1. 20.1 Oxidation/Reduction (REDOX)
Chapter 20 - Oxidation & Reduction Reactions
20.1 Oxidation/Reduction (REDOX)

2. Chemistry
Today we are learning to:- 1. Understand two basic definitions of oxidation and reduction
2. Recognise from a balanced chemical equation where oxidation and reduction is taking place

3. Oxidation and Reduction
Why name the process oxidation?
Originally oxidation was seen as a chemical reaction where oxygen was added to an element or compound
2Mg + O2  2MgO
Magnesium and hydrogen are oxidized
2H2 + O2  2H2O

4. Oxidation and Reduction
Why name the process reduction?
Originally, reduction was seen as a chemical reaction where oxygen was removed from a compound. The compound is made simpler/smaller/reduced
Iron oxide
FeO + CO  Fe +CO2
Iron and hydrogen are reduced
2H2O2  2H2O + O2
Hydrogen peroxide

5. Oxidation and Reduction
Oxidation  gain of oxygen
Reduction  loss of oxygen

6. Mg O  MgO Oxidation and Reduction
Today we look at how electrons are transferred in oxidation/reduction reactions. Look at the previous reactions again.
Mg is a group 2 metal. It has 2 valence electrons.
Oxygen has 6 valence electrons
2Mg + O2  2MgO
2 electrons lost
2 electrons gained +2 Mg O -2 
MgO
When something is oxidized it loses electrons.
Similarly, when something is reduced it gains electrons.
Loss of Electrons  Oxidation
Gain of Electrons  Reduction

7. Oxidation and Reduction
LEO GER

8. Oxidation and Reduction
Oxidation and reduction occur in any reaction where electrons are lost or gained.
They always occur together
So they are called REDOX reactions
Mg + Cl2  MgCl2
Oxidized
Reduced

9. Oxidation Numbers
Number of electrons lost or gained by an atom called oxidation number.
Oxidation number can be positive, negative or neutral.
Use the periodic table in the Reference tables to find common oxidation numbers
Use the list of oxidation number rules to find common oxidation numbers
What are the oxidation numbers of carbon and oxygen in carbon monoxide?
What are the oxidation numbers of carbon and oxygen in carbon dioxide?
C =+2, O=-2
C =+4, O=-2
FeO + CO  Fe +CO2
What are the oxidation numbers of iron and oxygen in iron oxide
What is the oxidation number of elemental iron
Fe=+2, O=-2
Fe=0

10. Oxidation Numbers
Ex1: Determine the oxidation number of each element in K2Cr2O4
K=+1 from periodic table or rule 2
O=-2 from periodic table or rule 5
K2Cr2O4
K 2×+1=+2
Cr 2×? = ?
O 4×-2=-8
From rule 6 the sum of oxidation numbers = 0 for this particular species so:
2(+1) + 2Cr + 4(-2) = 0
2Cr = +6
Cr = + 3

11. Oxidation Numbers
Ex1: Determine the oxidation number of each element in Ba(NO3)2
Ba=+2 from periodic table or rule 2
O=-2 from periodic table or rule 5
Ba(NO3)2
Ba 1×+2=+2
N 2×? = ?
O 6×-2=-12
From rule 6 the sum of oxidation numbers = 0 for this particular species so:
1(+2) + 2N + 6(-2) = 0
2N = +10
N = + 5
Questions 1-10 in review book

12. REDOX Reactions
A change in oxidation number for any type of atom in a reaction means it is a REDOX reaction
Assign oxidation numbers to all the atoms in a reaction
If an oxidation number changes the reaction is REDOX
No change in oxidation number means it is not REDOX +1 -1 +2 -1
Zn + 2HCl  ZnCl2 + H2
Zinc is oxidized as it has lost 2 electrons
Hydrogen is reduced as each has gained 1 electron

13. REDOX Reactions
A change in oxidation number for any type of atom in a reaction means it is a REDOX reaction
If you recognize a double replacement reaction, it is not REDOX +4 -2 +1 -1 +2 -1 +1 -2
MnO2 + 4HCl  MnCl2 + 2H2O +Cl2
Mn has gone from Mn+4 to Mn+2  gained electrons  reduced
Some Cl have gone from Cl-1 to Cl0 in Cl2  lost electrons  oxidized

14. Oxidizing Agents and Reducing Agents
A substance that causes another to be reduced is called a reducing agent.
This gets oxidized
A substance that causes another to be oxidized is called an oxidizing agent.
This gets reduced
Oxidizing agent as is oxidizes HCl to Cl2
MnO2 + 4HCl  MnCl2 + 2H2O +Cl2
Reducing agent as is reduces MnO2 to MnCl2

15. END OF SHOW