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
2H2O2 → 2H2O + O2
Oxidation Number of O: -1 → -2 0
O is both oxidised and reduced during this reaction
- 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
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