Chemical Engineering Tips – Redox Reactions

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The concept of oxidation-reduction originally referred to reactions in which a substance combines with oxygen as oxidation, while reactions in which a substance loses oxygen were called reduction. However, in reality, not all redox reactions necessarily involve oxygen. Thus, it was further expanded to “losing hydrogen is also oxidation, and binding hydrogen is also reduction.” Defined in this way, it still has certain limitations. The essence of redox reaction is the transfer of electrons (including the gain and loss of electrons as well as the shift of electron pairs). The process in which an atom or ion loses electrons is called oxidation; the process in which an atom or ion gains electrons is called reduction. Oxidation and reduction are two opposing processes that occur simultaneously and are interdependent. The gain and loss of electrons occur simultaneously, and the total number of electrons gained and lost is equal. Due to the transfer of electrons, the oxidation states of the elements change before and after the reaction. In redox reactions, the substance that gains electrons is called an oxidizing agent. Oxidizing agents possess oxidizing properties; they can oxidize a reducing agent while being reduced themselves. The substance that loses electrons in a reaction is called a reducing agent. Reducing agents have reducing properties; they can reduce an oxidizing agent while being oxidized themselves. Oxidants and reductants are present simultaneously in a redox reaction. Based on the electrode potential, oxidizing agents and reducing agents can be identified, as well as their relative strengths. The standard electrode potentials E for several electrode pairs are listed below: substances in the oxidized state generally act as oxidizing agents, and the higher the E value, the stronger the oxidizing agent. Common oxidants are reactive non-metallic elements such as Cl2, O2, etc.; compounds or ions of elements in higher oxidation states such as HNO3, concentrated H2SO4, KMnO4, K2Cr2O7, Fe3+, etc. Reduced substances generally act as reducing agents. The lower the E value, the stronger the reducing agent. Common reducing agents include reactive metal elements such as Na, Mg, Al, etc.; some unreactive non-metal elements such as H2, C, etc.; and compounds or ions of low-valent non-metal elements such as H2S, HI, NH3, etc. Some substances whose oxidation states are in an intermediate state, such as H2O2, Fe2+, SO2, etc., can act as both oxidants and reducers. As in the reaction: H2O2 acts as an oxidizing agent, and it is itself reduced by Fe2+ to form H2O. It can be seen that when encountering a stronger oxidizing agent, it acts as a reducing agent. When faced with a stronger reducing agent, it acts as an oxidizing agent. The key factor depends on the strength of the redox nature of the substance that reacts with it.