Thermal decomposition

For the biological process, see decomposition.

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 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.

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.

Equipment used by Priestley in his experiments on gases

• Some oxides, especially of weakly electropositive metals decompose when heated to high enough temperatures. 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, leading Antoine Lavoisier to explore the nature of the gas.

• 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 continue to produce smoke even well after being removed from the heat. In films, where stuntmen have to jump through breaking windows, the window panes traditionally were breakaway glass made of sugar, which is safer than real glass.^[1][2] Melting the sugar is a tricky business, however; an error of just a few degrees will start a caramelisation process that will ruin the product, so nowadays suitable plastics are commonly used instead.^[3]

• When water is heated to well over 2000 °C, a small percentage of it will decompose into its constituent elements:

2 H₂O → 2 H₂ + O₂

• The compound with the highest known decomposition temperature is carbon monoxide at ≈3870 °C (≈7000 °F).^{[citation needed]}.

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 nitrate on heating yields sodium nitrite and oxygen gas.

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 break less easily. For example, copper is near the bottom of the reactivity series, and copper sulfate (CuSO₄), begins to decompose at about 200°C, increasing rapidly at higher temperatures to about 560°C. In contrast potassium is near the top of the reactivity series, and potassium sulfate (K₂SO₄) does not decompose at its melting point of about 1069°C, nor even at its boiling point.^[4]

See also

• Ellingham diagram
• Thermochemical cycle
• Thermal depolymerization
• Chemical thermodynamics
• Pyrolysis - thermolysis of organic material

References