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Methods for Determining the Strength of Oxidizing and Reducing Properties

(I) Determination Based on Chemical Equations

(1) Oxidizing agent (oxidizing property) + Reducing agent (reducing property) ===> Reduced product + Oxidized product

Oxidizing agent ----> Reduced product: gains electrons, oxidation state decreases, is reduced, undergoes a reduction reaction

Reducing agent ----> Oxidized product: loses electrons, oxidation state increases, is oxidized, undergoes an oxidation reaction

Oxidizing property: Oxidizing agent > Oxidized product
Reducing property: Reducing agent > Reduced product

(2) Determination based on the oxidizing and reducing agents in the same reaction

Oxidizing property: Oxidizing agent > Reducing agent
Reducing property: Reducing agent > Oxidizing agent

(II) Comparison Based on the Activity Series of Substances

(1) For metallic reducing agents, the strength of reduction of the metal element generally corresponds to the metal activity series; the further back the metal is located, the less easily it loses electrons, and the weaker its reducing property.

Reducing property: K > Ca > Na > Mg > Al > Mn > Zn > Cr > Fe > Ni > Sn > Pb > (H) > Cu > Ag > Pt > Au

(2) Order of oxidizing properties of metal cations
K+ < Ca2+ < Na+ < Mg2+ < Al3+ < Mn2+ < Zn2+ < Cr3+ < Fe2+ < Ni2+ < Sn2+ < Pb2+ < (H+) < Cu2+ < Ag+ < Pt2+ < Au2+

Note: Fe2+, Sn2+, Pb2+, not Fe3+, Sn4+, Pb4+

(3) Activity order of nonmetals (common elements)
F ---- Cl ---- Br ---- I ---- S
The oxidizing property of the atom (or element) gradually decreases, corresponding to an increase in the reducing property of the related anion.

(III) Determination Based on Reaction Conditions
When different oxidizing agents react with the same reducing agent, if the valence states of the oxidized products are the same, the difficulty of the reaction can be used to determine the strength. The easier the reaction, the stronger the oxidizing property of the oxidizing agent.

For example:
16 HCl (concentrated) + 2 KMnO4 === 2 KCl + 2 MnCl2 + 8 H2O + 5 Cl2 (gas)
4 HCl (concentrated) + MnO2 === (heating) MnCl2 + 2 H2O + Cl2 (gas)
4 HCl (concentrated) + O2 === (heating, CuCl2 catalyst) 2 H2O + 2 Cl2 (gas)

Oxidizing property: KMnO4 > MnO2 > O2

(IV) Determination Based on the Valence of Oxidized Products
When a reducing agent containing a variable-valence element reacts with different oxidizing agents under similar conditions, the strength of the oxidizing agent can be judged according to the valence of the oxidized product.For example: 2Fe + 3Cl₂ ==(ignite)== 2FeCl₃ Fe + S ==(heat)== FeS Oxidizing ability: Cl₂ > S (5) Judging according to the Periodic Table of Elements (1) Elements in the same main group (from top to bottom) Non-metal atoms (or elements) gradually decrease in oxidizing ability, and the corresponding anions' reducing ability gradually increases. Metal atoms gradually increase in reducing ability, and the corresponding cations' oxidizing ability gradually decreases. (2) Elements in the same main group within the same period (from left to right) The elemental reducing ability gradually increases, and oxidizing ability gradually increases. The cation oxidizing ability gradually increases, and the anion reducing ability gradually decreases. (6) Comparing the acidity or basicity of the hydrated oxides of elements with their highest oxidation states The stronger the acidity, the stronger the corresponding element's oxidizing ability. The stronger the basicity, the stronger the corresponding element's reducing ability. (7) Judging based on the electrode reactions in galvanic cells For a galvanic cell composed of two different metals, the negative electrode metal is the site of electron flow out, and the positive electrode metal is the site of electron flow in. Its reducing ability: negative electrode metal > positive electrode metal (8) Judging based on the concentration of a substance The greater the concentration of a substance with oxidizing (or reducing) ability, the stronger its oxidizing (or reducing) ability; conversely, the weaker it is. (9) Judging based on the element's valence state Generally, a variable-valence element in its highest oxidation state has only oxidizing ability, in its lowest oxidation state has only reducing ability, and in intermediate states has both oxidizing and reducing abilities. Generally, in the highest oxidation state, the oxidizing ability is the strongest; as the valence decreases, oxidizing ability weakens and reducing ability strengthens.