The Haber–Weiss reaction generates •OH (

hydroxyl radical The hydroxyl radical, •HO, is the neutral form of the hydroxide ion (HO–). Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are pr ...

s) from H₂O₂ (

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

) and

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

(•O₂⁻)

catalyzed Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

ions. It was first proposed by

Fritz Haber Fritz Jakob Haber (; 9 December 1868 – 29 January 1934) was a German chemist who received the Nobel Prize in Chemistry in 1918 for his invention of the Haber process, a method used in industry to synthesize ammonia from nitrogen gas and hydrog ...

and his student Joseph Joshua Weiss in 1932. This reaction has long been studied and revived in different contexts, including

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabolic disorders Metabolism ...

radiochemistry Radiochemistry is the chemistry of radioactive materials, where radioactive isotopes of elements are used to study the properties and chemical reactions of non-radioactive isotopes (often within radiochemistry the absence of radioactivity leads t ...

, and water

radiolysis Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...

. In the 1970, with the emerging interest for the effect of free radicals onto the ageing mechanisms of living

cells Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a d ...

due to

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(O₂), it was proposed that the Haber–Weiss reaction was a source of radicals responsible for cellular

oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...

. However, this hypothesis was later disproved by several research works. The oxidative stress

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...

is not caused by the Haber–Weiss reaction as a whole, but by the

Fenton reaction Fenton's reagent is a solution of hydrogen peroxide (H2O2) and an iron catalyst (typically iron(II) sulfate, FeSO4). It is used to oxidize contaminants or waste water as part of an advanced oxidation process. Fenton's reagent can be used to destr ...

, which is one specific part of it. The reaction is kinetically slow, but is

by dissolved

ions. The first step of the

catalytic cycle In chemistry, a catalytic cycle is a multistep reaction mechanism that involves a catalyst. The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, bioinorganic chemistry, materials s ...

involves the reduction of the

ferric In chemistry, iron(III) or ''ferric'' refers to the chemical element, element iron in its +3 oxidation number, oxidation state. ''Ferric chloride'' is an alternative name for iron(III) chloride (). The adjective ''ferrous'' is used instead for i ...

(Fe³⁺) ion into the

ferrous In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...

(Fe²⁺) ion: :Fe³⁺ + •O₂⁻ → Fe²⁺ + O₂ The second step is the

: :Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + •OH Net reaction: :•O₂⁻ + H₂O₂ → •OH + OH⁻ + O₂

Haber-Weiss chain reaction

The main finding of Haber and Weiss was that hydrogen peroxide (H₂O₂) is decomposed by a chain reaction. The Haber–Weiss reaction chain proceeds by successive steps: (i) initiation, (ii) propagation and (iii) termination. The chain is initiated by the Fenton reaction: :Fe²⁺ + H₂O₂ → Fe³⁺ + HO^– + HO^• (step 1: initiation) Then, the reaction chain propagates by means of two successive steps: :HO^• + H₂O₂ → H₂O + O₂^•– + H⁺ (step 2: propagation) :O₂^•– + H⁺ + H₂O₂ → O₂ + HO^• + H₂O (step 3: propagation) Finally, the chain is terminated when the hydroxyl radical is scavenged by a ferrous ion: :Fe²⁺ + HO^• + H⁺ → Fe³⁺ + H₂O (step 4: termination) George showed in 1947 that, in water, step 3 cannot compete with the spontaneous disproportionation of superoxide, and proposed an improved mechanism for the disappearance of hydrogen peroxide. See for a summary. The reactions proposed therein are: :Fe²⁺ + H₂O₂ → Fe³⁺ + HO^– + HO^• (initiation) :Fe²⁺ + HO^• → Fe³⁺ + HO^– (termination) :H₂O₂ + HO^• → H₂O + HO₂^• (propagation) :Fe²⁺ + HO₂^• → Fe³⁺ + HO₂⁻ (termination) :Fe³⁺ + HO₂^• → Fe²⁺ + O₂ + H⁺ (termination)

Hydroperoxyl and superoxide radicals

With time, various chemical notations for the

hydroperoxyl The hydroperoxyl radical, also known as the hydrogen superoxide, is the protonated form of superoxide with the chemical formula HO2, also written HOO•. This species plays an important role in the atmosphere and as a reactive oxygen species ...

(perhydroxyl) radical coexist in the literature. Haber, Wilstätter and Weiss simply wrote HO₂ or O₂H, but sometimes HO₂^• or ^•O₂H can also be found to stress the radical character of the species. The hydroperoxyl radical is a weak acid and gives rise to the

superoxide radical In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of th ...

(O₂^•–) when it loses a proton: :HO₂ → H⁺ + O₂^– :sometimes also written as: :HO₂^• → H⁺ + O₂^•– A first pK_a value of 4.88 for the dissociation of the hydroperoxyl radical was determined in 1970. The presently accepted value is 4.7. This pK_a value is close to that of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

. Below a pH of 4.7, the protonated hydroperoxyl radical will dominate in solution while at pH above 4.7 the superoxide radical anion will be the main species.

Effect of pH on the reaction rate

As the Haber–Weiss reaction depends on the presence of both Fe³⁺ and Fe²⁺ in solution, its kinetics is influenced by the respective solubilities of both species whose are directly function of the solution pH. As Fe³⁺ is about 100 times less soluble than Fe²⁺ in natural waters at near-neutral pH, the ferric ion concentration is the limiting factor for the reaction rate. The reaction can only proceed with a fast enough rate under sufficiently acidic conditions. At high pH, under alkaline conditions, the reaction considerably slows down because of the precipitation of Fe(OH)₃ which notably lowers the concentration of the Fe³⁺ species in solution. Moreover, the pH value also directly influences the acid-base dissociation equilibrium involving the

and the

radicals (pK_a = 4.7) as mentioned above.

References

{{DEFAULTSORT:Haber-Weiss reaction Catalysis Environmental chemistry Free radical reactions Fritz Haber Iron compounds Name reactions Oxidizing agents Peroxides Radiation effects

Haber-Weiss chain reaction

Hydroperoxyl and superoxide radicals

Effect of pH on the reaction rate

See also

References