AQA A Level Chemistry

Revision Notes

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

Electrochemistry OIL RIG Diagram, downloadable AS & A Level Chemistry revision notes

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+

Answers:

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 agents

Four 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

Answer

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

Oxidising & Reducing Agents Worked Example equation, downloadable AS & A Level Chemistry revision notes

    • 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 compounds

Give the full systematic names of the following compounds:

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

Answer

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

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