A-Level Chemistry AQA Notes

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Oxidation Numbers

Oxidation number is a number representing the number of electrons lost or gained by an atom in a compound.
Oxidation is a loss of electrons during a reaction or an increase in oxidation number.
Reduction is a gain of electrons during a reaction or a decrease in oxidation number
The rules for assigning oxidation numbers:
- An uncombined element has an oxidation number of 0
- A simple ion (of a single element) has an oxidation number equal to the charge on the ion
- The sum of oxidation numbers of the elements in a compound is equal to the overall charge of the compound
- The charge on a complex ion, e.g. NH4+, is equal to the sum of the oxidation numbers
- The most electronegative element in a compound always has a negative oxidation number
- Oxygen is always -2 except in peroxides where its -1
- Hydrogen is +1 except in metal hydrides where its -1
Oxidation numbers are represented by Roman numerals when naming compounds

Redox Reactions

Redox reactions involve both oxidation and reduction
Oxidising agents cause oxidation of other species, and so are themselves reduced
Reducing agents cause reduction of other species, and so are themselves oxidised
Disproportionation reactions involve both oxidation and reduction of the same element

e.g. The decomposition of hydrogen peroxide
2H2O2 → 2H2O + O2
Oxidation Number of O: -1 → -2 0
O is both oxidised and reduced during this reaction

Redox Equations

A redox reaction can be constructed from two half-equations; one representing an oxidation process, and the other a reduction process
To construct a full equation from half equations:
- Balance the electrons
- Combine the equations
- Cancel the electrons
- Check the charge balance and stoichiometry

e.g. Mg -> Mg²⁺ + 2e^-

Cu²⁺ + 2e^--> Cu

1. Mg -> Mg²⁺ + 2e^-

Cu²⁺ + 2e^--> Cu

2. Mg + Cu²⁺ + 2e^-->Mg²⁺+2e^-+Cu

3. Mg + Cu²⁺ -> Mg²⁺+Cu

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