In this lesson we will learn about different oxidation reactions, including combustion and corrosion.
By the end of this lesson you will be able to:
- Describe combustion, including some of its uses in society.
- Describe corrosion, including ways it can be prevented.
- Compare the chemical processes of combustion and corrosion.
- Chemical reactions where oxygen is one of the reactants are examples of a type of chemical reaction known as an oxidation reaction.
- (Oxidation reactions don’t always oxygen, but we will mainly be looking at those that do.)
- Oxidation reactions can involve metals or non-metals reacting with oxygen.
- Corrosion reactions are an example of oxidation reactions that involve metals.
- Combustion reactions are an example of oxidation reactions that usually involve non-metals.
- Oxidation reactions are exothermic, which means they release energy.
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- Corrosion is the slow oxidation of a metal to form compounds such as metal oxides, metal hydroxides and metal sulfides.
- Example 1: Corrosion involving oxygen.
- Aluminium metal reacts with oxygen in the air to form aluminium oxide, as shown in the following word and formula equations:
- aluminium + oxygen → aluminium oxide
- Al + O2 → Al2O3
- Example 2: Corrosion involving substances other than oxygen.
- Silver metal reacts with hydrogen sulfide gas to form silver sulfide and hydrogen gas, as shown in the following word and formula equations:
- silver + hydrogen sulfide → silver sulfide + hydrogen
- Ag + H2S → Ag2S + H2
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- The process of rusting specifically refers to the corrosion of iron.
- Iron metal reacts with oxygen in the air (in the presence of water) to form iron oxide, otherwise known as rust.
- The rusting of iron involves several steps, but can be summarised by the following word and formula equations:
- iron + oxygen → iron (III) oxide
- Fe + O2 → Fe2O3
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- Corrosion is a gradual process, often taking place over many years.
- In most cases, is it is undesirable process. For example, the corrosion of metals in motor vehicles, buildings, bridges and storage containers is a huge and costly problem.
- Corrosion usually occurs when metals react with oxygen in the air, but the rate of corrosion increases in the presence of water and other chemicals, such as salts and acids.
- There are a variety of methods that can be used to protect metallic structures against corrosion.
- They mostly involve forming some sort of physical barrier to prevent air, water and other substances from reacting with the metal.
- Anodising involves coating a metal with a protective oxide layer.
- The oxide layer is impervious to air and water, thus protecting the underlying metal from further corrosion.
- Anodising is often used to protect aluminium, for example, in aircraft parts, architectural materials and electronics.
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- Electroplating involves coating a metal, such as iron, with a less reactive metal, such as tin.
- The less reactive coating metal will corrode much slower, thus protecting the more reactive metal underneath.
- Electroplating is used in the manufacture of tin cans, which are actually steel with just a thin coating of tin.
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- Galvanising involves coating a metal, such as iron, with a more reactive metal, such as zinc.
- The coating metal forms a protective barrier for the underlying metal.
- Even though galvanising coats a metal with a more reactive metal, it has an advantage over electroplating, as the galvanising metal acts as a ‘sacrificial’ layer.
- Even if a scratch in the coating layer exposes the underlying metal, the underlying metal is still protected.
- This is because all of the more reactive galvanising layer will corrode before the underlying metal begins to corrode.
- In electroplating, the opposite will take place. A scratch in electroplating layer means that the underlying metal will begin to corrode in preference to the coating layer. This is why tin cans that are dented should be avoided as rust may have leached into their contents.
- Galvanising is often used in a range of building materials, ranging from nails to storage tanks.
(Image: Idarek, Wikimedia Commons)
Other Protective Coatings
- Coating metals with plastic, paint, grease and oil can also protect metals.
- These methods all involve unreactive substances forming physical barriers against corrosion of an underlying metal.
- They are generally more cost-effective than the methods described above.
- Plastics have the disadvantage of being subject to splitting and cracking, particularly when exposed to sunlight, which renders them ineffective.
- Paint, grease and oil all require continued reapplication to maintain their effectiveness.
(Images: Pubic Domain Pictures; Ken LaRock, US Air Force; terimakasih0, Pixabay)
- Combustion is the chemical reaction of a fuel with oxygen.
