Decomposition Reactions

FIELDS OF STUDY: Organic Chemistry; Chemical Engineering; Environmental Chemistry

ABSTRACT

Decomposition reactions are defined, and their importance in various chemical processes is elaborated. Decomposition reactions have broad value, both as a means of destroying unwanted materials and as a useful step in producing desired materials. Through their distinctive products, they also provide a means of identifying the source of materials in forensic investigations.

Chemical Synthesis in Reverse

A decomposition reaction can be defined as the breaking apart of a compound or substance into smaller and less complex components. This breakdown must obey the same rules of chemical behavior as the formation of a compound or substance from its lesser components. In essence, decomposition reactions are the exact opposite of synthesis reactions. When a compound or material is synthesized, the component materials, or reactants, interact with each other in specific ways that are determined by the basic rules of chemical behavior. Decomposition of the material often occurs by reversing the direction of the formation reaction, resulting in products that are identical to the original reactants.

Decomposition reactions can be used to produce a material that is difficult to prepare by normal chemical methods. While it is possible in some cases to prepare a material that decomposes by a different mechanism to produce the desired material, devising and controlling such an application demands a thorough understanding of the electronic principles of reaction mechanisms. In other cases, decomposition reactions make it possible to easily prepare something that cannot be reproduced by synthetic means, such as the caramelization that occurs in cooking when the sugars and other carbohydrates begin to undergo thermal decomposition.

The Occurrence of Decomposition Reactions

The term "decomposition" has somewhat different meanings depending on the context in which it is used. In chemistry, it refers to a specific type of chemical reaction, while in forensic analysis and ecological science, for example, materials are described as being in a state of decomposition when the original structure of the material has broken down and is no longer in its living or original state. Thus, in the broadest sense, decomposition can be defined as a process in which materials are broken down to their component parts and elements.

In the natural environment, decomposition is normally caused by chemical reactions such as oxidation (a reaction with oxygen in the atmosphere) or by the activity of bacteria and other living things that actively consume the material. In the context of chemistry, decomposition takes place when a chemical compound splits apart into component pieces that essentially retain the structures they had in the parent molecule. Decomposition reactions can thus be represented by the following general symbolic equation:

AB → A + B

In this representation, the compound AB has two component structures, A and B. The molecule AB decomposes when it splits apart to liberate both A and B.

Decomposition reactions occur in solid, liquid, and gas phases and can pose a problem for purified laboratory chemicals, which often become discolored over time. The essential characteristic of such reactions is that the material undergoing decomposition obeysthe general equation given above. It is important to note thatthe parent material can split into more than two parts, though this is a rare situation. It is far more commonly the case that a product component, once formed, quickly degrades further.

Decomposition Reactions and Activation Energy

All chemical compounds will undergo decomposition reactions when given sufficient activation energy. In the vast majority of cases, the activation energy for the process is provided by either thermal excitation or photolysis. The mechanisms by which these affect the material are essentially the same: electrons in specific chemical bonds become excited to a higher energy level in a molecular orbital that does not support bonding. When this happens, the parent molecule is free to separate at that location. Thermal excitation acts to stimulate the vibrations that are always taking place in the bonds between atoms. These vibrations have several different forms, including stretching and relaxation, in the same manner that a spring can be stretched and relaxed. The energy of this mode of vibration can become great enough to overpower the strength of the chemical bond and separate the components of the molecule.

The ability of a material to undergo a decomposition reaction is strongly affected by the stability of the product material. One example of this is the equilibrium between carbon dioxide, water, and carbonic acid. One molecule of carbonic acid, H₂CO₃, forms when one molecule of carbon dioxide (CO₂) and one molecule of water (H₂O) undergo a combination reaction. This is the method by which aerobic cells eliminate the carbon dioxide produced during glycolysis, which is the anaerobic metabolism of glucose: an enzyme catalyzes both the formation and the decomposition of carbonic acid, allowing carbon dioxide gas, a by-product of the process, to be expelled. In carbonated beverages, however, the simple chemical equilibrium

H₂O + CO₂ ⇌ H₂CO₃

is established between water and dissolved carbon dioxide. When the cap is removed and the pressure inside the can or bottle is released, numerous bubbles of carbon dioxide appear as the carbonic acid decomposes back to the more stable molecules of water and carbon dioxide. It is easy to demonstrate that the warmer the liquid is when the pressure is released, the more rapid the decomposition. Who has not seen a warm bottle of soda spew its contents violently when opened?

Another example of chemical decomposition due to thermal excitation is the caramelization of white sugar, also known as "sucrose." This is an example of pyrolysis, which is a form of thermal decomposition, or thermolysis, that takes place at high temperatures in the absence of oxygen. The decomposition of sucrose occurs at temperatures around 160 degrees Celsius and higher, when heat drives out the components of water from the carbohydrate structure of the sugar.

Decomposition in the Atmosphere

One alternative to thermolysis is photodecomposition, or photolysis, which occurs when electrons in a chemical bond absorb a photon of light energy and enter an excited state. In this excited state, the strength of the chemical bond is greatly weakened, if not completely eliminated. The normal vibrations of the weakened bond are then sufficient to separate the two components of the molecule. When this occurs, the products are unstable radical entities, which are essentially normal molecules that have all of their electrons but are missing an atom, leaving one free electron that would normally be involved in a chemical bond but for which such a bond does not exist.

This is the typical process that takes place in the atmosphere, and it is especially important in regard to the chemistry of atmospheric pollution. A common example of photolysis is when a molecule of chlorine gas, Cl₂, separates into two neutral chlorine atoms. This phenomenon is also an example of homolytic cleavage, which is when a chemical bond breaks and the resulting products each retain one of the two electrons that formed the bond. Photolysis of a molecule such as chloromethane, CH₃Cl, results in the separation of the neutral chlorine atom Cl• from the neutral methyl radical CH₃• (the dot is used in standard notation to indicate the free electron). The corresponding reaction equations are as follows:

Cl₂ ⇌ 2Cl•

CH₃Cl ⇌ CH₃• + Cl•

These are shown as equilibria because the process can be easily reversed by simply reforming the bond, and indeed this is primarily what occurs, unless there is some other chemical species available to react with one of the radicals when it is formed. In the atmosphere, molecular oxygen (O₂) and ozone (O₃), as well as several other gaseous materials, are available to enter into a chain reaction that is either a normal process of atmospheric chemistry or one that produces various atmospheric pollutants.

The ozone layer that surrounds Earth has been seriously affected by the release of various halocarbons known as "chlorofluorocarbons," or CFCs, which are similar to chloromethane and were widely used as refrigerants for several decades. The ozone in Earth’s stratosphere interacts with incoming ultraviolet radiation from the sun, preventing much of it from reaching the surface of the planet, where it would pose a danger to exposed life forms. This ozone is regenerated from molecular oxygen through a series of radical chain reactions in which decomposition reactions are an essential feature. However, when CFCs reach the planet’s upper atmosphere, that same ultraviolet radiation causes a decomposition reaction that frees a radical chlorine atom from the compound, which in turn interacts with existing ozone molecules in such a way that it turns ozone into molecular oxygen, thus depleting the protective ozone layer between Earth and the sun.

PRINCIPAL TERMS

• activation energy: the amount of energy a system requires for the formation of an activated complex from which a reaction can proceed.
• combination reaction: a chemical reaction in which two or more reactants combine to form a single product.
• product: a chemical species that is formed as a result of a chemical reaction.
• reactant: a chemical species that takes part in a chemical reaction.
• reaction mechanism: the sequence of electron and orbital interactions that occurs during a chemical reaction as chemical bonds are broken, made, and rearranged.

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