1. Oxidation & Reduction
Ch 20

2. Why do we call it “oxidation” ?
Intro to Redox
Why do we call it “oxidation” ?
In the past scientists believed there was a gain or loss of oxygen. This is FALSE!

3. Definition of Redox Reaction
Oxidation Reduction Reaction (Redox):
A reaction in which electrons are transferred from one substance to another.

4. Definition of Redox Reaction
Oxidation cannot occur without reduction
Definition of Oxidized:
1. Losing of electrons (LEO)
2. Increasing the ox. number, ie: 0 to +1
or -2 to -1
Definition of Reduced:
1. Gaining of electrons (GER)
2. Reducing the ox. number, ie: -1 to -2
or 1 to 0

5. Redox Rules to memorize!
The Statue of Liberty’s outer layer of copper has undergone redox

6. Oxidation Rule #1
Oxidation number of a neutral atom, molecule, or compound = zero i.e. atom: Mg, H2, Na i.e. All H2 O2 N2 Cl2 Br2 I2 F2 i.e. compounds: (the total ox#s adds up to 0 NH3 SiCl4 K3PO4 H2O

7. Rule #2
Oxidation number of an ion = charge of ion i.e. monotomic: Mg+2 , O–2, Al +3 , Br – i.e. Polyatomic: NH4+, CO3–2, SO4–2, NO3 –

8. Rule #3 The oxidation number for H is +1
Exception: when bonded to a less
electronegative element (metal),
then it is -1.
i.e. LiH, NaH, MgH2
i.e. HF, HCl +1 -1 +1 -1 +2 -1 +1 -1 +1 -1

9. Rule#4 Two exceptions: peroxides and fluorine in H2O2 with OF2
Oxygen’s oxidation number is -2, in a compound
Two exceptions: peroxides and fluorine
in H2O2
with OF2 +1
? = -1
where O is -1
2(+1)
+ 2( ?) = 0 +2 -1
where O is +2
? = 0
2(-1)
fluorine (more electronegative)

10. Rule #5
The most electronegative element fluorine ALWAYS has an oxidation number of -1 when it is bonded to another element.
i.e. HF

11. Question Time Assign oxidation numbers to each element Cl2 Cl – MgCl2
NaH
HCl
K3PO4
CaSO4 KF

12. Classwork Redox Rules Oxidation numbers of the nitrogen
N2O b. NO c. N2O4 d. N2O5 e. NO2- f. NO3-
2. Oxidation number of the sulfur atom:
H2S b. S c. H2SO d. S e. HS f. SO g. SO3
3. Indicate the oxidation number of phosphorus in each of the following compounds:
a. HPO3 b. H3PO2 c. H3PO3 d. H3PO4 e. H4P2O7 f. H5P3O10

14. Half Reactions

15. Example of a complete electron transfer reaction
2Mg + O2  2MgO
Mg atom transfers two electrons to each oxygen atom.
As a result Mg atoms become Mg2+ and oxygen atoms become O2-
Mg loses two electrons
O gains two electrons

16. Example of an electron transfer reaction
2Mg + O2  2MgO
Mg atom transfers two electrons to each oxygen atom.
As a result 2 Mg atoms become Mg2+ and two oxygen atoms become O2-
Mg loses two electrons
O gains two electrons

17. General examples of Oxidation ½ Reaction
When an electron is lost (An electron is found on the product side) there is oxidation
X0  X e –
When an electron is gained (An electron is found on the reactant side) there is reduction
e – + Y+1  Y0

18. Using the Rules Oxidation Number
If oxidation number goes up it is oxidized.
If oxidation number goes down it is reduced.

19. Two ways to remember oxidation/reduction (redox)
L E O goes G E R O I L R I G
X I D AT I O N O S E L E C T R O N S
X I D AT I O N A I N L E C T R O N S
E D U C T I O N S
E D U C T I O N
O S I N G S
A I N I N G

20. ½ Reaction Examples: start finish Ex1 Na0  Na+1 +1e – oxidized
Fill in the electrons, then tell if it is oxidation or reduction.
start finish
Ex1 Na0  Na+1
Ex Fe+3  Fe+2
Ex3 N  N+2
Ex4 Mg+2 Mg0
+1e –
oxidized
1e – +
reduced
+1e –
oxidized
2e – +
reduced
start homework

21. Using the Rules-Last Lesson for CP
Balance the reactions,
write the oxidation numbers for each atom.
Ex5 HCl + Zn  ZnCl2 + H2
Loss of electron oxidation (LEO) +1 -1 +2 -1 2
Gain of electron reduction (GER)

22. Using the Rules (last lesson this year for CP)
Balance the reactions, then write oxidation # for each atom.
Ex6 Mg N  Mg3N2
Ex7 H2S Cl  HCl + S
Ex8 Fe O  Fe2O3

23. Summary of Oxidation/Reduction
There are two definitions of oxidation and reduction:
Oxidation Reduction
1._______________ 1._______________
2._______________ 2._______________

24. Classwork with ½ reactions
Assign oxidation numbers for each element. Then, for the following balanced redox reactions answer the following questions:
1) AgNO3 + Cu  CuNO3 + Ag
Write all the oxidation numbers. Identify what charges change.
Write the ½ reactions on the lines.
_______________________ ________________________
(oxidation or reduction) (oxidation or reduction)
Add the electrons to each ½ reaction.
Identify each ½ reaction as oxidation or reduction.
AgNO3 + 1e –  Ag
Cu  CuNO3 + 1e –
Finish classwork, then start homework.

25. 2) Zn(s) + HCl(aq)  H2(g) + ZnCl2(aq)
Write all the oxidation numbers.
Write the ½ reactions.
__________________ ___________________
(oxidation or reduction)(oxidation or reduction)
Add the electrons to each ½ reaction.
Identify each ½ reaction as oxidation or reduction.

26. 3) SbCl5 + 2 KI  SbCl3 + I2 + 2 KCl
Write all the oxidation numbers.
Write the ½ reactions.
__________________ ___________________
(oxidation or reduction)(oxidation or reduction)
Add the electrons to each ½ reaction.
Identify each ½ reaction as oxidation or reduction.

27. H Balancing Redox Reactions using Half-Reaction method in Acidic Conditions
Assign oxidation numbers to each atom
Write the 2 half reactions
Balance all elements except O & H
Balance O by adding H2O
Balance H by adding H+ (Acidic solution)
Add OH- ions to each side of the equation to neutralize H+ ions. Cancel H2O molecules. (Basic solution)
Balance charge by adding e-
Balance e- by multiplying by a factor
Add the ½ reactions, cancel e-
Check that atoms and charges balance

28. Ex 1: MnO4 – + Fe+2 → Fe+3 + Mn+2

29. Cl2 → ClO – + Cl –

30. Cl2 + S2O32– → Cl – + SO42–

31. MnO4 – + SO32– → Mn SO42–