Redox Reactions (Summary of Respiration Reactions, Generalization of Half-Reaction Equations)

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  • The following is a summary of the half-reaction equation for respiration of living organisms.
    

                                                                                                                   

    ①Organic matter oxidation   CO₂  +  4H⁺ + 4e- =   CH₂O + H₂O                    

                                                                                                                   

    ②Oxygen respiration：               O₂  +  4H⁺ + 4e^-  =  2H₂O                                  

    ③Nitrste respiration：               4NO₃^- + 24H⁺ + 20e^-  =   2N₂ + 12 H₂O 

    　（There is no direct generation of N2 by nitrate respiration, so there is no this respiratory reaction）

                        NO₃^- + 2H⁺ + 2e^-   =  NO₂^- + H₂O                       

    　　　                （In nitrate respiration, nitrite is produced first.）

    ④Sulphate respiration：    SO₄^2- + 10H⁺ + 8e^-   =  H₂S + 4H₂O                 

    ⑤Methane fermentation：　CO₂ + 8H⁺  + 8e^-  =   CH₄ + 2H₂O                               

     

    Since the electrons are in a state of limbo in the half-reaction alone, it is not possible to decide in which direction the reaction will go by only the individual half-reactions. Therefore, the left side (original form) and the right side (product form) are connected by an equal (=) and arranged so that the electrons are on the left side.　

    
    

    Generalized description of half-reaction equation：

    　The substance on the left side is called the oxidizer (the substance that receives electrons) and is abbreviated as Ox. The substance on the right side is called the reductant (the substance that gives electrons) and is abbreviated as Red. The half-reaction equation is generalized as follows.

    
    

                     Ox + e-　＝　Red

    
      　A single half-reaction alone does not constitute a redox reaction. In the half-reaction equation, Ox receives an electron to become Red. It is not that the electrons of Red in the half-reaction equation are passed to Ox. There is an exchange of electrons with another half-reaction, and a redox reaction takes place. For example, the redox reaction of sulfate respiration proceeds when the half-reactions ① and ④ are paired. In other words, the direction of reaction ① is determined as right→left, and the direction of reaction ④ is determined as left→right. If ② and ④ are paired, the direction of ② is determined as left→right and the direction of ④ is determined as right→left.
    
    

    　However, just as in photosynthesis the reaction proceeds in the opposite direction, depending on the surrounding environment (redox potential and concentration of substances), it is possible for the reaction to proceed in the opposite direction to that described above.