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Redox (portmanteau of reduction and oxidation) reactions include all chemical reactions in which atoms have their oxidation state changed; in general, redox reactions involve the transfer of electrons between species.


  • Hydrogen combines with oxygen with great vigor. A stream of hydrogen when ignited burns in oxygen or air with a very hot, almost colorless flame (Figure 6-8), and a mixture of hydrogen and oxygen when ignited explodes with great violence.
    When hydrogen burns in air or oxygen, forming hydrogen oxide (water), the hydrogen is said to have been oxidized. The process is called oxidation, and oxygen is called the oxidizing agent.
    The tendency of hydrogen to combine with oxygen to form water is so great that the gas will even remove oxygen from many metallic oxides. Thus when a stream of hydrogen is passed over hot copper oxide, CuO, in a heated tube, the copper oxide is converted into metallic copper (Figure 6-9):

    CuO + H2 → Cu + H2O

    This reaction is described as the reduction of copper oxide by hydrogen. Hydrogen is called the reducing agent in the reaction. Copper oxide is said to have been reduced to metallic copper.
    In the reaction of hydrogen and copper oxide the copper oxide is the oxidizing agent. In every reaction of this sort there is a reducing agent which is oxidized and an oxidizing agent which is reduced.

    • Linus Pauling (1957), College Chemistry (2nd ed.), Chapter 6. Hydrogen and Oxygen, p. 109-111
  • Although different kinds of chemical reactions are thus easily recognized, it has not been found very useful in general to attempt to classify reactions in a rigorous way. Nevertheless, there is one very important class of chemical reactions that deserves special study. These reactions are oxidation-reduction reactions, to which we now turn our attention.
    • Linus Pauling (1957), College Chemistry (2nd ed.), Chapter 12. Oxidation-Reduction Reactions, p. 243
  • The investigation of redox metalloenzymes by UEM should shed new light on the mode of action because it will be possible to record electron-energy-loss spectra (Chap. 6) and to reveal oxidation states of the metal ions at active centers, even though the measurements may be complicated by the reducing environment created by the secondary electrons within the sample. These and other promising features of UEM were realized since the first UEM publication 27 (Sec. 6.1) and currently represent one of the major areas of study at Caltech.
    • Ahmed H. Zewail, John M. Thomas (2010), 4D Electron Microscopy: Imaging in Space and Time, Chap. 8. 4D Visualization: Past, Present, and Future, p. 324
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