Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.
TOPIC 3: REDOX I
1. know what is meant by the term ‘oxidation number’
The oxidation number is also referred to as the ‘oxidation state.’
It is defined as the formal charge on an atom calculated on the assumption that it is in a
wholly ionic compound.
- Oxidation number is written with the charge before the number, whereas charge is
written with the number before the charge.
o E.g. The charge of Mg2+ is 2+.
But the oxidation number is +2.
Consider the two processes :
- Fe Fe2+ + 2e-
Fe Fe3+ + 3e-
- In both cases, iron atoms have been oxidised (i.e. they have lost electrons). In one
case, the atoms have been oxidised to Fe2+ ions; in the other case, they have been
oxidised to Fe3+ ions.
- This demonstrates that the phrase ‘iron atoms have been oxidised’ is ambiguous.
- Oxidation numbers are therefore assigned to atoms or ions to indicate the extent to
which they are oxidised or reduced.
2. be able to calculate the oxidation number of elements in compounds and ions
Rules for finding the oxidation number of an element:
Scenario Oxidation number Examples
Uncombined elements 0 Cl2 = 0
Fe = 0
Simple ions (one element) Charge on the ion Cl– = -1
Fe2+ = +2
Fe3+ = +3
Neutral compound Sum of the oxidation numbers of all NaCl: overall = 0
elements in compound = 0 MgF2: overall = 0
Complex ions (ions Sum of oxidation numbers of all NH4+: overall = +1
containing more than one elements in compound = charge on ion SO42–: overall = -2
atom)
Some elements (particularly transition metals) can assume a range of different oxidation
numbers in their compounds, but there are some general rules that hold true:
- Na, K = +1
- Mg, Ca = +2
- Al = +3
- H = +1 (except in metal hydrides e.g. NaH, MgH2)
- F = -1
- O = -2 except in peroxides e.g. H2O2, superoxides e.g. KO2, and fluorides)
- Cl = -1 (except in compounds with oxygen e.g. Cl2O7 and fluorine e.g. ClF)
, o In any species, the more electronegative atom is assigned a negative
oxidation number and the less electronegative atom is assigned a positive
oxidation number.
o This is why Cl has a positive oxidation number when in compounds with
oxygen or fluorine
Examples: give the oxidation number of the element in bold in each of these substances.
Example Oxidation number Comments
Fe Ox(Fe) = 0 Uncombined elements always have
an oxidation number of 0.
Fe3+ Ox(Fe3+) = +3 Simple ions have the same oxidation
number as their charge.
PbCl2 Ox(PbCl2) = Ox(Pb) + 2 x Ox(Cl) The sum of all oxidation numbers of
0 = Ox(Pb) + (2 x -1) elements in a compound is 0.
Ox(Pb) = +2
Fe2O3 Ox(Fe2O3) = 2 x Ox(Fe) + 3 x Ox(O) Oxidation numbers are quoted or
0 = 2 x Ox(Fe) + (3 x -2) atom or ion present.
Ox(Fe) = +3
NH4+ Ox(NH4+) = Ox(N) + 4 x Ox(H) The sum of all oxidation numbers of
+1 = Ox(N) + (4 x +1) elements in a complex ion is equal to
Ox(N) = -3 its charge.
3. understand oxidation and reduction in terms of electron transfer and changes in
oxidation number, applied to reactions of s- and p- block elements
Oxidation numbers are used by chemists to keep track of electrons within a compound.
- Changes in oxidation number during a reaction tell us that there is a transfer of
electrons, and they also tell us whether a species has been oxidised or reduced.
4. understand oxidation and reduction in terms of electron loss or electron gain
Historically, oxidation has been defined as:
- The gain of oxygen:
o E.g. 2 Cu (s) + O2 2 CuO (s)
o The copper has been oxidised.
- The loss of hydrogen:
o E.g. MnO2 (s) + 4 HCl (aq) MnCl2 (aq) + 2 H2O (l) + Cl2 (g)
o The hydrochloric acid here has been oxidised.
Conversely, reduction has been defined as:
- The loss of oxygen:
o E.g. Fe2O3 (s) + 3 CO (g) 2 Fe (l) + 3 CO2 (g)
o The iron (III) oxide has been reduced here.
- The gain of hydrogen:
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