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The main finding of Haber and Weiss was that hydrogen peroxide (H 2 O 2) is decomposed by a chain reaction. [2]The Haber–Weiss reaction chain proceeds by successive steps: (i) initiation, (ii) propagation and (iii) termination.
The reaction 2 H 2 + O 2 → 2 H 2 O provides an example of chain branching. The propagation is a sequence of two steps whose net effect is to replace an H atom by another H atom plus two OH radicals. This leads to an explosion under certain conditions of temperature and pressure. [6] H• + O 2 → •OH + •O•
For example, at 2,200 °C (2,470 K; 3,990 °F) about three percent of all H 2 O are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H 2, O, O 2, and OH. Other reaction products like H 2 O 2 or HO 2 remain minor. At the very high temperature of 3,000 °C (3,270 K; 5,430 °F) more than half of the water molecules are ...
2H 2 O → O 2 + 4H + + 4e − Oxidation (generation of dioxygen) 4H + + 4e − → 2H 2 Reduction (generation of dihydrogen) 2H 2 O → 2H 2 + O 2 Total Reaction Of the two half reactions, the oxidation step is the most demanding because it requires the coupling of 4 electron and proton transfers and the formation of an oxygen-oxygen bond.
Trioxidane (H 2 O 3) is rare and readily decomposes into water and singlet oxygen. Tetraoxidane (H 2 O 4) has been synthesized by reaction among peroxy radicals at low temperature. [1] Pentaoxidane (H 2 O 5) is a byproduct of trioxidane production [2] and has also been synthesized by reaction among peroxy radicals at low temperature. [1]
In the process, water loses an electron and becomes highly reactive. Then through a three-step chain reaction, water is sequentially converted to hydroxyl radical (• OH), hydrogen peroxide (H 2 O 2), superoxide radical (• O − 2), and ultimately oxygen (O 2). [citation needed]
Net reaction: 2 H 2 O → 2 H 2 + O 2. The sulfur and iodine compounds are recovered and reused, hence the consideration of the process as a cycle. This S–I process is a chemical heat engine. Heat enters the cycle in high-temperature endothermic chemical reactions 2 and 3, and heat exits the cycle in the low-temperature exothermic reaction 1.
Elements with low electronegativities, such as most metals, easily donate electrons and oxidize – they are reducing agents. On the contrary, many oxides or ions with high oxidation numbers of their non-oxygen atoms, such as H 2 O 2, MnO − 4, CrO 3, Cr 2 O 2− 7, or OsO 4, can gain one or two extra electrons and are strong oxidizing agents.