Half reactions - reactions of oxidation or reduction of a component of a redox reaction . Half-reactions take into account changes in the oxidation states of individual substances involved in the redox reaction [1] . Each half-reaction is characterized by an electrode redox potential, the value of which determines the ease of electron transfer [2] .
Often the concept of half-reactions is used to describe what happens in an electrochemical cell , for example, in a galvanic battery . Half reactions can be written to describe both the metal being oxidized ( anode ) and the metal being reduced ( cathode ).
Half reactions are often used as a method of balancing redox reactions. For redox reactions in acidic conditions, after the atoms and oxidation states are balanced, you will need to add H + ions to balance the hydrogen ions in the half-reaction. Redox reactions under basic conditions, after the atoms and oxidation states are balanced, first consider this as an acid solution, and then OH - ions are added to balance the amount of H + ions in the half-reaction (which would give H 2 O).
When the redox reaction occurs, we do not see the redistribution of electrons. What we see are reagents (source material) and final products. In any redox reaction, there are two half reactions: the half reaction of oxidation and the half reaction of reduction. The sum of these half-reactions is a redox reaction.
Content
Example: Zn and Cu are galvanic cells.
Consider the galvanic cell shown in the adjacent image: it is built from a piece of zinc (Zn) immersed in a solution of zinc sulfate (ZnSO 4 ), and a piece of copper (Cu) immersed in a solution of copper sulfate (II) (CuSO 4 ).
Oxidation occurs at the anode (Zn) (metal loses electrons).
At the cathode (Cu), reduction occurs (electrons are received).
Half-reaction balancing method
Consider the reaction:
Two elements are involved - iron and chlorine. For each, the degree of oxidation changes: for iron from +2 to +3, for chlorine from 0 to −1. That is, two half-reactions actually proceed:
Decomposition of the reaction into half-reactions is the key to understanding various chemical processes. For example, for the above reaction, it can be shown that this is a redox reaction in which Fe is oxidized and Cl is reduced. Note the electron transfer from Fe to Cl. Decomposition is also a way to simplify the balancing of the chemical equation .
For example:
- becomes
- is added
- and finally it becomes
See also
- Electrode potential
Notes
- ↑ Half-Reaction .
- ↑ CHEMISTRY p.11