AQA AS Chemistry

Revision Notes

1.9.3 Redox Equations

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, eg:

Fe(III) = iron with ox. no. +3

Worked Example

Writing overall redox reactions

Manganate(VII) ions (MnO4) react with Fe2+ ions in the presence of acid to (H+) 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 ox. no.

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

  • Step 2: Deduce the ox.no. changes

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

  • Step 3: Balance the ox.no. 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, eg:

Cu + H2O → CuO + H2

(Cu has gained an oxygen and is oxidised)

  • Oxidation is also the loss of a hydrogen, eg:

2NH3 + 3Br2 → N2 + 6HBr

(N has lost a 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 ox. no., 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

(O has been reduced)

  • Reduction is also the gain of a 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

(change in ox. no. of Cu is -2 thus Cu is reduced)

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, eg:

Cu2++ Mg → Mg2+ + Cu

(Cu has been reduced and Mg has been oxidised)

Worked Example

Oxidation and reduction

In each of the following equation, 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 ox. no. has increased by 1

Reduced: Na+ as the ox. no. has decreased by 1

Answer 2:
Oxidised: Mg as the ox. no. has increased by 2

Reduced: Fe2+ as the ox. no. 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 ox. no.

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

  • Step 2: Deduce the ox. no. changes

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

  • Step 3: Balance the ox. no. 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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