Oxidation Number (Edexcel A Level Chemistry): Revision Note
Defining Oxidation Number
There are three definitions of oxidation and reduction used in different branches of chemistry
Oxidation and reduction can be used to describe any of the following processes
Definitions and Examples of Oxidation & Reduction
Use the acronym "Oil Rig" to help you remember the definitions of oxidation and reduction
Oxidation Number
The oxidation number of an atom is the charge that would exist on an individual atom if the bonding were completely ionic
It is like the electronic ‘status’ of an element
Oxidation numbers are used to
Tell if oxidation or reduction has taken place
Work out what has been oxidised and/or reduced
Construct half equations and balance redox equations
Oxidation Numbers of Simple Ions
Worked Example
What are the oxidation numbers of the elements in the following species?
a) C b) Fe3+ c) Fe2+
d) O2- e) He f) Al3+
Answers:
a) 0 b) +3 c) +2
d) -2 e) 0 f) +3
So, in simple ions, the oxidation numbers of the atom is the charge on the ion:
Na+, K+, H+ all have an oxidation number of +1
Mg2+, Ca2+, Pb2+ all have an oxidation number of +2
Cl–, Br–, I– all have an oxidation number of -1
O2-, S2- all have an oxidation number of -2
Roman numerals are also used to indicate the oxidation state of ions
Iron(II) sulfate implies that the iron is in a +2 oxidation state, i.e. Fe2+
Iron(III) sulfate implies that the iron is in a +3 oxidation state, i.e. Fe3+
More complicated examples include other atoms / ions as part of the formula
Potassium manganate(VII) implies that the manganese is in a +7 oxidation
Potassium manganate(VII) contains the potassium ion K+ and the manganate ion MnO4–
Since the oxygen in the manganate ion is in the -2 oxidation state, there is a total of -8 from the oxygen
The manganate ion has an overall -1 charge, which means that the manganese ion must be in the +7 oxidation state
Calculating Oxidation Numbers
Oxidation Number Rules
A few simple rules help guide you through the process of determining the oxidation number of any element
Remember, you are determining the oxidation number of a single atom
The oxidation number (ox.no.) refers to a single atom in a compound
Oxidation Number Rules Table
Molecules or Compounds In molecules or compounds, the sum of the oxidation numbers on the atoms is zero
Oxidation Number in Molecules or Compounds
Because CO2 is a neutral molecule, the sum of the oxidation numbers must be zero
For this, one element must have a positive oxidation number and the other must be negative
How do you determine which is the positive one?
The more electronegative species will have the negative value
Electronegativity increases across a period and decreases down a group
O is further to the right than C in the periodic table so it has the negative value
How do you determine the value of an element’s oxidation number?
From its position in the periodic table and / or
The other element(s) present in the formula
The oxidation numbers of all other atoms in their compounds can vary
By following the oxidation number rules, the oxidation number of any atom in a compound or ion can be deduced
The position of an element in the periodic table can act as a guide to the oxidation number
Oxidation Numbers & the Periodic Table
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