chapter 20 - oxidation & reduction reactions 20.1 oxidation/reduction (redox)
TRANSCRIPT
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