AQA A Level Chemistry复习笔记1.9.1 Oxidation & Reduction

Oxidation & Reduction


  • 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 States

  • The oxidation state 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 states 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 States of Simple Ions



Worked Example

What are the oxidation states of the elements in the following species?a) C                b)  Fe3+                       c)  Fe2+d) O2-             e)  He                          f)  Al3+


a) 0     b) +3    c) +2    d) -2    e) 0    f) +3

  • So, in simple ions, the oxidation stateof the atom is the charge on the ion:
    • Na+, K+, H+ all have an oxidation state of +1
    • Mg2+, Ca2+, Pb2+ all have an oxidation state of +2
    • Cl–, Br–, I– all have an oxidation state of -1
    • O2-, S2- all have an oxidation state of -2


Exam Tip

Oxidation state and oxidation number are often used interchangeably, though IUPAC does not distinguish between the two terms.Oxidation states are represented by Roman numerals according to IUPAC

Oxidising & Reducing Agents

Oxidising agent

  • An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons
  • An oxidising agent itself gets reduced – it gains electrons
  • Therefore, the oxidation state of the oxidising agent decreases



Example of an oxidising agent in a chemical reaction

Reducing agent

  • A reducing agent is a substance that reduces another atom or ion by causing it to gain electrons
  • A reducing agent itself gets oxidised – it loses/donates electrons
  • Therefore, the oxidation state of the reducing agent increases




Example of a reducing agent in a chemical reaction

  • For a reaction to be a redox reaction, there must be both an oxidising and reducing agent present
  • Some substances can act both as oxidising and reducing agents - look at the two roles of H2O2 in the previous examples
  • The role they take is dependent on what they are reacting with and the reaction conditions

Worked Example

Oxidising & reducing agentsFour reactions are shown. In which reaction is the species in bold acting as an oxidising agent?

  1. Cr2O72-+ 8H+ + 3SO32- →  2Cr3+ + 4H2O + 3SO42-
  2. Mg + Fe2+ →  Mg2+ + Fe
  3. Cl2 + 2Br- →  2Cl- + Br2
  4. Fe2O3+ 3CO →  2Fe + 3CO2


The correct option is 2

    • Oxidising agents are substances that oxidise other species, gain electrons and are themselves reduced.
    • Write down the oxidation numbers of each species in the reaction




    • In equation 2, Fe2+ oxidises Mg(0) to Mg2+(+2) and is itself reduced from Fe2+(+2) to Fe(0)


Roman numerals

  • Roman numerals are used to show the oxidation states of transition metals which can have more than one oxidation state
  • Iron can be both +2 and +3 so Roman numerals are used to distinguish between them
    • Fe2+ in FeO is written as iron(II) oxide
    • Fe3+ in Fe2O3 is written as iron(III) oxide


Worked Example

Systematic names of compoundsGive the full systematic names of the following compounds:

  1. FeCl2
  2. HClO4
  3. NO2
  4. Mg(NO3)2
  5. K2SO4


Answer 1: Iron(II) chloride: the oxidation state of 2 Cl atoms is -2 and FeCl2 has no overall charge so the oxidation state of Fe is +2

Answer 2: Chloric(VII) acid: the oxidation state of H is +1, 4 O atoms is -8 and HClO4 has no overall charge so the oxidation state of Cl is +7

Answer 3: Nitrogen(IV) oxide: the oxidation state of 2 O atoms is -4 and NO2 has no overall charge so the oxidation state of N is +4

Answer 4: Magnesium nitrate: this is a salt of the common acid, so it is named without including the oxidation state of the non-metal

Answer 5: Potassium sulfate: this is a salt of the common acid, so it is named without including the oxidation state of the non-metal