Oxidation Number Guidelines

Oxidation number(s) for…

  1. Any uncombined element is 0.
  2. Monoatomic ions is equal to its charge
  3. A neutral compound, sum of numbers must equal zero.

A polyatomic ion, sum of numbers equals the charge of ion.

  1. A Group 1 metal is always +1.

A Group 2 metal is always +2

A Group 17 nonmetal is usually –1 (F is always –1)

  1. H is usually +1 (except in metal hydrides, H is –1)
  2. O is usually –2 (except in peroxides, O is –1
  1. Other elements can usually be found by deduction.

Oxidation Number Method for balancing redox reactions

1.  Assign oxidation numbers

2.  Identify what has been oxidized and reduced

  1. Place brackets
  2. Show gain and loss of electrons on brackets
  3. Multiply gain and loss of electrons by any subscripts
  4. Multiply by factors to make the gain and loss of electrons equal
  5. Add coefficients based on the multipliers on the brackets

3.  Balance other atoms (except H and O)

4.  If necessary, balance charges

  1. In acidic solution, add H+ where necessary
  2. In basic solution, add OH- where necessary

5.  Balance O atoms by adding H2O where necessary

6.  The H should check out.

Half-Reaction Method (Ion-Electron Method) for balancing redox reactions

Acidic solutions

1.  Assign oxidation numbers to see what was oxidized and reduced.

2.  Write the half-reactions.

3.  For each half-reaction:

  1. Balance the “main” element M
  2. Balance the O, using H2O O
  3. Balance the H, using H+ H
  4. Balance the charge, using e- e

4.  If necessary, multiply each half-reactions by integer to equalize e-.

5.  Simplify (by cancelling out e-, sometimes H+, H2O), add half-reactions, and check.

Basic Solutions…do everything you do for acidic solutions and then…

6.  Add OH- ions to both side of the reaction

(equal to the amount of H+ on the one side; H+ and OH- = H2O)

7.  Simplify (by maybe cancelling out H2O), and check.