AQA A Level Chemistry

Revision Notes

1.9.3 Redox Equations

Balancing Redox Reactions

  • Balancing equations using redox principles is a useful skill and is best illustrated by following an example
  • It is important to follow a methodical step-by-step approach so that you don’t get lost:

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 state

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

Step 2: Deduce the oxidation state changes

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

Step 3: Balance the oxidation state 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: Finally, 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, eg:

Cu + H2O → CuO + H2

(Cu has gained an oxygen and is oxidised)

  • Oxidation is also the loss of hydrogen, eg:

2NH3 + 3Br2 → N2 + 6HBr

(NH3 has lost hydrogen and is oxidised)

  • Oxidation is also the loss of electrons, eg:

Cu2+ + Mg → Mg2+ + Cu

(Mg has lost two electrons and is oxidised)

  • Oxidation causes an increase in oxidaiton state, eg:

Cu2+ + Mg → Mg2+ + Cu

(change in ox. no. of Mg is +2 thus Mg is oxidised)

Reduction

  • Reduction is the loss of oxygen, eg:

Cu+ H2O → 2CuO + H2

(H2O has been reduced)

  • Reduction is also the gain of hydrogen, eg:

2NH3+ 3Br2 → N2 + 6HBr

(Br has been reduced)

  • Reduction is also the gain of electrons, eg:

Cu2+ + Mg → Mg2+ + Cu

(Cu has been reduced)

  • Reduction causes a decrease in oxidation number, eg:

Cu2+ + Mg → Mg2+ + Cu

(the change in oxidation state of Cu is -2 thus Cu is reduced)

Redox reactions

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

Cu2++ Mg → Mg2+ + Cu

(Cu has been reduced and Mg has been oxidised)

Worked Example

Oxidation and reduction

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

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

Answer

Answer 1:

    • Oxidised: Cl as the oxidation state has increased by 1
    • Reduced: Na+ as the oxidation state has decreased by 1

Answer 2:

    • Oxidised: Mg as the oxidation state has increased by 2
    • Reduced: Fe2+ as the oxidation state 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

 

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

Example of a disproportion reaction in which the same species (chlorine in this case) 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 Cland ClO3ions and water

Answer

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

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

Step 2: Deduce the oxidation state changes:

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

Step 3: Balance the oxidation state 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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