reaction with oxygen.
Combustion is a highly exothermic process, releasing significant energy as heat and light. This energy can be used for heating, generating electricity in power stations, or powering internal combustion engines in vehicles.
Complete vs. Incomplete Combustion
- Complete Combustion: Occurs when there is a plentiful supply of oxygen.
The only products are carbon dioxide and water.
Example (Propane): $C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$ - Incomplete Combustion: Occurs when the oxygen supply is limited. It
produces carbon monoxide (CO) and/or solid carbon (soot) in addition to water.
Example (Propane): $2C_3H_8(g) + 7O_2(g) \rightarrow 6CO(g) + 8H_2O(l)$
Incomplete combustion is less efficient as it releases less energy than complete combustion.
Solved Examples:
- What is combustion?
Solution: A rapid reaction between a substance and an oxidant, usually oxygen, to produce heat and light. - Write the balanced equation for the complete combustion of methane
($CH_4$).
Solution: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$. - What are the two main products of complete combustion of any
hydrocarbon?
Solution: Carbon dioxide ($CO_2$) and water ($H_2O$). - Under what condition does incomplete combustion
occur?
Solution: When there is a limited or insufficient supply of oxygen. - Name two possible products of the incomplete combustion of an
alkane.
Solution: Carbon monoxide (CO) and carbon (soot). - Write an equation for the incomplete combustion of ethane ($C_2H_6$) to form
carbon monoxide and water.
Solution: $2C_2H_6(g) + 5O_2(g) \rightarrow 4CO(g) + 6H_2O(l)$. - Why are alkanes such good fuels?
Solution: Their combustion is highly exothermic, releasing a large amount of energy per mole. - What is the physical appearance of the carbon produced during incomplete
combustion?
Solution: It is a black solid known as soot. - Which process releases more energy: complete or incomplete
combustion?
Solution: Complete combustion. - What is the main component of natural
gas?
Solution: Methane ($CH_4$).
7.18 Environmental Problems
While alkanes are vital fuels, their widespread combustion contributes to several major environmental issues.
- Carbon Monoxide (CO): A product of incomplete combustion. It is a toxic gas because it binds irreversibly to haemoglobin in red blood cells, reducing their ability to transport oxygen around the body.
- Greenhouse Gases: Carbon dioxide ($CO_2$) is a major greenhouse gas. It is transparent to incoming solar radiation but absorbs outgoing infrared (heat) radiation, trapping heat in the atmosphere. The rising levels of $CO_2$ from burning fossil fuels are a primary cause of global warming and climate change.
- Acid Rain: Crude oil contains sulphur impurities. When fuels are burned, this sulphur is oxidised to sulphur dioxide ($S + O_2 \rightarrow SO_2$). Additionally, the high temperatures in engines cause atmospheric nitrogen and oxygen to react, forming oxides of nitrogen ($N_2 + O_2 \rightarrow 2NO$). These acidic gases ($SO_2$ and $NO_x$) dissolve in atmospheric water droplets to form acid rain, which damages buildings, harms aquatic life, and kills forests.
- Other Pollutants: Unburned hydrocarbons and soot particles can contribute to smog and respiratory problems.
Solved Examples:
- Why is carbon monoxide dangerous to
humans?
Solution: It is toxic because it reduces the oxygen-carrying capacity of the blood. - What is the greenhouse effect?
Solution: The process by which certain gases in the atmosphere trap heat, warming the planet. - Name the two main acidic gases responsible for acid
rain.
Solution: Sulphur dioxide ($SO_2$) and oxides of nitrogen ($NO_x$). - How is sulphur dioxide produced from burning fossil
fuels?
Solution: From the combustion of sulphur impurities present in the fuel. - What is a major environmental consequence of increased carbon dioxide
levels?
Solution: Global warming and climate change. - Under what conditions are oxides of nitrogen formed in a car
engine?
Solution: The high temperature and pressure inside the engine cause atmospheric nitrogen and oxygen to react. - What is one negative effect of acid
rain?
Solution: It can corrode limestone buildings and statues, or lower the pH of lakes, harming fish. - Is water vapour a greenhouse gas?
Solution: Yes, it is a significant greenhouse gas, though its concentration is primarily controlled by the water cycle rather than direct human emissions. - What is soot?
Solution: Fine particles of solid carbon produced during incomplete combustion. - What is the source of nitrogen that forms $NO_x$ in car
engines?
Solution: The air, which is approximately 78% nitrogen.
7.19 Quality of Petrol (Octane Number)
The efficiency of an internal combustion engine depends on the quality of the petrol. In an engine, a fuel-air mixture is compressed and then ignited by a spark plug. If the fuel ignites too early under compression, before the spark, it causes an engine "knock" or "pinking," which reduces power and can damage the engine.
The octane number is a measure of a fuel's ability to resist knocking.
- Branched-chain alkanes and aromatic compounds have high octane numbers because they are very resistant to auto-ignition.
- Straight-chain alkanes have low octane numbers and are prone to knocking.
The scale is based on two reference compounds:
- 2,2,4-trimethylpentane (an isomer of octane) is very resistant to knocking and is assigned an octane number of 100.
- Heptane (a straight-chain alkane) knocks very easily and is assigned an octane number of 0.
A fuel with an octane number of 95 has the same knocking characteristics as a mixture of 95% 2,2,4-trimethylpentane and 5% heptane. Industrial processes like reforming are used to convert low-octane straight-chain alkanes into high-octane branched and aromatic compounds.
Solved Examples:
- What does the octane number of a fuel
measure?
Solution: Its resistance to knocking or pre-ignition in an engine. - Which type of alkane has a higher octane number: branched or
straight-chain?
Solution: Branched-chain alkanes. - What compound is assigned an octane number of
100?
Solution: 2,2,4-trimethylpentane. - What compound is assigned an octane number of
0?
Solution: Heptane. - What is "knocking" in an engine?
Solution: The premature ignition of the fuel-air mixture due to compression rather than the spark plug. - Which industrial process is used to increase the octane number of
petrol?
Solution: Reforming. - A fuel is rated at 98 octane. What does this
mean?
Solution: It has the same anti-knocking properties as a mixture containing 98% 2,2,4-trimethylpentane and 2% heptane. - Which would be a better fuel component: hexane or
benzene?
Solution: Benzene, as aromatic compounds have high octane numbers. - Why is a high octane number desirable for
petrol?
Solution: It allows for higher compression ratios in engines, leading to greater power and efficiency without engine damage. - What is the relationship between hexane and 2-methylpentane? How would their
octane numbers compare?
Solution: They are chain isomers. 2-methylpentane (the branched isomer) would have a higher octane number than hexane (the straight-chain isomer).