1. rEDOX

2. Burning Magnesium in air

3. Oxidation is gain of oxygen
When magnesium reacts with oxygen in the air:
2Mg + O2 2MgO
It is easy to see that the magnesium has gained oxygen and an oxidation reaction has occurred.
The magnesium has been OXIDISED.

4. Reduction of lead(II)oxide

5. Reduction is loss of oxygen
When lead is extracted from lead (II) oxide , carbon is used to remove the oxygen
2PbO + C Pb + CO2
The oxygen has been removed from the lead (II) oxide
The lead (II) oxide has been REDUCED.

6. Lets look at this further…
What actually happens when the magnesium joins with oxygen?
The magnesium atom LOSES ELECTRONS to become a magnesium ion
We can write what happens like this:
Mg Mg2+ + 2e-

7. Oxidation is loss of electrons
So, when magnesium takes part in an oxidation reaction it loses electrons.
When magnesium reacts with chlorine it also loses electrons.
Mg + Cl2 MgCl2
This is an oxidation reaction too!

8. Reduction is gain of electrons
In the compound lead (II) oxide the lead is an ion, Pb2+
In order to form lead the Pb2+ ion has to GAIN 2 electrons
We can write this as follows:
Pb2+ + 2e- Pb

9. Redox
You may have noticed that when the magnesium loses its electrons they are gained by the oxygen
Oxidation (loss of electrons) and reduction (gain of electrons) happen simultaneously.
We call these REDOX reactions

10. OILRIG
Oxidation Is
Loss
Reduction
Gain
(of electrons)

11. Oxidation Numbers When magnesium lost electrons it got more POSITIVE
Mg Mg2+ + 2e- (0 to +2)
When lead gained electrons it became more NEGATIVE
Pb2+ + 2e- Pb (+2 to 0)

12. Oxidation Numbers
For a reaction to be a redox reaction, different reactants have to be oxidised or reduced.
To identify these reactants, OXIDATION NUMBERS are assigned to the reactants and products
These are also called OXIDATION STATES

13. Assigning oxidation numbers Rule 1
The oxidation number of an element is 0
Zn + CuSO4 ZnSO4 + Cu
The oxidation numbers of Zn and Cu in this reaction are 0

14. Rule 2
The oxidation number of a monatomic ion equals the charge on the ion
Mg + Cl2 MgCl2
In this example the oxidation number of Mg2+ and Cl- are +2 and -1 respectively

15. Rule 3
The oxidation number of oxygen is -2 (except in certain compounds called peroxides)
MgO Fe2O3 Na2O
In each case the O2- ion is formed and the oxidation number is +2

16. Rule 4
The oxidation number of hydrogen is +1, except in metal hydrides when it is -1
H2O HCl
In both cases hydrogen forms H+ ions and has an oxidation number of +1
In NaH is has an oxidation number of -1

17. Rule 5
The oxidation number of fluorine is always -1 and the oxidation number of the other halogens -1 (except for a few compounds)
NaCl LiBr CaF2
In each case the halide ion has a charge of -1 (e.g. Cl-) and the oxidation number is -1

18. Rule 6 The oxidation number of the atoms in a compound is zero
The sum of the oxidation numbers of the atoms in a polyatomic ion equals the charge in the ion

19. Rule 6 Examples
e.g. in the hydroxide ion (OH-) the oxidation number of oxygen is -2 and the oxidation number of hydrogen is +1
-2 + (+1) = -1
The overall charge on the hydroxide ion is -1
See C.I pg 195

20. Practice 1. What is the oxidation number of oxygen in O2?
2. What is the oxidation number of sodium in NaCl?
3. What is the oxidation number of iron in Fe2O3?
4. What is the oxidation number of chromium in CrO3?
5. What is the oxidation number of gallium in Ga(OH)3?
6. What is the oxidation number of phosphorous in PO43-
7. What is the oxidation number of sulphur in H2SO4?

21. Recognising oxidation and reduction in reactions
Look at the equation
Write out the oxidation numbers of the atoms involved
When the oxidation number of an atom increases (gets more positive) oxidation has occurred
When the oxidation number of an atom decreases (gets more negative) reduction has occurred.

22. Example
H2 + Cl2 2HCl
Hydrogen and chlorine both have oxidation numbers of o in H2 and Cl2 as they are elements
Hydrogen forms the H+ ion so has an oxidation number of +1
Chlorine forms the Cl- ion and has an oxidation number of -1
The oxidation number of hydrogen has changed from 0 to +1 so hydrogen has been oxidised
The oxidation number of chlorine has changed from 0 to -1 so chlorine has been reduced

23. Now try these:
In the reactions below, which element is being oxidised and which reduced?
1. 2Ca + O2 2CaO
NaBr + Cl2 NaCl + Br2
2Al + Fe2O3 2Fe + Al2O3
Pg 197 Q 1-8 for HL

24. Oxidising and Reducing Agents
A substance which donates electrons easily and causes another substance to be reduced is called a REDUCING AGENT
A substance which accepts electrons easily and helps another substance to become oxidised is called an OXIDISING AGENT
Think of some examples for each!