Thermal decomposition
Chemical decomposition caused by heat
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Through Temperature, Endothermic process and Chemical bond
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 (TGA) 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 (DSC), 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 (like water) 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.
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Examples
- Calcium carbonate (limestone or chalk) decomposes into calcium oxide and carbon dioxide when heated. The chemical reaction is as follows:
- CaCO3 → CaO + CO2
- The reaction is used to make quick lime, which is an industrially important product.
- Another example of thermal decomposition is 2Pb(NO3)2 → 2PbO + O2 + 4NO2.
- Some oxides, especially of weakly electro positive metals decompose when heated to high enough temperature. A classical example is the decomposition of mercuric oxide to give oxygen and mercury metal. The reaction was used by Joseph Priestley to prepare samples of gaseous oxygen for the first time.
- When water is heated to well over 2,000 °C (2,270 K; 3,630 °F), a small percentage of it will decompose into OH, monatomic oxygen, monatomic hydrogen, O2, and H2.
- The compound with the highest known decomposition temperature is carbon monoxide at ≈3,870 °C (4,140 K; 7,000 °F).
Decomposition of nitrates, nitrites and ammonium compounds
- Ammonium dichromate on heating yields nitrogen, water and chromium(III) oxide.
- Ammonium nitrate on strong heating yields dinitrogen oxide ("laughing gas") and water.
- Ammonium nitrite on heating yields nitrogen gas and water.
- Barium azide on heating yields barium metal and nitrogen gas.
- Sodium azide on heating at 300 °C (573 K; 572 °F) violently decomposes to nitrogen and metallic sodium.
- Sodium nitrate on heating yields sodium nitrite and oxygen gas.
- Organic compounds such as tertiary amines undergo Hofmann elimination when heated and yield secondary amines and alkenes.
Ease of decomposition
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 (CuSO4), begins to decompose at about 200 °C (473 K; 392 °F), increasing rapidly at higher temperatures to about 560 °C (833 K; 1,040 °F). In contrast potassium is near the top of the reactivity series, and potassium sulfate (K2SO4) does not decompose at its melting point of about 1,069 °C (1,342 K; 1,956 °F), 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 100 °C (373 K; 212 °F) and for lactic acid, the decomposition process started around 50 °C (323 K; 122 °F). These components are necessary for further testing, so in the forensics discipline, decomposition of fingerprints is significant.