AQA A Level Chemistry复习笔记1.9.1 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

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

   

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

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

   

• • 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

   

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