CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.6.2 Redox Reactions

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

Electrochemistry Worked Example - Writing overall redox reactions, downloadable AS & A Level Chemistry revision notes

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

Electrochemistry Worked example - Oxidation and reduction, downloadable AS & A Level Chemistry revision notes

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

Electrochemistry Worked example - Balancing disproportionation reactions, downloadable AS & A Level Chemistry revision notes

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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