Redox Equations (AQA A Level Chemistry): Revision Note
Exam code: 7405
Balancing Redox Reactions
The most powerful way to understand redox (reduction and oxidation) is by tracking the oxidation states of the elements involved
This method works for all redox reactions.
Oxidation States & Redox: The Core Rules
A redox reaction is one where the oxidation states of elements change.
Oxidation Is increase
If an element's oxidation state increases (becomes more positive), it has been oxidised
This corresponds to a loss of electrons.
Reduction Is decrease:
If an element's oxidation state decreases (becomes more negative), it has been reduced
This corresponds to a gain of electrons.
Examiner Tips and Tricks
A good mnemonic to remember this is OIL RIG:
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
Balancing Equations Using Oxidation States
Balancing complex redox equations is a key skill
The method involves balancing the oxidation state changes for the species being oxidised and reduced
It is best illustrated by following a step-by-step example:
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:
MnO4- (aq) + Fe2+ (aq) + H+ (aq) → Mn2+ (aq) + Fe3+ (aq) + H2O (l)
Step 2: Identify the oxidation states of the atoms:
Manganese:
+7 in MnO4- (aq)
+2 in Mn2+ (aq)
Iron:
+2 in Fe2+ (aq)
+3 in Fe3+ (aq)
Hydrogen:
+1 in H+ (aq) AND H2O (l)
Oxygen:
-2 in MnO4- (aq) AND H2O (l)
Step 3: Deduce the changes in oxidation state:
Manganese:
+7 to +2
A decrease in oxidation state of 5
Iron:
+2 to +3
An increase in oxidation state of 1
Hydrogen and oxygen:
No change
Step 4: Balance the oxidation state changes:
5Fe2+ (aq) are needed to balance the manganese change in oxidation state:
MnO4- (aq) + 5Fe2+ (aq) + H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + H2O (l)
Step 5: Balance the charges:
Ignoring H+ (aq):
Reactants:
1- from MnO4- (aq)
5 x 2+ from 5Fe2+ (aq)
Total = (1-) + (5 x 2+) = 9+
Products:
2+ from Mn2+ (aq)
5 x 3+ from 5Fe3+ (aq)
Total = (2+) + (5 x 3+) = 17+
Therefore, 8H+ (aq) are needed to balance the charge:
MnO4- (aq) + 5Fe2+ (aq) + 8H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + H2O (l)
Step 6: Balance the atoms:
1 Mn in MnO4- (aq) forms 1 Mn in Mn2+ (aq) - balanced
4 O in MnO4- (aq) forms 1O in H2O (l)
Forming 4H2O (l) balances the oxygen atoms
MnO4- (aq) + 5Fe2+ (aq) + 8H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
5Fe in 5Fe2+ (aq) forms 5Fe in 5Fe3+ (aq) - balanced
8H in 8H+ (aq) now forms 8H in 4H2O (l) - balanced
So, the final fully balanced redox equation is:
MnO4- (aq) + 5Fe2+ (aq) + 8H+ (aq) → Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l)
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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