Chemical decomposition

Chemical decomposition is the separation (or breakdown) of a chemical compound into smaller compounds or elements. It is sometimes defined as the opposite of chemical synthesis. The stability that a chemical compound ordinarily has is eventually limited when exposed to extreme environmental conditions like heat, radiation, humidity or the acidity of a solvent. The details of decomposition processes are generally not well defined, as a molecule may break up into a host of smaller fragments. There are broadly three types of decomposition reactions: Thermal, electrolytic, and catalytic.

Chemical decomposition is often an undesired chemical reaction. However, chemical decomposition is exploited in several analytical techniques, notably mass spectrometry, traditional gravimetric analysis, and thermogravimetric analysis.

Broader definition

A broader definition of the term decomposition also includes the breakdown of one phase into two or more phases.

Reaction formulas

The generalized reaction formula for chemical decomposition is:

AB → A + B

with a specific example being the electrolysis of water to gaseous hydrogen and oxygen:

2H₂O → 2H₂ + O₂

Additional examples

An example of spontaneous decomposition is that of hydrogen peroxide, which will slowly decompose into water and oxygen:

2H₂O₂ → 2H₂O + O₂

Carbonates will decompose when heated, a notable exception being that of carbonic acid, H₂CO₃. Carbonic acid, the "fizz" in sodas, pop cans and other carbonated beverages, will decompose over time (spontaneously) into carbon dioxide and water

H₂CO₃ → H₂O + CO₂

Other carbonates will decompose when heated producing the corresponding metal oxide and carbon dioxide. In the following equation M represents a metal:

MCO₃ → MO + CO₂

A specific example of this involving calcium carbonate:

CaCO₃ → CaO + CO₂

Metal chlorates also decompose when heated. A metal chloride and oxygen gas are the products.

MClO₃ → MCl + O₂

A common decomposition of a chlorate to evolve oxygen utilizes potassium chlorate as follows:

2KClO₃ → 2KCl + 3O₂

Thermal decomposition

Thermal decomposition, also called thermolysis, is defined as a chemical reaction whereby a chemical substance breaks up into at least two chemical substances when heated. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. The decomposition temperature of a substance is the temperature at which the substance decomposes into smaller substances or into its constituent atoms.

For example, calcium carbonate decomposes into calcium oxide and carbon dioxide. Some compounds, on the other hand, simply decompose into their constituent elements. Water, when heated to well over 2000 degrees Celsius, breaks up into its components - hydrogen and oxygen.

A common example is the decomposition of copper carbonate into copper oxide and carbon dioxide, seen here:

CuCO₃ → CuO + CO₂

The copper carbonate turns from a green powder into a black copper oxide, and carbon dioxide is released in a gaseous state.

Decomposition may be aided by the presence of a catalyst. For example, hydrogen peroxide decomposes more quickly with the use of manganese(IV) oxide:

2H₂O₂(aq) → 2H₂O(l) + O₂(g)

High temperatures can also induce polymerization, which produces larger molecules, possibly also causing thermal decomposition and evaporation of smaller molecules in the process. Such reactions are called pyrolysis reactions. A common example is coking, which is the formation of an amorphous carbon structure along with the evaporation of hydrogen and other pyrolysis gases.

If thermal decomposition of a substance is significantly exothermic, then the substance is thermodynamically unstable. If initiated, its decomposition forms a positive feedback loop and undergoes thermal runaway up to the point of causing an explosion.

This process can be seen in almost every office as a coffee pot is left on the hot plate. When examined, one can see an oily substance on the top that is the organic components of the coffee coming out of solution due to over or re-heating.

See also

• Analytical chemistry
• Carbonate
• Catalysis
• Chemical reaction
• Combustion
• Electrolysis
• Heat

References

ISBN links support NWE through referral fees

• McMurry, John. 2004. Organic Chemistry, 6th ed. Belmont, CA: Brooks/Cole. ISBN 0534420052.
• Solomons, T.W. Graham, and Craig B. Fryhle. 2004. Organic Chemistry, 8th ed. Hoboken, NJ: John Wiley. ISBN 0471417998.
• Zumdahl, Steven S. 2005. Chemical Principles. New York, NY: Houghton Mifflin. ISBN 0618372067.

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