Oxidation Numbers - Introduction (Edexcel International AS Chemistry): Revision Note

Oxidation Numbers - Introduction

• 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

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

Roman Numerals 

• Transition metals are characterised by having variable oxidation numbers.

• Oxidation numbers can be used in the names of compounds to indicate which oxidation number a particular element in the compound is in

• Where the element has a variable oxidation number, the number is written afterwards in Roman numerals.

• This is called the STOCK NOTATION (after the German inorganic chemist Alfred Stock), but is not as widely used for non-metals, so SO₂ is often referred to as sulfur dioxide rather than sulfur(IV) oxide

• For example, iron can be both +2 and +3 so Roman numerals are used to distinguish between them

  • Fe²⁺ in FeO can be written as iron(II) oxide

  • Fe³⁺ in Fe₂O₃ can be written as iron(III) oxide

• 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

Oxidation Numbers - Calculations

Molecules or Compounds

• In molecules or compounds, the sum of the oxidation numbers on the atoms is zero

Oxidation Number in Molecules or Compounds Table

• Because CO₂ is a neutral molecule, the sum of the oxidation states 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

Hydrogen in metal hydrides, H^-

• An example of a metal hydride is sodium hydrogen, NaH which is a neutral compound 

• The hydrogen atom is present as a hydride ion 

  • This has the symbol H^- and therefore the oxidation state -1

• We can also think of this as a sum

  • Since group 1 metals always have an oxidation state of +1 in their compounds, it follows that the hydrogen must have an oxidation state of -1 (+1 -1 = 0)

Oxygen in peroxides, O^-

• Peroxides include hydrogen peroxide, H₂O₂, which again is a neutral compound 

• The sum of the oxidation states of the hydrogen and oxygen must be 0 

• Since each hydrogen has an oxidation state of +1, each oxygen must have an oxidation state of -1 to accommodate the neutral charge of the compound

Ions

• The oxidation number of elements within an ion can be calculated using the rules shown in the Oxidation Number Rules Table (above)