OxidationReduction
Topic 9
Review Book
Oxidation Numbers
• Oxidation is the loss of electrons; Reduction is the gain of
electrons
• Oxidation and reduction go together. Whenever a
substance loses electrons and another substance gains
electrons
• Oxidation Numbers are a system that we can use to keep
track of electron transfers
LEO
says
GER!
Loss of Electrons = Oxidation
Gain of Electrons = Reduction
Rules for Assigning Oxidation
Numbers
• Free elements are 0
• Ions have the charge indicated
• Metals : Group 1 = +1; Group 2= +2
• Hydrogen is +1 unless in binary compound with
metal, then it is -1
• Halogens in binary compounds are usually -1, in
ternary compounds move on and go back to do
the algebra
• Oxygen is -2 unless in peroxide when it is -1 or
with fluorine when it is +2
Practice Assigning
Oxidation Numbers
NO2
N2O5
HClO3
HNO3
Ca(NO3)2
KMnO4
Practice Assigning
Oxidation Numbers
NO2
N= +4, O = -2
N2O5
N = +5, O = -2
HClO3
H=+1, Cl=+5, O = -2
HNO3
H=+1, N = +5, O = -2
Ca(NO3)2
Ca=+2, N =+5, O= -2
KMnO4
K=+1, Mn=+7, O= -2
Practice Assigning
Oxidation Numbers
CaCl2
K2Cr2O7
KH
NaHCO3
H2O
MgO
Practice Assigning
Oxidation Numbers
CaCl2
K2Cr2O7
Ca = +2, Cl = -1
K=+1, Cr=+6, O=-2
KH
K = +1, H= -1
NaHCO3
Na= +1, H= +1, C=+4, O= -2
H2O
H= +1, O= -2
MgO
Mg = +2, O = -2
Using Oxidation Numbers
•
Careful examination of the oxidation
numbers of atoms in an equation allows
us to determine what is oxidized and
what is reduced in an oxidation-reduction
reaction
Using Oxidation Numbers
• An increase in the oxidation number indicates that
an atom has lost electrons and therefore oxidized.
• A decrease in the oxidation number indicates that
an atom has gained electrons and therefore
reduced
• Example
Mg +
ZnSO4  MgSO4 + Zn
Exercise
For each of the following reactions find the element
oxidized and the element reduced
Cl2
Mg
+
KBr

KCl +
Br2
+ HNO3  Mg(NO3)2 + NO2 + H2O
HNO3 +
I2
 HIO3 +
NO2
Exercise
For each of the following reactions find the element
oxidized and the element reduced
Cl2
+
KBr

KCl +
Br increases from –1 to 0 -- oxidized
Cl decreases from 0 to –1 -- Reduced
K remains unchanged at +1
Br2
Exercise
For each of the following reactions find the element oxidized
and the element reduced
Mg + HNO3  Mg(NO3)2 + NO2 + H2O
• Cu increases from 0 to +2. It is oxidized
• Only part of the N in nitric acid changes from +5
to +4. It is reduced
• The nitrogen that ends up in copper nitrate
remains unchanged
Exercise
For each of the following reactions find the element
oxidized and the element reduced
HNO3 +
•
•
•
I2
 HIO3 +
NO2
N is reduced from +5 to +4. It is reduced
I is increased from 0 to +5 It is oxidized
The hydrogen and oxygen remain unchanged.
Balancing Redox Reactions
There are several basic steps
1. Assign oxidation numbers to the species in the
reaction
2. Find the substance oxidized and the substance
reduced
3. Write half reactions for the oxidation and reduction
4. Balance the atoms that change in the half reaction
5. Determine the electrons transferred and balance the
electrons between the half reactions
6. Check your work. Make sure that both the atoms and
charges balance
Balancing Redox Equations 1
1.
Assign oxidation numbers
to the species in the
reaction
2.
Find the substance
oxidized and the substance
reduced
3.
Write half reactions for the
oxidation and reduction
4.
Balance the atoms that
change in the half reaction
5.
Determine the electrons
transferred and balance
the electrons between the
half reactions
6.
Check your work. Make
sure that both the atoms
and charges balance
*It may be helpful to balance the
equation before starting the
redox process
• Zn + HCl  ZnCl2 + H2
Balancing Redox Equations 2
1.
Assign oxidation numbers
to the species in the
reaction
2.
Find the substance oxidized
and the substance reduced
3.
Write half reactions for the
oxidation and reduction
4.
Balance the atoms that
change in the half reaction
5.
Determine the electrons
transferred and balance the
electrons between the half
reactions
6.
Check your work. Make
sure that both the atoms
and charges balance
MnO2 + HCl  MnCl2 + Cl2 + H2O
Balance, Assign, ID,
write ½ rxns for the following
• 1. Mg + O2  MgO
• 2. KClO3  KCl + O2
• 3. HCl + O2  H2O + Cl2
Balance the following ionic
equations
• ____Cr +6 + ____ S  ___ Cr +3 +____ S +4
• ____Sn2+ + ____Hg+ 2  ___Sn+4 + ___Hg +1
• _____Cu + _____Ag+1  ___Cu+2 + ___Ag