Oxidation and Reduction


Oxidation can be considered in a number of ways. In a reaction with oxygen, a substance that gains oxygen has been oxidised and a substance that loses oxygen is reduced.

In electron transfer reactions, oxidation is the loss of electrons by a species and reduction is the gain of electrons (OIL RIG). 

Another method for identifying oxidation is to assign oxidation numbers to species in a reaction. Oxidation occurs in this context  when an element has increased its oxidation number on going from reactant to product. Conversely, reduction occurs when an element has decreased in oxidation number.

The oxidation number of an atom is a measure of the number electrons lost or gained by an atom in a compound compared to the uncombined atom. These numbers are assigned using the following rules:

           Species                                                 Oxidation number                                                   example

  1. Uncombined element                                           0                                                      C, Na, hyrogen, fluorine
  2. Simple ion                                                charge on ion                                                      Na+ =+1, Cl-  = -1
  3. Compound                                      total oxidation number = 0                                      methane, water
  4. Polyatomic ion                           total oxidation number = charge on ion                      sulfate = -2
  5. Combined oxygen                                                  -2                                                                    H2O, CaO
  6. Combined hydrogen                                             +1                                                                    H2O, CH4
  7. Combined fluorine                                                  -1                                                                  NaF, AlF3

Exceptions: Oxygen becomes +1 when bonded to fluorine. -1 in peroxides. When bonded to a metal H becomes -1 (hydride ion)

Not discussed in the video:  Disproportionation occurs when an atom simultaneously undergoes both oxidation and reduction in a reaction. E.g. when chlorine (O.N = 0) is added to water, Hydrochloric acid (HCl, O.N. = -1) and  chloric (I) acid (HOCl, O.N. = +1) are formed.

Using half equations to balance REDOX equations

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