- It can also be described as burning.
- Fuels are usually carbon-based compounds, such as wood, coal, petrol (gasoline) and natural gas.
- Combustion reactions are usually fast and highly exothermic.
- This means they releases large amounts of energy – mostly heat, but also light – over a short period of time.
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Combustion of Hydrocarbons
- The combustion of hydrocarbons (substances containing only hydrogen and carbon) can be described as complete or incomplete, depending on the supply of oxygen and the products formed.
- Complete combustion takes place when there is a non-limiting supply of oxygen.
- These reactions produce carbon dioxide and water, as shown by the following word and formula equations:
- hydrocarbon + oxygen → carbon dioxide + water
- CxHy + O2 → CO2 + H2O
- Incomplete combustion takes place when there is a limiting supply of oxygen.
- These reactions produce either carbon monoxide and water or carbon (soot) and water, as shown by the following word and formula equations:
- hydrocarbon + oxygen → carbon monoxide + water
- CxHy + O2 → CO + H2O
- hydrocarbon + oxygen → carbon + water
- CxHy + O2 → C + H2O
- Carbon monoxide is produced when oxygen is slightly limiting, and carbon is produced when oxygen is very limiting.
- (In reality, most combustion reactions will produce a mixture carbon dioxide, carbon monoxide and carbon, but in different ratios, depending on the oxygen supply.)
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Combustion of Fossil Fuels and Global Warming
- Much of the human world relies heavily on the combustion of fossil fuels, such as coal, oil and natural gas, for purposes such as electricity generation, transportation and heating.
- The burning of fossil fuels has resulted in a dramatic increase in the amount of carbon dioxide in the atmosphere.
- Carbon dioxide is a greenhouse gas, which means it traps heat in the atmosphere.
- Scientists believe that this increase in carbon dioxide is causing an increase in average global temperatures, known as global warming.
- If it continues, global warming is predicted to have disastrous consequences for the natural world and human societies.
Combustion of Metals
- Many reactive metals also undergo combustion when they are heated in air (oxygen).
- For example, heating a piece of magnesium metal over a blue Bunsen burner flame will cause the magnesium to burn brightly (the same glow produced by burning flares).
- The magnesium reacts with oxygen to produce magnesium oxide, as shown by the following word and formula equations:
- magnesium + oxygen → magnesium oxide
- Mg + O2 → MgO
- (The magnesium also reacts with the nitrogen in the air, to produce magnesium nitride.)
- Another example of metal combustion is the use of powdered aluminium as a rocket fuel.
(Image: US Army, Wikimedia Commons)
- Aerobic respiration is a chemical process that takes place in the cells of living things.
- It can be considered as a slow, controlled form of combustion, as it involves the reaction of glucose with oxygen to produce energy.
- Aerobic respiration can be summarised by the following word and formula equations:
- glucose + oxygen → carbon dioxide + water (+ energy)
- C6H12O6 + O2 → CO2 + H2O (+ energy)
- (We will learn more about respiration in the following lesson.)
- Corrosion and combustion are both examples of oxidation reactions involving substances reacting with oxygen.
- Corrosion is the slow reaction of a metal with the air.
- Corrosion can be prevented by physical barriers that prevent the exposure of metals to air and water.
- Methods used include anodising, electroplating, galvanising, plastic coating, paint, grease and oil.
- Combustion is the rapid burning of a fuel with oxygen.
- Fuels are usually hydrocarbons, but can include reactive metals.
- Combustion of hydrocarbons can be described as complete or incomplete, depending the supply of oxygen and the products formed.
- Complete combustion occurs when oxygen is not limited, and produces carbon dioxide and water.
- Incomplete combustion occurs when oxygen is limited, and produces either carbon monoxide and water or carbon and water.
- The combustion of fossil fuels by humans is causing global warming due to the build-up of carbon dioxide in the atmosphere.
- Aerobic respiration is the reaction of glucose with oxygen that takes place in the cells of living things, providing them with energy.
(Header image: Nejron Photo, Adobe Stock)