•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Oxidation-Reduction Reactions
Chapter 20
Oxidation-Reduction Reactions
The Meaning of Oxidation and Reduction
– What happens to magnesium and oxygen when they react to form
magnesium oxide(MgO?)
• Mg+2 and O-2
• Oxidation-Reduction Reactions
Why does iron corrode easily while gold doesn’t?
– What is transferred in chemical reactions?
If H2O is converted to H2O2, has H2O been oxidized?
– What does oxidize imply?
Oxidation is the gain of oxygen or the loss of electrons.
– List some examples of when this process occurs.
• Combustion reactions, bleaching, rusting
Oxidation-Reduction Reactions
Combustion reactions
– CH4(g) + 2O2(g) ----> CO2(g) + 2H2O(g)
– Products of incomplete combustion.
• CO
carbon monoxide
• CH2O2 formic acid
• CH2O formaldehyde
• CH4O methanol
– Oxidation-Reduction Reactions
Reduction
– The opposite of oxidation.
– 2Fe2O3(s) + 3C(s) ----> 4Fe(s) + 3CO2(g)
– This is the process for reducing iron ore to iron which was used 2500
years ago.
– Do you notice anything peculiar about this process?
• Oxidation and reduction take place at the same time.
Oxidation-Reduction Reactions
Oxidation-reduction reactions occur simultaneously. No oxidation can occur
without reduction and the opposite is true.
– These reactions are called redox reactions.
What is the electron structure of oxygen?
– 1s2, 2s2, 2p2
– How many electrons does oxygen want/need?
– When combined with a metal, the metal gives up electrons to the
oxygen. When oxygen is removed, the metal gains the electrons.
Oxidation-Reduction Reactions
Electron Transfer in Redox Reactions
The current definition of oxidation is the complete or partial Loss of
Electrons or gain of Oxygen. The complete or partial Gain of Electrons or the
loss of oxygen is Reduction.
2Fe(s) + 6H+(aq) --->2Fe3+(aq) + 3H2(g)
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
What has been oxidized and what reduced?
The substance that loses electrons is the reducing agent. The substance that
gains electrons is the...
Oxidation-Reduction Reactions
Another example
– 2H2(g) + O2(g) ---> 2H2O(l)
– Identify the reducing agent.
• hydrogen because it was oxidized. LEO
– Identify the oxidizing agent.
• Oxygen is the oxidizing agent because it was reduced. GRE
• A water molecule is polar. The oxygen pulls the electrons close
to it’s nucleus. Because the hydrogen has partially lost the
electrons it is/ has been oxidized.
Oxidation-Reduction Reactions
– 2C2H2(g) + 5O2(g) ---> 4CO2(g) + 2H2O(g)
– Identify the reducing agent.
• Which was oxidized(LEO?)
– C2H2(g)
– Identify the oxidizing agent.
• Which was reduced(GER?)
– O2(g)
– Oxidation-Reduction Reactions
Corrosion
The oxidation of iron by oxygen. The presence of water accelerates the
process.
– 2Fe(s) + O2(g) + 2H2O(l) ---> 2Fe(OH)2(s)
– 4Fe(OH)2(s) + O2(g) + 2H2O(l) ---> 4Fe(OH) 3(s)
Why does the presence of salts and acids increase the rate of corrosion?
Oxidation-Reduction Reactions
Corrosion
How can you reduce the rate of corrosion?
Determining Oxidation Numbers
Oxidation numbers are positive or negative numbers assigned to an atom.
Most of the time we will use the periodic chart.
Ca is +2, Cl is -1, Fe is +3
H is +1. With a metal it is -1.
When in its elemental form the oxidation number is 0. K, Cl2
Determining Oxidation Numbers
Compounds must be zero.
Polyatomic ions must equal the charge of the ion. SO4 2- S= +6, O = -2 x 4 = -8
Practice: S2O3 O2 Al2(SO4)3
Determining Oxidation Numbers in Reactions
+1 +5 -2
0
+2 +5 -2
0
2AgNO3(aq) + Cu(s)  Cu(NO3)2 + 2Ag(s)
What has been oxidized?
What has been reduced?