1. Introduction to Oxidation-Reduction Reactions
Electron Transfer Reactions

2. Types of Chemical Reactions
There are four types of chemical reactions:
Acid/Base
Precipitation/Solubility
Complex Formation/Complex Dissociation
Oxidation/Reduction
Any chemical reaction consists of one (or more) of these basic categories.

3. Oxidation/Reduction Reactions
Acid/Base reactions
involve a donation /acceptance of protons
Precipitation/ Solubility reactions
involve a donation/ acceptance of negative charge
what is being donated and accepted in a redox reaction?

4. Oxidation/Reduction Reactions
Electrons!
Consider the reaction taking place in a disposable battery: Zn + 3MnO2  Mn3O4 + 2ZnO How can you tell that electrons are being donated and accepted? Which species is donating electron( s) and which is accepting electron (s)?

5. REDOX REACTIONS
Redox reactions are characterized by ELECTRON TRANSFER between an electron donor and electron acceptor.

6. REDOX REACTIONS
Transfer leads to—
increase in oxidation number of some element = OXIDATION
2. decrease in oxidation number of some element = REDUCTION

7. Electron Transfer in Redox Reactions
Oxidation
Loss of electrons
Gain in oxygen
Reduction
Gain of electrons
Loss of oxygen
“LEO the lion goes Ger”

8. Mg S Mg 2+ S2- Example The reaction of a metal and non-metal
All the electrons must be accounted for! Mg S
Mg 2+
S2- + → +

9. Oxidation-Reduction
Oxidation means an increase in oxidation state - lose electrons.
Reduction means a decrease in oxidation state - gain electrons.
The substance that is oxidized is called the reducing agent.
The substance that is reduced is called the oxidizing agent.

10. Assigning Oxidation States
An Oxidation-reduction reaction involves the transfer of electrons.
You should memorize these rules

11. Rules for Oxidation States
The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred.
The oxidation state of elements in their standard states is zero.
Example: Na, Be, K, Pb, H2, O2, P4 = 0

12. Assigning Oxidation States
Oxidation state for monatomic ions are the same as their charge.
Example: Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2
Oxygen is assigned an oxidation state of -2 in its covalent compounds except as a peroxide.

13. Rules for Oxidation States
The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1.
Group IA metals are +1, IIA metals are +2 and fluorine is always –1.
The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.

14. Practice in Oxidation States
Assign the oxidation states to each element in the following.
K2SO4
NO3-
H2SO4
Fe2O3
Fe3O4

15. Identify the Oxidizing agent Reducing agent Substance oxidized
Substance reduced
On the worksheet

16. Types of Oxidation-Reduction Reactions
Combination Reaction
A + B C +4 -2
S + O SO2
Decomposition Reaction
C A + B +1 +5 -2 +1 -1
2KClO KCl + 3O2

17. Types of Oxidation-Reduction Reactions
Displacement Reaction a.k.a Single Replacement
A + BC AC + B +1 +2
Sr + 2H2O Sr(OH)2 + H2
Hydrogen Displacement +4 +2
TiCl4 + 2Mg Ti + 2MgCl2
Metal Displacement -1 -1
Cl2 + 2KBr KCl + Br2
Halogen Displacement

18. The Activity Series for Metals
Hydrogen Displacement Reaction
M + BC AC + B
M is metal
BC is acid or H2O
B is H2
Ca + 2H2O Ca(OH)2 + H2

19. Copper Demonstration Copper Pennies reacting with nitric acid.
Can you figure out the equation?

20. Types of Oxidation-Reduction Reactions
Disproportionation Reaction
Element is simultaneously oxidized and reduced. +1 -1
Cl2 + 2OH ClO- + Cl- + H2O
Chlorine Chemistry

21. Classify the following reactions.
Ca2+ + CO CaCO3
NH3 + H NH4+
Zn + 2HCl ZnCl2 + H2
Ca + F CaF2

22. Half-Reactions
All redox reactions can be thought of as happening in two halves.
One produces electrons - Oxidation half.
The other requires electrons - Reduction half.

23. Half-Reactions Write the half reactions for the following.
Na + Cl2 → Na+ + Cl-
SO3- + H+ + MnO4- → SO4- + H2O + Mn+2

24. Balancing Redox Equations
In aqueous solutions the key is the number of electrons produced must be the same as those required.
For reactions in acidic solution an 8 step procedure.

25. Balancing Redox Equations
Write separate half reactions
For each half reaction balance all reactants except H and O
Balance O using H2O

26. Acidic Solution
Balance H using H+
Balance charge using e-

27. Acidic Solution Multiply equations to make electrons equal
Add equations and cancel identical species
Check that charges and elements are balanced.

28. Practice Balance the following reactions:
Sn 2+ (aq) + 2Fe 3+ → Sn 4+ (aq) + 2Fe 2+
MnO4- (aq) + C2O4-2 (aq) → Mn2+ (aq) + CO2 (g)

29. Practice
The following reactions occur in aqueous solution. Balance them
Cr(OH)3 + OCl- + OH- ® CrO4-2 + Cl- + H2O
MnO4- + Fe+2 ® Mn+2 + Fe+3

30. Now for a tough one
Fe(CN)6-4 + MnO4- ® Mn+2 + Fe+3 + CO2 + NO3-

31. Basic Solution
Do everything you would with acid, but add one more step.
Add enough OH- to both sides to neutralize the H+
CrI3 + Cl2 ® CrO4- + IO4- + Cl-
CN MnO4- → CNO- + MnO2

32. Redox Titrations Same as any other titration.
the permanganate ion is used often because it is its own indicator. MnO4- is purple, Mn+2 is colorless. When reaction solution remains clear, MnO4- is gone.
Chromate ion is also useful, but color change, orangish yellow to green, is harder to detect.