IB Chemistry home > Syllabus 2016 > Redox processes > Oxidation state

Syllabus ref: 9.1

The terms oxidation number and oxidation state are often used interchangeably to indicate the apparent charge of an atom within a species.

The IUPAC does differentiate between the two terms, representing oxidation numbers usign Roman numerals and oxidation states with numbers preceded by a sign.

Oxidation state

The oxidation state this is the apparent valency of an atom within a compound.

This can be determined by treating all atoms as if they are ionic, to allow the apparent number of electrons gained or lost to be assessed.

The redox rules

Examples

  • The oxidation state of iron in iron(II) sulfate is +2
  • The oxidation state of phosphorus in phosphorus(V) oxide is +5
  • The oxidation state of chromium in potassium dichromate(VI) is +6

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Calculating the oxidation state

There are some elements that nearly always have the same oxidation state. These can be used to calculate the oxidation states of the atoms to which they are bonded.

Hydrogen, for example has an oxidation state of -1 when bonded to a metal (more electropositive element) and +1 when bonded to a more electronegative element (non-metal). Oxygen is nearly always -2 (the exception is when it is in the form of the peroxide ion, O-O2-, it has an oxidation state of -1).

Group 1 and 2 metals have an oxidation state of +1 and +2 respectively.

Example: Calculate the oxidation state of sulfur in sulfuric acid H2SO4

  • Hydrogen = +1 oxidation state
  • Oxygen = -2 oxidation state

(2 x H) + S + (4 x O) = 0

2 + S - 8 = 0

S = +6

Example: Calculate the oxidation state of nitrogen in calcium nitrate Ca(NO3)2

  • Calcium is in group 2 = +2 oxidation state
  • Oxygen = -2 oxidation state

Therefore:

(+2) + [(2 x N) + (6 x -2)] = 0

+2 + 2N -12 = 0

2N = 10

N = +5


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Oxidation state in ions

When you are dealing with an ion, the process for working out the oxidation state of the atoms within the ion is the same, with the important exception that the sum of the oxidation states must add up to the charge on the ion.

Example: Calculate the oxidation state of the chloride atom in the chlorate ion, ClO3-

Each of the oxygen atoms has an oxidation state of -2

Therefore as Cl + (3 x -2) = -1

Then Cl = -1 + 6 = +5


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