Chemistry - redox reactions

Oxidation

• defined as the loss of electrons

Reduction

• defined as the gain of electrons

OIL RIG

• oxidation is loss

• Reduction is gain

Oxidation state changes

• Oxidation → oxidation state increases/becomes more positive

• Reduction → oxidation state decreases/becomes more negative

Rule for oxidation and reduction

• they always occur together

Oxidising agent

• a species that accepts electrons and is reduced, causing another species to be oxidised

Reducing agent

• A species that donates electrons and is oxidised, causing another species to be reduced.

Relationship between oxidation and memorisation

Oxidising agent → gains e⁻ → reduced

Reducing agent → loses e⁻ → oxidised

Oxidation numbers/states

• a theoretical number representing the charge an atom would have if electrons were assigned according to specific rules

Oxidation’s states example

Mg(s) + O₂(g) → MgO(s)

Before the reaction:

Mg = 0 O = 0

After the reaction:

Mg in MgO = +2 O in MgO = −2

So:

Mg: 0 → +2

Its oxidation state increased.

Therefore:

Mg was oxidised.

O: 0 → −2

Its oxidation state decreased.

Therefore:

O was reduced.

Oxidation pattern to learn

Oxidation = oxidation number increases

Reduction = oxidation number decreases

Oxidation state rules

• RULE 1 - free elements= 0 → eg: Fe(s)=0

• RULE 2 - monatomic ions = a single atom with a charge has an oxidation state equal to its charge - eg: Na+=+1

• RULE 3 - group 1 = +1 - Li,Na,K,etc - in compounds they have +1

• RULE 4 - group 2 = +2 - Mg,Ca,etc - in compunds - +2

• RULE 5 - oxygen is usually -2 - eg: H2O → o=-2

• RULE 6 = hydrogen is usually +1 - eg: H2O → H=+1

• RULE 7 - fluorine = -1

→ BIG RULE - for neutral compounds - all oxidation states must add to 0