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Redox Reactions (CIE AS Chemistry)

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Balancing Redox Reactions

  • Oxidation numbers can be used to balance chemical equations
  • Roman numerals between brackets are used to show the ox. no. of an atom that can have multiple oxidation states, e.g.:
    • Fe(II) = iron with an oxidation number of +2
    • Fe(III) = iron with an oxidation number of +3

Worked example

Writing overall redox reactions

Manganate(VII) ions, MnO4, react with Fe2+ ions in the presence of acid, H+, to form Mn2+ ions, Fe3+ ions and water.

Write the overall redox equation for this reaction.

Answer

  • Step 1: Write the unbalanced equation and identify the atoms which change in oxidation number:

Electrochemistry Step 1 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

  • Step 2: Deduce the oxidation number changes:

Electrochemistry Step 2 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

  • Step 3: Balance the oxidation number changes:

Electrochemistry Step 3 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

  • Step 4: Balance the charges:

Electrochemistry Step 4 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

  • Step 5: Balance the atoms:

Electrochemistry Step 5 Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

Redox & Disproportionation Reactions

Oxidation

  • Oxidation is the gain of oxygen, e.g.:
    • Cu + H2O → CuO + H2
    • Cu has gained an oxygen and is oxidised
  • Oxidation is also the loss of a hydrogen, e.g.:
    • 2NH3 + 3Br2 → N2 + 6HBr
    • N has lost a hydrogen and is oxidised
  • Oxidation is also the loss of electrons, e.g.:
    • Cu2+ + Mg → Mg2+ + Cu
    • Mg has lost two electrons and is oxidised
  • Oxidation causes an increase in oxidation number, e.g:
    • Cu2+ + Mg → Mg2+ + Cu
    • The oxidation number of Mg changes from 0 to +2, thus Mg is oxidised

Reduction

  • Reduction is the loss of oxygen, e.g.:
    • Cu+ H2O → 2CuO + H2
    • O has been reduced
  • Reduction is also the gain of a hydrogen, e.g.:
    • 2NH3+ 3Br2 → N2 + 6HBr
    • Br has been reduced
  • Reduction is also the gain of electrons, e.g.:
    • Cu2+ + Mg → Mg2+ + Cu
    • Cu has been reduced
  • Reduction causes a decrease in oxidation number, e.g.:
    • Cu2+ + Mg → Mg2+ + Cu
    • The oxidation number of Cu changes from +2 to 0, thus Cu is reduced

OIL RIG acronym

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

Redox reactions

  • Redox reactions are reactions in which oxidation and reduction take place together
    • While one species is oxidising, another is reducing in the same reaction
  • For example:

Cu2++ Mg → Mg2+ + Cu

  • Cu has been reduced from +2 to 0
  • Mg has been oxidised from 0 to +2

Worked example

Oxidation and reduction

In each of the following equations, state which reactant has been oxidised and which has been reduced.

  1. 2Na + Cl2 → 2NaCl
  2. Mg + Fe2+ → Mg2+ + Fe
  3. CO + Ag2O → 2Ag + CO2 

Answer

  • Answer 1:
    • Oxidised: Na as the oxidation number has increased from 0 to +1
    • Reduced: Cl2 as the oxidation number has decreased from 0 to –1
  • Answer 2:
    • Oxidised: Mg as the oxidation number has increased by 2
    • Reduced: Fe2+ as the oxidation number has decreased by 2
  • Answer 3:
    • Oxidised: C as it has gained oxygen
    • Reduced: Ag as it has lost oxygen

Disproportionation reactions

  • A disproportionation reaction is a reaction in which the same species is both oxidised and reduced at the same time

 

Example disproportion reaction

Electrochemistry Disproportion Reaction, downloadable AS & A Level Chemistry revision notes

Example of a disproportion reaction in which chlorine has been both oxidised and reduced

Worked example

Balancing disproportionation reactions

Balance the disproportionation reaction which takes place when chlorine is added to hot concentrated aqueous sodium hydroxide.

The products are Cl ions, ClO3 ions and water.

Answer

  • Step 1: Write the unbalanced equation and identify the atoms that change in oxidation number:

Electrochemistry Step 1 Balancing disproportionation reactions, downloadable AS & A Level Chemistry revision notes

  • Step 2: Deduce the oxidation number changes:

Electrochemistry Step 2 Balancing disproportionation reactions, downloadable AS & A Level Chemistry revision notes

  • Step 3: Balance the oxidation number changes:

Electrochemistry Step 3 Balancing disproportionation reactions, downloadable AS & A Level Chemistry revision notes

  • Step 4: Balance the charges:

Electrochemistry Step 4 Balancing disproportionation reactions, downloadable AS & A Level Chemistry revision notes

  • Step 5: Balance the atoms:

Electrochemistry Step 5 Balancing disproportionation reactions, downloadable AS & A Level Chemistry revision notes

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.