What is slow oxidation?

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)

Com­bus­tion

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.