Thermal decomposition
Encyclopedia
For the biological process, see decomposition
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...

.


Thermal decomposition, or thermolysis, is a chemical decomposition
Chemical decomposition
Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...

 caused by heat. The decomposition temperature of a substance is the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 at which the substance chemically decomposes.

The reaction is usually endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...

 as heat is required to break chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

s in the compound undergoing decomposition. If decomposition is sufficiently exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...

, a positive feedback loop is created producing thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

 and possibly an explosion.

Examples

  • Calcium carbonate
    Calcium carbonate
    Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

     (Limestone
    Limestone
    Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

     or chalk
    Chalk
    Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....

    ) decomposes into calcium oxide
    Calcium oxide
    Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

     and carbon dioxide
    Carbon dioxide
    Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

     when heated:
CaCO3 → CaO + CO2
The reaction is used to make quick lime, which when hydrated becomes slaked lime and is used a building material.

  • Many oxides decompose at high enough temperatures, an example being the decomposition of mercuric oxide to give oxygen
    Oxygen
    Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

     and mercury
    Mercury (element)
    Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

    . The reaction was used by Joseph Priestley
    Joseph Priestley
    Joseph Priestley, FRS was an 18th-century English theologian, Dissenting clergyman, natural philosopher, chemist, educator, and political theorist who published over 150 works...

     to make the gas for the first time.

  • Some foods will decompose exothermically at cooking temperatures; anyone who has overheated sugar or syrupy foods will know how long they take to cool. Mild versions of the process will produce caramelised dishes that are pleasant, but cannot be tasted safely before they have cooled to a comfortable temperature. Once they start to char, such dishes commonly will continue in a positive feedback loop; they become dangerously hot and continue to blacken from the inside out, and smoke even well after being removed from the heat. In films, where stuntmen have to jump through breaking windows, the window panes are made of sugar, which is safer than glass. Melting the sugar however, is a tricky business; an error of just a few degrees will start a caramelisation process that will ruin the product.

  • Water
    Water
    Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

    , when heated to well over 2000 °C, decomposes to its constituent elements:
2 H2O → 2 H2 + O2

  • The compound with the highest known decomposition temperature is carbon monoxide
    Carbon monoxide
    Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

     at ≈3870 °C (≈7000 °F)..

See also

  • Ellingham diagram
    Ellingham diagram
    An Ellingham diagram is a graph showing the temperature dependence of the stability for compounds. This analysis is usually used to evaluate the ease of reduction of metal oxides and sulphides. These diagrams were first constructed by Harold Ellingham in 1944...

  • Thermal depolymerization
    Thermal depolymerization
    Thermal depolymerization is a depolymerization process using hydrous pyrolysis for the reduction of complex organic materials into light crude oil. It mimics the natural geological processes thought to be involved in the production of fossil fuels...

  • Chemical thermodynamics
    Chemical thermodynamics
    Chemical thermodynamics is the study of the interrelation of heat and work with chemical reactions or with physical changes of state within the confines of the laws of thermodynamics...

  • Pyrolysis
    Pyrolysis
    Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

    - thermolysis of organic material
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