Redox Reactions (Cambridge (CIE) AS Chemistry)
Revision Note
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, e.g.:
Fe(II) = iron with an oxidation number of +2
Fe(III) = iron with an oxidation number of +3
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 number:
Step 2: Deduce the oxidation number changes:
Step 3: Balance the oxidation number changes:
Step 4: Balance the charges:
Step 5: Balance the atoms:
Redox & Disproportionation Reactions
Oxidation
Oxidation is the gain of oxygen, e.g.:
Cu + H2O → CuO + H2
Cu has gained an oxygen and is oxidised
Oxidation is also the loss of a hydrogen, e.g.:
2NH3 + 3Br2 → N2 + 6HBr
N has lost a hydrogen and is oxidised
Oxidation is also the loss of electrons, e.g.:
Cu2+ + Mg → Mg2+ + Cu
Mg has lost two electrons and is oxidised
Oxidation causes an increase in oxidation number, e.g:
Cu2+ + Mg → Mg2+ + Cu
The oxidation number of Mg changes from 0 to +2, thus Mg is oxidised
Reduction
Reduction is the loss of oxygen, e.g.:
Cu+ H2O → 2CuO + H2
O has been reduced
Reduction is also the gain of a hydrogen, e.g.:
2NH3+ 3Br2 → N2 + 6HBr
Br has been reduced
Reduction is also the gain of electrons, e.g.:
Cu2+ + Mg → Mg2+ + Cu
Cu has been reduced
Reduction causes a decrease in oxidation number, e.g.:
Cu2+ + Mg → Mg2+ + Cu
The oxidation number of Cu changes from +2 to 0, thus Cu is reduced
OIL RIG acronym
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
For example:
Cu2++ Mg → Mg2+ + Cu
Cu has been reduced from +2 to 0
Mg has been oxidised from 0 to +2
Worked Example
Oxidation and reduction
In each of the following equations, state which reactant has been oxidised and which has been reduced.
2Na + Cl2 → 2NaCl
Mg + Fe2+ → Mg2+ + Fe
CO + Ag2O → 2Ag + CO2
Answer
Answer 1:
Oxidised: Na as the oxidation number has increased from 0 to +1
Reduced: Cl2 as the oxidation number has decreased from 0 to –1
Answer 2:
Oxidised: Mg as the oxidation number has increased by 2
Reduced: Fe2+ as the oxidation number 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 at the same time
Example disproportion reaction
Example of a disproportion reaction in which chlorine 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– ions, ClO3– ions and water.
Answer
Step 1: Write the unbalanced equation and identify the atoms that change in oxidation number:
Step 2: Deduce the oxidation number changes:
Step 3: Balance the oxidation number changes:
Step 4: Balance the charges:
Step 5: Balance the atoms:
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