What is slow oxidation?
Oxidation of organic and inorganic matter
In discussing oxidation, we often mean the reaction of a certain substance with oxygen, with the formation of new substances. In the broader sense, oxidation is an oxidation-reduction reaction, in which the oxidation state of the oxidized substance’s atom increases (the electron donor is the substance that gives up electrons), with the simultaneous reduction of the oxidation state of the oxidizer (the electron acceptor is the atom that accepts electrons). The oxidizer is not always oxygen (for example, halogens, especially fluorine F₂, are also strong oxidizers).
The reaction of any substance (besides fluorine and practically all inert gases) with oxygen can be called oxidation. Whether the process takes place rapidly or slowly, whether it requires a catalyst and how much heat is released during the reaction. But all of these factors are significant in classifying oxidation. It may be rapid, slow, catalytic etc.
What is slow oxidation?
Oxidation is called slow when the formation of new substances takes place quite slowly, and in the course of the reaction an insignificant quantity of heat is released without light. In slow oxidation, the substance does not reach flashpoint, and so does not ignite.
A typical example of inorganic slow oxidation is the rusting of iron in air in a damp environment:
4Fe + 6H₂O + 3O₂ = 4Fe(OH)₃ – orange-brown iron(III) hydroxide forms (the product of the slow oxidation of iron)
Iron does not burn in this case under the impact of air, but an orange coating of iron(III) hydroxide slowly forms on its surface (when the hydroxide is dehydrated, as it is quite unstable in this case, on the surface only iron oxide Fe₂O₃ remains, also as a product of slow oxidation). Any similar reaction which does not take place instantaneously, without strong heating or ignition, is classified as slow oxidation.
Slow oxidation of organic matter
Many organic compounds are capable of slowly oxidizing (decaying). For example, slow oxidation is often used in the manufacture of fertilizers from organic waste – composting. Microorganisms take part in the process – both aerobic (which receive energy with access to oxygen) and anaerobic (which do not require air for life activity),
On contact with oxygen and/or under the impact of microorganisms, organic waste oxidizes – it heats up, as the life activity of bacteria involves the release of gases (for aerobic oxidation this is carbon dioxide CO₂, for anaerobic it is methane CH₄, which causes the unpleasant smell). Usually in composting both processes take place at once – aerobic and anaerobic.
Other examples of organic slow oxidation include:
- breathing of aerobic creatures;
- rotting of leaves;
- oxidation of oil (rancidification).
Slow oxidation of organic matter often involves the attachment of oxygen. For example, from primary alcohol carbonic acid may be obtained in several stages (as long as it does not ignite):
CH₃-CH₂-OH + [O] = CH₃COH + H₂O
(with the impact of atomic oxygen on alcohol, an aldehyde forms – ethanal; this is the product of the incomplete slow oxidation of alcohol, as aldehydes can be oxidized to carbonic acids)
CH₃COH + [O] = CH₃COOH (acetic acid forms)
In the subsequent oxidation by oxygen, acid breaks down into carbon dioxide and water. Oxidizers than can be used include oxygen, potassium permanganate KMnO₄, copper(II) oxide CuO and others. Click here for amazing experiments with copper. In the equivalent oxidation of a secondary alcohol, ketone may be obtained:
CH₃-CH(OH)-CH₃ + [O] = (CH₃)₂C=O (from isopropyl alcohol, acetone forms)
In oxygen (pure or in the air) practically all substances can burn: simple, complex, organic and inorganic. Any organic substances burn especially well. The combustion of oil and natural gas has found practical significance – products of their rapid oxidation are used as fuel.
Combustion is a type of oxidation process. It differs fundamentally from slow oxidation, as combustion is the process of the rapid oxidation of a substance, in which a large quantity of heat is released, and light appears (a flame). For example, if the product of slow oxidation of iron in a damp environment is rust Fe(OH)₃, on the combustion of iron in oxygen, mixed iron(II, III) oxide is formed, Fe₃O₄ (FeO·Fe₂O₃) – “iron cinder”:
3Fe + 2O₂ = Fe₃O₄
In the case with organic matter, the product of this reaction is usually carbon dioxide and water:
C₂H₅OH + 3О₂ = 2CO₂ + 3H₂O
(the reaction takes place at a high temperature with access of oxygen, which supports combustion)
In some cases (for example, in the combustion of amines), the release of additional products is possible:
4CH₃NH₂ + 9O₂ = 4CO₂ + 10H₂O + 2N₂
(as amines contain nitrogen atoms, on combustion molecular nitrogen is present among the products of reaction)
In certain conditions, slow oxidation can move to combustion. This usually happens at an increased temperature: if heat is released in slow oxidation that is sufficient for ignition, the substance begins to burn with the release of a considerable quantity of heat and light. The main difference of slow oxidation from combustion lies in the speed of the reaction.
Slow oxidation has great biological and practical significance: it is an essential part of the process of breathing and decay; slow oxidation is often used in the manufacture of fertilizers for plants.