Thermal decomposition , or thermolysis , is a chemical decomposition caused by heat . The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic because heat is required to break the chemical bonds in the compound undergoing decomposition . If the decomposition is sufficiently exothermic , a positive feedback loop is created , generating thermal runaway and possibly an explosion or other chemical reaction .
Example
- Calcium carbonate (limestone or chalk) on heating decomposes into calcium oxide and carbon dioxide . The chemical reaction is as follows:
CaCO 3 → CaO + CO 2
The reaction is used to make quick lime , an industrially important product.
Another example of thermal decomposition is : – 2Pb(NO 3 ) 2 —-> 2PbO + O 2 +4NO 2
- Some oxides , especially weakly electropositive metals, decompose upon heating to high temperatures. A classical example is the decomposition of mercuric oxide to give oxygen and mercury metal . For the first time the reaction was used by Joseph Priestley to prepare samples of gaseous oxygen .
- When water is heated to more than 2000 °C, a small percentage of it will decompose into OH, monoatomic oxygen, monoatomic hydrogen, O 2 and H 2 . [1]
- The compound with the highest known decomposition temperature is carbon monoxide at 3870 °C (≈7000 °F) .
Decomposition of nitrates, nitrites and ammonium compounds
- Ammonium dichromate on heating gives nitrogen, water and chromium(III) oxide.
- Ammonium nitrate on strong heating produces dinitrogen oxide (” laughing gas “) and water.
- On heating ammonium nitrite , nitrogen gas and water are released.
- Barium metal and nitrogen gas are released when barium azide is heated.
- Sodium azide on heating at 300 °C gives nitrogen and sodium.
- On heating sodium nitrate , sodium nitrite and oxygen gas are evolved .
- On heating organic compounds such as tertiary amines, Hoffmann is removed and secondary amines and alkenes are obtained.
Ease of decomposition
When metals are near the bottom of the reactivity chain , their compounds usually decompose readily at high temperatures. This is because stronger bonds form between atoms towards the top of the reactive chain , and stronger bonds are more difficult to break. For example, copper is near the bottom of the reactivity chain, and copper sulfate (CuSO 4 ), begins to decompose at about 200 °C, rapidly increasing to about 560 °C at higher temperatures. In contrast potassium is near the top of the reactivity chain, and potassium sulfate (K 2 SO 4 ).) does not decompose at its melting point around 1069 °C, nor at its boiling point.
