Decomposition (chemistry)

Decomposition (or degradation , degradation in) means chemistry and biology the decomposition of a chemical compound into smaller molecules or even in the elements by physical, chemical or biological agents. A distinction is therefore made between physical decomposition and thermal decomposition , chemical decomposition and biological decomposition or biological degradation .

Types of decomposition

Physical decomposition

A chemical substance often decomposes when energy is supplied in the form of heat (see pyrolysis and calcination ). These processes are also known as thermal decomposition. Other sources of energy such as electrical current ( electrolysis , lightning , electric arcs ), ultraviolet radiation ( photolysis ) or X-rays also cause bonds within molecules to split . This often creates radicals , which then continue to react as unstable, high-energy particles. With the exclusion of oxidizing agents such as atmospheric oxygen , this decomposition can take place down to the elements from which the compound is built up (e.g. formation of coal deposits ) or to compounds that are thermodynamically most stable under the selected conditions . In synthetic chemistry, the decomposition of starting materials under low pressure and high temperatures to a desired end product is used as a " flash vacuum pyrolysis " process.

Examples: ${\ displaystyle \ mathrm {CH_ {4} \ {\ xrightarrow {\ Delta T}} \ C + \ 2 \ H_ {2}}}$; Thermal decomposition of methane to carbon and hydrogen

{\ displaystyle \ mathrm {2 \ BaO_ {2} \ {\ xrightarrow {\ Delta T}} \ 2 \ BaO + \ O_ {2}}}

Thermal decomposition of barium peroxide to barium oxide and oxygen

{\ displaystyle \ mathrm {CaCO_ {3} \ {\ xrightarrow {\ Delta T}} \ CaO + \ CO_ {2}}}

Thermal decomposition of calcium carbonate to calcium oxide and carbon dioxide

{\ displaystyle \ mathrm {2 \ H_ {2} O \ {\ xrightarrow {Electrolysis}} \ O_ {2} \ + \ 2 \ H_ {2}}}

Electrolysis of water to oxygen and hydrogen

Chemical decomposition

If oxidizing agents are present during decomposition, the most stable compounds with the oxidizing agent are formed from combustible substances under combustion , sometimes even without energy input. If the oxidizing agent is oxygen (for example from the air), organic compounds form water H ₂ O and carbon dioxide CO ₂ (possibly also sulfur oxides , nitrogen oxides or nitrogen as gas), in inorganic substances mostly oxides or other oxygen-containing compounds such as sulfates , phosphates or silicates .

Some compounds also decompose when heated, splitting off stable compounds such as carbon dioxide ( decarboxylation ) or water ( dehydration , e.g. the release of the water of crystallization from hydrates ). If metals are attacked during decomposition , this is called corrosion ; in the case of iron, in particular, the resulting product from various iron oxides and water is called rust . When biologically important, polymeric biomolecules such as proteins and polysaccharides are broken down by enzymes , dilute acids or bases , one speaks of hydrolysis ( breakdown by water ). The breakdown products of a substance can provide information about the composition of the starting substances. The thermal degradation therefore plays in the analytical chemistry a significant role.

Examples: ${\ displaystyle \ mathrm {CH_ {4} \ + \ 2 \ O_ {2} \ {\ xrightarrow {\ Delta T}} \ CO_ {2} \ + \ 2 \ H_ {2} O}}$; Combustion of methane with oxygen to produce carbon dioxide and water

{\ displaystyle \ mathrm {HOOC-CH_ {2} -COOH \ {\ xrightarrow {\ Delta T}} \ HOOC-CH_ {3} \ + \ CO_ {2}}}

Thermal decomposition of malonic acid to acetic acid with the elimination of carbon dioxide

Biotic decomposition

Living beings that break down organic substances are grouped under the name saprobionts or destructors . These are animals such as mites , woodlice , and microorganisms such as bacteria (e.g. actinomycetes ) and fungi . Organic substances, mostly high-polymer proteins or carbohydrates, are first broken down mechanically by these organisms, then chemically either with the exclusion of air ( anaerobic ) or in the presence of air ( aerobic ). The saprobionts use the organic compounds as an energy source.

Badges of DIN CERTCO and European Bioplastics according to EN 13432

The decomposition of organic substances in waste by saprobionts or their enzymes that feed waste into the natural material cycle is also generally called biological degradation . The biodegradability of industrially produced chemicals , u. a. on the basis of renewable raw materials as in oleochemicals , and materials (e.g. plastics) are examined with (partly) specific test procedures. Labeling systems exist, especially for biodegradable plastics . The terms biodegradability or biodegradability are also often used. Substances are said to be resistant to degradation if they are not subject to biological degradation. If they are not broken down by other chemical or physical processes, they are referred to as persistent .

The corrosion of substances by living things is known as bio- corrosion . This includes both decomposition processes, which serve their metabolism , and excreted secondary metabolites , which lead to decomposition.

Web links

Biocatalysis / Biodegradation Database (flowcharts, English)
Investigation of the environmental impact of packaging made from biodegradable plastics (PDF; 3.1 MB), Federal Environment Agency , 2012

Individual evidence

↑ Hartmut Kainer: Coupling of heat and material exchange with chemical kinetics in the decomposition of natural carbonates. Dissertation TU Clausthal, December 1982.

[1] Hartmut Kainer: Coupling of heat and material exchange with chemical kinetics in the decomposition of natural carbonates. Dissertation TU Clausthal, December 1982.