What is a Decomposition Reaction?

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, in which elements or relatively simple compounds combine to produce one that is more complex. Since a decomposition reaction involves the breaking of chemical bonds, it requires the addition of energy; this may come from heat, an electrical current or other sources. Sometimes, a catalyst will speed up the reaction or allow it to take place 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 is commonly used to bring about a decomposition reaction. When a compound heats up, its atoms move about more vigorously, and this movement can break chemical bonds. For example, if calcium carbonate (CaCO₃) is strongly heated, it will decompose into calcium oxide (CaO) and carbon dioxide (CO₂). The temperature required to decompose a compound depends on the strength of the bonds that hold it together. In this example, calcium carbonate loses an atom of carbon and two atoms of oxygen as CO₂, but the calcium holds on to one oxygen atom because the calcium-oxygen bond is very strong and cannot be broken by heating to any easily achievable temperature.

The more reactive elements tend to form stronger bonds and are, therefore, more difficult to separate from their compounds. In contrast to the above example, the oxides of less reactive metals, such as silver and mercury, can be decomposed by relatively moderate 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.

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 electrodes, which are placed in the liquid. Negatively charged electrons flow into one electrode, known as the cathode, and out of the other, which is known as the anode. The cathode therefore has a negative charge, and the anode, a positive charge. Ions in the liquid move toward 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 very poor conductor, but the introduction of even a very small amount of an ionic compound, such as sodium sulfate, greatly improves its conductivity and allows electrolysis to take place. At the cathode, water (H₂O) is split into hydrogen gas (H₂) and hydroxide (OH^-) ions, which are attracted to the positively charged anode. At the anode, water is split into oxygen gas and hydrogen (H⁺) ions, which are attracted to the cathode.

In some compounds, the energy needed for decomposition is small and can be supplied 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 gas if subjected to a fairly small impact. Sodium azide is a similar, but slightly less sensitive, compound that is used to inflate car airbags in a collision.

Light can cause the decomposition of some compounds. For example, silver chloride is converted to silver and chlorine gas on exposure to light. This phenomenon was crucial to the development of photography.

In many cases, a decomposition reaction can be prompted or speeded up by the use of a catalyst. These substances do not take part in the reaction, and are therefore unchanged by it, but they encourage the reaction to take place. A good example is the decomposition of dilute solutions of hydrogen peroxide (H₂O₂) into water and oxygen. This reaction can be promoted by the addition of powdered manganese dioxide, which acts as a catalyst to produce oxygen gas.

Thermal decomposition is used in the industrial production of quicklime for cement manufacture 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. Decomposition reactions involving electrolysis are also used to make the extremely reactive element fluorine, and as a “clean” way of generating hydrogen for fuel.

There are some scientific applications that depend on decomposition reactions in order to analyze materials. In mass spectrometry, for example, a small sample of the material of interest is split into ions, which are separated according to their charges and masses. The composition of the material can then be determined.