All chemical reactions can be divided into two groups. In those of the first group, the oxidation state of all the elements in the reactants remains constant, while in those of the second group, the oxidation state of one or several elements changes.
We can exemplify the reactions of the first group by the neutralization reaction:
HCl + NaOH = NaCl + H2O
An example of a reaction of the second group is the reaction of metal with an acid:
Zn + 2HC1 = ZnCl2 + H2↑.
If in the neutralization reaction no element changes its oxidation number, in the second example the oxidation number of the Zinc changes from 0 to +2, and of the Hydrogen from +1 to 0.
Reactions as a result of which the oxidation numbers of elements change are called oxidation-reduction (redox) reactions.
Oxidation-reduction reactions are of great importance in biological systems. Photosynthesis, breathing, digestion - all these are chains of oxidation-reduction reactions. In engineering, the significance of redox reactions is also great. For example, the entire metallurgical industry is based on oxidation-reduction processes during which metals are recovered from natural compounds.
Simple example of an oxidation-reduction reaction is the reaction of formation of an ionic compound from elementary substances, for example, the reaction of Sodium with Chlorine:
2Na + Cl2 = 2NaCl.
This reaction, like any heterogeneous one, proceeds in several steps. In one of them, the Sodium atoms transform into positively charged ions; the oxidation number of Sodium changes from 0 to -1:
Na = Na+ + .
Such a process - the losing of electrons attended by an increase in the oxidation number of element - is called oxidation.
The Chlorine atoms, which transform into negatively charged ions, gain the electrons lost by Sodium; the oxidation number of Chlorine changes from 0 to -1:
Cl2 + 2 = 2C1-.
The gaining of electrons attended by a decrease in the oxidation number of an element is called reduction.
Hence, in the reaction considered above, Sodium is oxidized, and Chlorine is reduced.
Substance containing an element that is oxidized is called reducing agent, and substance containing an element that is reduced is called oxidizing agent (oxidant).
Consequently, in the above example, Sodium is a reducing agent, and Chlorine an oxidizing one.
Inspection of the oxidation-reduction equations shows that one molecule of Chlorine when reduced gains two electrons, while the oxidation of one Sodium atom is attended by the loss of one electron. The total number of electrons in the system in chemical reactions does not change: the number of electrons lost by the molecules (atoms, ions) of the reducing agent is equal to the number of electrons gained by the molecules (atoms, ions) of the oxidizing agent. Therefore, one molecule of Chlorine can oxidize two Sodium atoms.
Date: 2015-01-12; view: 843