Everything in this world eventually decomposes. In living organisms, decomposition is simply the breaking down of dead, organic matter. In chemistry terms, it is when a single compound breaks down into at least 2 simpler products.
With this in mind, there are 2 main categories of decomposition: biotic, or organic decomposition is characterised by the metabolic breakdown of organic matter into simpler components, and this process can also be called biodegradation. This affects all living things, including animals, plants and foods.
The second category is abiotic, or chemical decomposition. This is characterised by a chemical or physical reaction that breaks apart a single compound into its elemental products or even simpler compounds. This is what will be discussed in this post.
When bonds in a single chemical compound are broken, it results in it breaking apart to form two or more products. When this happens, it is known as chemical decomposition and it takes place as a chemical reaction.
Why does it happen?
Every chemical compound is limited to certain conditions, whether it’s heat, radiation or acidity. When a compound decomposes, therefore, it is often an undesired reaction that happens when a substance is exposed to extreme conditions that surpass its physical threshold.
However, chemical decomposition reactions are also incurred because of how valuable they can be in analytical techniques, including:
- Mass Spectrometry
- Gravimetric Analysis
- Thermogravimetric Analysis
How does chemical decomposition occur?
The general formula of a chemical decomposition reaction shows that when the bonds in a compound are broken, it causes it to decompose into either its elemental part or simpler compounds. The equation can be written as:
AB → A + B
In order for this reaction to occur, a lot of energy is required to break the existing bonds in the compound. This energy usually comes from heat, making the majority of decomposition reactions endothermic. But this energy can also be supplied by other things like an electric current or an acid.
Therefore, there are 3 main types of chemical decomposition reactions based on these energy sources:
- Electrolytic: where electricity supplies the energy
- Thermal: where heat supplies the energy
- Catalytic: where a catalyst, like an acid, supplies the energy
When electricity is used to decompose a molecule or compound, it is known as electrolytic decomposition. It usually takes place in aqueous solutions when an electric current is sent through them. A common example of this type of reaction is the electrolysis of water, which decomposes into hydrogen and oxygen:
2H20 + electricity → 2H2 + O2
The reason why electricity is used to break the bonds in water is that heat is not effective at causing water to dissociate. Passing an electric current through it, however, supplies enough energy to cause the solution to separate into its elemental products.
When a chemical decomposes as a result of heat, it is called thermal decomposition or thermolysis. This is an endothermic reaction that breaks apart the chemical bonds in a compound by causing friction between molecules. Thermolysis is the most common form of chemical decomposition. Thermal energy can also cause compounds to decompose in 2 ways.
Thermal energy can cause molecules or compounds that are physically attached to separate. This is a physical rather than chemical reaction and can be shown when hydrated copper (II) sulphate is heated:
CuSO4 • 5H2O (s) + heat → CuSO4 (s) + 5H2O (g)
Hydrated copper (II) sulphate simply means that water molecules are physically attached to the CuSO4 molecule. In this form, copper (II) sulphate appears as a blue crystal. However, when heat causes the water molecules to physically separate from the compound, the appearance changes to a white solid and it is simply called copper (II) sulphate.
This is not a chemical reaction because, even when the water molecules are not attached, copper (II) sulphate remains the same physical substance before and after heat has been introduced.
If, after it has decomposed, a compound has been broken down into simpler components, it is a chemical decomposition reaction because it is no longer the same physical substance. An example of this is copper (II) carbonate, which decomposes to form copper (II) oxide and carbon dioxide:
CuCO3 (s) + heat → CuO (s) + CO2 (g)
When copper (II) carbonate decomposes into copper (II) oxide it changes colour from green to black. Another example of a chemical decomposition reaction caused by thermal energy is when potassium permanganate decomposes to form potassium manganate, magnesium oxide and oxygen:
2KMnO4 (s) + heat → K2MnO4 (s) + MnO2 (s) + O2 (g)
There are dozens of thermal decomposition reactions that take place with a huge range of compounds, including calcium carbonate, sodium carbonate, metal oxides, nitrates, nitrites and even ammonium compounds.
When a catalyst is used to cause a chemical to break apart, it does not take part in the reaction but simply makes it happen more quickly by lowering the activation energy. Hydrogen peroxide, for example, is actually decomposing all the time but at a very slow rate. By introducing a catalyst, however, we can speed up the reaction.
Decomposition of Hydrogen Peroxide
Hydrogen peroxide (H2O2) is constantly decomposing because its chemical structure contains an incredibly weak and unstable peroxide bond (single oxygen-oxygen bond). Because this bond is so weak, it can easily break, causing hydrogen peroxide to decompose into water and oxygen.
Therefore, the decomposition of hydrogen peroxide happens naturally and gives it a finite shelf-life. This is why it is stored very carefully in dark, plastic containers. But when a catalyst is added, the decomposition reaction is sped up and has actually become one of the most popular science experiments in and out of the classroom.
Several catalysts can be added to hydrogen peroxide in order to speed up this reaction:
When a catalyst is added to hydrogen peroxide, the solution begins fizzing very quickly and will rise out of the container like a big cylinder of foam. This happens because of the rapid liberation of oxygen gas, which forms bubbles. If you add dish soap and food colouring to the solution, the foam will be thicker and colourful, making an attractive experiment known as Elephant’s Toothpaste. The equation for the decomposition of hydrogen peroxide is:
2H2O2 + catalyst → 2H2O + O2
Another example of a catalytic decomposition reaction is when sugar is catalysed by a strong, concentrated acid like sulphuric acid. This causes sugar to decompose into carbon and water, and the reaction causes the sugar to change from white to black.
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