11. Redox - Oxidation

Oxidation is traditionally seen as what happens when something reacts with the oxygen in the air.

The rusting of the iron nails that we put in different conditions was due to the oxidation of the iron.

Fe(s) + O₂(g) --> Fe₂O₃(s)

Oxygen makes up 21% of the Earth's atmosphere, so it is very easy for things to react with it:

Activity: Cut an apple in half. Treat one side with an 'anti-oxidant' such as citric acid or bicarbonate solution. Place the two halves in clear view of the iPad camera where they won't be disturbed for at least 30 minutes and start the timelapse camera.

Demonstration:

Hot copper metal will react with sulfur to make copper sulfide.

Cu(s) + S(s) --> CuS(s)

As this is an an oxidation reaction without oxygen we need a new definition for oxidation.

An oxidation reaction is now defined as one where a species loses electrons.

Oxidation: the loss of electrons

Oxidation

Is

Loss

Cu(s) + S(s) --> Cu²⁺S^2-(s)

In the reaction, the copper metal loses two electrons to become the Cu²⁺ ion, this is oxidation.

The sulfur gains two electrons to become the S^2- ion.

The Cu²⁺ and S^2- attract each other to become the compound copper sulfide, (CuS).

Activity: calculating oxidation states

• When using oxidation states, we effectively imagine everything to be an ion - the oxidation state is the charge it would have if it was an ion.

RULES for assigning oxidation states:

a) On simple ions, the oxidation state is the charge on the ion

e.g. Cu²⁺ (Cu +2); Cl^- (Cl -1); Al₂O₃ (Al +3, O -2)

b) In elements, the oxidation state is always zero

e.g. Cl₂ (Cl 0)

c) The total of all the oxidation states must always equal the overall charge on the species.

d) H is nearly always +1 and oxygen -2

e.g. CH4 (C -4, H +1); CO₂ (C +4, O -2); H₂O (H +1, O -2)

1) Calculate the oxidation state of the stated element in each of the following species: