Redox Equations (AQA AS Chemistry): Revision Note
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
Step 2: Deduce the oxidation state changes
Step 3: Balance the oxidation state changes
Step 4: Balance the charges
Step 5: Finally, balance the atoms
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 oxidation 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.
Na++ Cl- → NaCl
Mg + Fe2+ → Mg2+ + Fe
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
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 Cl- and ClO3- ions and water
Answer
Step 1: Write the unbalanced equation and identify the atoms that change in oxidation state:
Step 2: Deduce the oxidation state changes:
Step 3: Balance the oxidation state changes:
Step 4: Balance the charges
Step 5: Balance the atoms
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