1. Oxidation-Reduction
Topic 9
Review Book

2. 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

3. GER! LEO says Loss of Electrons = Oxidation
Gain of Electrons = Reduction

4. Rules for Assigning Oxidation Numbers
Free elements are 0
Ions have the charge indicated
Metals : Group 1 = +1; Group 2= +2
Halogens in binary compounds are usually -1, in ternary compounds move on and go back to do the algebra
Hydrogen is +1 unless in binary compound with metal, then it is -1
Oxygen is -2 unless in peroxide when it is -1 or with fluorine when it is +2
(FIMHHO)

5. Practice Assigning Oxidation Numbers
NO2
N2O5
HClO3
HNO3
Ca(NO3)2
KMnO4

6. 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

7. Practice Assigning Oxidation Numbers
CaCl2
K2Cr2O7 KH
NaHCO3
H2O
MgO

8. Practice Assigning Oxidation Numbers
CaCl2
Ca = +2, Cl = -1
K2Cr2O7
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

9. 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

10. 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

11. Exercise Cl2 + KBr  KCl + Br2 Mg + HNO3  Mg(NO3)2 + NO2 + H2O
For each of the following reactions find the element oxidized and the element reduced
Cl KBr  KCl Br2
Mg HNO3  Mg(NO3) NO2 + H2O
HNO I  HIO NO2

12. Exercise Cl2 + KBr  KCl + Br2 Br increases from –1 to 0 -- oxidized
For each of the following reactions find the element oxidized and the element reduced
Cl KBr  KCl Br2
Br increases from –1 to oxidized
Cl decreases from 0 to – Reduced
K remains unchanged at +1

13. Exercise Mg increases from 0 to +2. It is oxidized
For each of the following reactions find the element oxidized and the element reduced
Mg HNO3  Mg(NO3) NO2 + H2O
Mg increases from 0 to It is oxidized
Only N in nitric acid changes from +5 to +4. It is reduced

14. Exercise HNO3 + I2  HIO3 + NO2
For each of the following reactions find the element oxidized and the element reduced
HNO I  HIO 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.

15. Balancing Redox Reactions
There are several basic steps
Assign oxidation numbers to the species in the reaction
Find the substance oxidized and the substance reduced
Write half reactions for the oxidation and reduction
Balance the atoms that change in the half reaction
Determine the electrons transferred and balance the electrons between the half reactions
Check your work. Make sure that both the atoms and charges balance

16. Balancing Redox Equations 1
Assign oxidation numbers to the species in the reaction
Find the substance oxidized and the substance reduced
Write half reactions for the oxidation and reduction
Balance the atoms that change in the half reaction
Determine the electrons transferred and balance the electrons between the half reactions
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

17. Balancing Redox Equations 2
Assign oxidation numbers to the species in the reaction
Find the substance oxidized and the substance reduced
Write half reactions for the oxidation and reduction
Balance the atoms that change in the half reaction
Determine the electrons transferred and balance the electrons between the half reactions
Check your work. Make sure that both the atoms and charges balance
MnO2 + HCl  MnCl2 + Cl2 + H2O

18. Balance, Assign, ID, write ½ rxns for the following
1. Mg + O2  MgO
2. KClO3  KCl + O2
3. HCl + O2  H2O + Cl2

19. Balance the following ionic equations
____Cr ____ S  ___ Cr ____ S +4
____Sn ____Hg+ 2  ___Sn ___Hg +1
_____Cu + _____Ag+1  ___Cu+2 + ___Ag