Chapter 20 - Oxidation & Reduction Reactions

20.1 Oxidation/Reduction (REDOX)

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

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

2H2 + O2 2H2O

Magnesium and hydrogen are oxidized

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

FeO + CO Fe +CO2

2H2O2 2H2O + O2

Iron and hydrogen are reduced

Hydrogen peroxide

Iron oxide

Oxidation and Reduction

1. Oxidation gain of oxygen

2. Reduction loss of oxygen

Today we look at how electrons are transferred in oxidation/reduction reactions.

Look at the previous reactions again.

Oxidation and Reduction

2Mg + O2 2MgOMg is a group 2 metal. It has 2 valence electrons.Oxygen has 6 valence electrons

+2Mg O MgO-2

1. Loss of Electrons Oxidation

2. Gain of Electrons Reduction

When something is oxidized it loses electrons.

Similarly, when something is reduced it gains electrons.

2 electrons lost 2 electrons gained

Oxidation and Reduction

LEO GER

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

ReducedOxidized

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

FeO + CO Fe +CO2

What are the oxidation numbers of iron and oxygen in iron oxide

What is the oxidation number of elemental iron

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

Fe=+2, O=-2 Fe=0

Ex1: Determine the oxidation number of each element in K2Cr2O4

K=+1 from periodic table or rule 2O=-2 from periodic table or rule 5

K2Cr2O4

K 2×+1=+2 O 4×-2=-8

From rule 6 the sum of oxidation numbers = 0 for this particular species so:

Cr 2×? = ?

2(+1) + 2Cr + 4(-2) = 0

2Cr = +6Cr = + 3

Oxidation Numbers

Ex1: Determine the oxidation number of each element in Ba(NO3)2

Ba=+2 from periodic table or rule 2O=-2 from periodic table or rule 5

Ba(NO3)2

Ba 1×+2=+2 O 6×-2=-12

From rule 6 the sum of oxidation numbers = 0 for this particular species so:

N 2×? = ?

1(+2) + 2N + 6(-2) = 0

2N = +10N = + 5

Oxidation Numbers

Questions 1-10 in review book

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

Zn + 2HCl ZnCl2 + H2

0 +2+1-1 -1 0

Zinc is oxidized as it has lost 2 electronsHydrogen is reduced as each has gained 1 electron

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

MnO2 + 4HCl MnCl2 + 2H2O +Cl2

+4 -1-2 +1

Mn has gone from Mn+4 to Mn+2 gained electrons reducedSome Cl have gone from Cl-1 to Cl0 in Cl2 lost electrons oxidized

+2 -2+1 -1 0

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

MnO2 + 4HCl MnCl2 + 2H2O +Cl2

Oxidizing agent as is oxidizes HCl to Cl2

Reducing agent as is reduces MnO2 to MnCl2

END OF SHOW