Oxidation / Reduction Reactions
A basic understanding of oxidation/reduction or redox reactions is fundamental to understanding chemical reactions in both organic and inorganic chemistry. Redox reactions always involve two components: an oxidizing agent and a reducing agent. The relationship between these components is summarized in the following table.
| Substance | Definition |
|---|---|
| Oxidizing Agent | An electron acceptor. The substance that is reduced by accepting electrons from the reducing agent. |
| Reducing Agent | An electron donor. The substance that is oxidized by donating electrons to the oxidizing agent. |
Half-Reactions
Redox reactions are reversible, and are expressed as two half-reactions, oxidation and reduction. The half-reactions of of hydrogen are shown in the following table:
| Half-Reaction | Definition | Description |
|---|---|---|
| H2 = 2H+ + 2e- | Oxidation | Hydrogen molecule losess electrons; it is oxidized. |
| 2H+ + 2e- = H2 | Reduction | Hydrogen ion gains electrons; it is reduced. |
Chemical Reactivity
The reactivity of chemical elements is expressed relative to the oxidation reaction of hydrogen, which is assigned an electromotive force (emf) value of zero volts.- Half reactions (referred to in the context of electrochemistry as half cells with a greater tendency to give up electrons than hydrogen are have a positive potential (electropositive).
- Half cells with a large positive potential are good reducing agents (they readily donate electrons).
- Half cells with a lower tendency to give up electrons than hydrogen are have a negative potential (electronegative).
- Half cells with a large negative potential are good oxidizing agents (the readily accept electrons).
A example of a strong reducing agent and a strong oxidizing agent is shown in the following table.
| Half-Reaction | EMF (volts) | Comments |
|---|---|---|
| K -> K+ + e- | +2.93 | Potassium is a strong reducing agent. |
| H2 -> 2H+ + 2e- | 0.00 | |
| 2Br -> Br2 + 2e- | -1.07 | Bromine is a moderately strong oxidizing agent. |
| 2Cl -> Cl2 + 2e- | -1.36 | Chlorine is a strong oxidizing agent. |
| 2F -> F2 + 2e- | -2.85 | Flourine is a strong oxidizing agent and the most reactive of all elements. |
Elemental potassium and flourine will combine directly to form potassium flouride:
2K + F2 = 2KF
In this example, potassium provides an electron that is accepted by flourine. A more familiar example of an alkali metal/halogeon combination is common salt: NaCl. Because chlorine is more reactive than bromine, NaCl is formed when chlorine: reacts with sodium bromine, NaBr:
2NaBr + Cl2 = 2NaBr + Br2