Thermal decomposition


Thermal decomposition, or thermolysis, is a chemical decomposition of a substance caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion or other chemical reaction.

Decomposition temperature definition

The thermal decomposition temperature is the specific temperature at which a material begins to undergo chemical breakdown into simpler substances when subjected to heat, usually identified experimentally by the initial significant mass loss in a thermogravimetric curve. This marks the onset of observable chemical decomposition, where chemical bonds cleave solely due to thermal energy.
Since chemical reactions are essentially always occurring, because molecules assume a probability distribution of energies, there is no distinct temperature value at which decomposition spontaneously occurs. Therefore, the onset temperature is often used experimentally; this is the temperature at which a chemical reaction occurs at a measurable rate, and is thus highly dependent on the sensitivity of the corresponding instrument. The onset temperature can be calculated using differential scanning calorimetry, which is also sensitive to the rate of heating. A higher rate of heating will often display a higher onset temperature due to further kinetic variables such as reaction order, activation energy, and autocatalytic behaviour.
A simple substance may exist in equilibrium with its thermal decomposition products, effectively halting the decomposition. The equilibrium fraction of decomposed molecules increases with the temperature.
Since thermal decomposition is a kinetic process, the observed temperature of its beginning in most instances will be a function of the experimental conditions and sensitivity of the experimental setup. For a rigorous depiction of the process, the use of thermokinetic modeling is recommended.

Examples

When metals are near the bottom of the reactivity series, their compounds generally decompose easily at high temperatures. This is because stronger bonds form between atoms towards the top of the reactivity series, and strong bonds are difficult to break. For example, copper is near the bottom of the reactivity series, and copper sulfate, begins to decompose at about, increasing rapidly at higher temperatures to about. In contrast potassium is near the top of the reactivity series, and potassium sulfate does not decompose at its melting point of about, nor even at its boiling point.

Practical applications

Many scenarios in the real world are affected by thermal degradation. One of the things affected is fingerprints. When anyone touches something, there is residue left from the fingers. If fingers are sweaty, or contain more oils, the residue contains many chemicals. De Paoli and her colleagues conducted a study testing thermal degradation on certain components found in fingerprints. For heat exposure, the amino acid and urea samples started degradation at and for lactic acid, the decomposition process started around. These components are necessary for further testing, so in the forensics discipline, decomposition of fingerprints is significant.