Redox

Oxidation State

Number of electrons which must be added to a positive ion or the number of electrons which must be removed from a negative ion to give a neutral atom.

The oxidation number of an uncombined element is 0.

In its compounds the oxidation state of Oxygen = -2, except in peroxides where it is -1.

Hydrogen has an oxidation state of +1 except in metal hydrides where it is -1.

Group 1 metals and Group 2 metals have an oxidation state of +1 and +2 respectively

Group 7 elements are usually -1, unless combined with a more electronegative element.

Fluorine is always -1.

Algebraic sum of oxidation numbers in an element in a compound is 0.

Oxidation number increasing = oxidation

Oxidation number decreasing = reduction.

Oxidation Agent

Substances which oxidise another substance; electron acceptors; they themselves are reduced

Reducing Agent

Substances which reduce another substance so are electron donors; they themselves are oxidised.

Disproportionation

When the same element is oxidised and reduced during a reaction

Construction of Half Equations for Redox

1. Write down formulae of the reactants and products and the relevant oxidation numbers

2. Balance the element changing oxidation state

3. Add necessary number of electrons to bring about change in oxidation number

4. If necessary add water molecules to balance the Oxygens

5. If necessary add sufficient H+ ions to balance charges and the number of Hs, if in acidic solution

6. Multiply half equations so that both reactions involve same number of electrons being transferred.