Transcript Chapter 19: Oxidation and Reduction

Chapter 19: Oxidation and Reduction
Oxidation originally was thought to be the gain of oxygen.
Reduction originally was thought to be the loss of oxygen.
Oxidation is really the loss of electrons. LEO Lose Electrons Oxidized
Reduction is really the gain of electrons. GER Gain Electrons Reduced
{
2Mg
+ O2  2MgO Do the dot diagrams for Mg and O and show
how they gain/lose electrons.
Redox Half-Reactions
2Na
+
Cl2 
2NaCl
The oxidation half reaction is the half of the reaction that involves loss of
electrons.
2Na 2 Na+ + 2e-
The reduction half-reaction is the half of the reaction that involves gain of
electrons.
Cl2 + 2 e-  2ClThe net ionic equation would be the 2 added together:
2Na +Cl2 + 2e-  2 Na+ + 2e- + 2Cl-
Oxidation Numbers
e- Oxidation numbers are numbers assigned to keep track of electron gain
and loss.
e- See page 686 in your MH text for rules guiding assignment of oxidation
numbers.
1.
2.
3.
4.
5.
6.
7.
The oxidation number of an uncombined atom is 0.
The oxidation number of a monatomic ion= its charge.
The oxidation number of oxygen is -2 except -1 in peroxides.
The oxidation number of + ions in IA,IIA, and IIIA are the + of the GP #.
The oxidation number of hydrogen is +1 except where #4/#6 cause -1.
The sum of oxidation numbers in a neutral compound=0.
The sum of oxidation numbers in a polyatomic ion = its charge.
Practice Assigning Oxidation #s

Assign oxidation numbers to each element in the following
compounds or polyatomic ions. To really keep on your toes, see if you
can name them as well. There should be a number/sign above each
capital letter.

MgBr2
Fe2O3

PO43-
Cr2O72-

Cl2
Na
AlN
HClO2
SO3
CuSO4
Ca2+
H2O2
Check Your Answers

Assign oxidation numbers to each element in the following
compounds or polyatomic ions. To really keep on your toes, see if
you can name them as well.

+2 -1
MgBr2


+5 -2

PO43-
0
Cl2
+3 -2
Fe2O3
+6
Cr2O720
Na
+3 -3
AlN
-2
+1 +3 -2
HClO2
+6 -2
SO3
+2 +6 -2
CuSO4
+2
Ca2+
-1 +1
H2O2
Oxidation # Change Method for
Determining Substance
Oxidized/Reduced
1. Assign oxidation numbers.
2. Connect the substance with
numbers that increase.
3. Connect the substances with
numbers that decrease.
4. The substance that decreases is
reduced…the one that increases is
oxidized.
Oxidation # Change Method for
Determining Substance
Oxidized/Reduced
Oxidation Number Change Method

Use the oxidation number
change method to determine
what is oxidized and reduced:

2Na
+
Br2

2NaBr