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A decomposition reaction breaks down a compound into simpler components, requiring energy from sources like heat or electric current. Catalysts can speed up the reaction. Examples include the production of quicklime and reactive metals, and scientific analysis of materials.
A decomposition reaction is a type of chemical reaction in which a compound is broken down into simpler components. It is the opposite of chemical synthesis, where relatively simple elements or compounds combine to produce a more complex one. Since a decomposition reaction involves breaking chemical bonds, it requires the addition of energy; this can come from heat, electric current or other sources. Sometimes, a catalyst will speed up the reaction or allow it to happen at a lower temperature. These reactions are used industrially in the production of some elements, especially reactive metals, and in the laboratory for the analysis of samples.
Heat decomposition
Heat is commonly used to cause a decomposition reaction. When a compound heats up, its atoms move more vigorously, and this movement can break chemical bonds. For example, if calcium carbonate (CaCO3) is heated strongly, it decomposes into calcium oxide (CaO) and carbon dioxide (CO2). The temperature required to decompose a compound depends on the strength of the bonds holding it together. In this example, the calcium carbonate loses one carbon atom and two oxygen atoms as CO2, but the calcium retains one oxygen atom because the calcium-oxygen bond is very strong and cannot be broken by heating to any easily achievable temperature.
More reactive elements tend to form stronger bonds and are, therefore, more difficult to separate from their compounds. In contrast to the previous example, less reactive metal oxides, such as silver and mercury, can be decomposed by relatively mild heating, releasing oxygen and leaving the pure metal. Highly reactive metals, such as sodium and potassium, cannot be separated from their compounds by heating alone.
Electrolysis
In the liquid state, elements can be separated from a compound by the application of a direct electric current in a process known as electrolysis. The current flows through the electrodes, which are placed in the liquid. Negatively charged electrons flow into one electrode, known as the cathode, and out the other, known as the anode. The cathode therefore has a negative charge and the anode a positive charge. The ions in the liquid move towards the oppositely charged electrode, allowing the current to flow.
An example is the decomposition of water into hydrogen and oxygen by electrolysis. Pure water is a bad conductor, but the introduction of even a very small amount of an ionic compound, such as sodium sulphate, greatly improves its conductivity and allows electrolysis. At the cathode, water (H2O) is split into hydrogen gas (H2) and hydroxide ions (OH-), which are attracted to the positively charged anode. At the anode, the water is split into oxygen gas and hydrogen ions (H+), which are attracted to the cathode.
other factors
In some compounds, the energy required for decomposition is small and can be provided by a minor shock, such as a physical impact. One such compound is lead azide (Pb(N3)2), which decomposes explosively into lead and nitrogen under relatively small impact. Sodium azide is a similar, but slightly less sensitive compound used to inflate automobile airbags in the event of a collision.
Light can cause some compounds to decompose. For example, silver chloride is converted to silver and chlorine gas upon exposure to light. This phenomenon was crucial to the development of photography.
catalysts
In many cases, a decomposition reaction can be initiated or accelerated by the use of a catalyst. These substances do not take part in the reaction, and are therefore unaffected by it, but favor the development of the reaction. A good example is the decomposition of dilute solutions of hydrogen peroxide (H2O2) in water and oxygen. This reaction can be promoted by the addition of powdered manganese dioxide, which acts as a catalyst to produce oxygen gas.
it is used
Thermal decomposition is used in the industrial production of quicklime for cement production and various other purposes. Electrolysis is used in the production of reactive metals. For example, sodium is produced by the electrolysis of molten salt (sodium chloride). This also produces chlorine gas, which has many industrial uses, although most chlorine is produced by the electrolysis of salt solutions in water. The decomposition reactions involving electrolysis are also used to make the highly reactive element fluorine and as a “clean” way of generating hydrogen for fuel.
There are some scientific applications that depend on decomposition reactions to analyze materials. In mass spectrometry, for example, a small sample of the material of interest is broken down into ions, which are separated based on their charges and masses. The material composition can then be determined.
