1. Ch. 19 Oxidation and Reduction
REDOX

2. 3 ways of looking at oxidation and reduction
1. oxidation is a gain of oxygen atoms, reduction is a loss of oxygen atoms
2. oxidation is a loss of hydrogen atoms, reduction is a gain of hydrogen atoms
3. oxidation is a loss of electrons, reduction is a gain of electrons
Most fundamental explanation, what we will be dealing with the most

3. Oxidation Reduction Gains oxygen Lose Oxygen Lose Hydrogen
Gains hydrogen
Lose Electrons
Gains electrons
Oxygen
Hydrogen e-

4. Leo the Lion! LEO the lion says GER OIL RIG
Loss of Electrons is Oxidation
Gain of Electrons is Reduction
OIL RIG
Oxidation Is the Lost of electrons
Reduction Is the Gain of electrons

5. Examples Is the reactant oxidized or reduced? Pb  PbO3 SnO2  SnO
KClO3 KCl
C2H6O  C2H4O
C2H2  C2H6

6. Pertaining to LEO… Mg + S  MgS Mg + S  Mg2+ + S2-
Magnesium is oxidized
Said to be the reducing agent
Substance in the reaction that loses electrons
Sulfide sulfur atom is reduced
Said to be the oxidizing agent
Substance in the reaction that gains electrons

7. Oxidation Numbers
A count of the electrons transferred or shared in the formation or breaking of chemical bonds
You must assign each element in the reaction an oxidation number
Follow a set of rules…

8. Oxidation Number Rules
1. The total of the oxidation numbers of all the atoms in a neutral molecule, an isolated atom, or a formula unit is 0
2. In their compounds, the Group 1A metals all have an oxidation number of +1, and the Group 2A metals have an oxidation number of 2+

9. Rules Con’t
In its compounds, hydrogen has an oxidation number of +1 (except in metal hydrides such as NaH, where it is -1)
4. In its compounds, oxygen has an oxidation number of -2 (except in peroxides such as H2O2, where it is -1)
5. In their binary compounds with metals, Group7A elements have an oxidation number of -1. Group 6A elements have an oxidation number of -2, and Groups 5A elements have an oxidation number of -3.

10. Problems What is the oxidation number of each element? I2 Cr2O3 AlCl3
Na2SO4
CaH2

11. Identifying Redox Reactions
2 Al + Fe2O3  2 Fe + Al2O3
Al increases from 0 to +3, it is ______
Oxidized!
Fe decreases from +3 to 0, it is _______
Reduced!

12. Oxidizing and Reducing Agents
Now the confusing part…
CuO + H2  Cu + H2O
Cu goes from +2 to 0
Cu is reduced, therefore it is called an oxidizing agent because it causes some other substance to be oxidized
H goes from 0 to +1
H is oxidized, therefore it is called a reducing agent because it causes some other substance to be reduced.

13. Identifying Agents in an Equation
CuO + H2  Cu + H2O
Reduction: CuO is the oxidizing agent
Oxidation: H2 is the reducing agent

14. Electrochemical Cells
An apparatus that allows a redox reaction to occur by transferring electrons through an external connector.
Product favored reaction ---> voltaic or galvanic cell ----> electric current
Reactant favored reaction ---> electrolytic cell ---> electric current used to cause chemical change.
Batteries are voltaic cells

15. Basic Concepts of Electrochemical Cells
Anode
Cathode

16. CHEMICAL CHANGE ---> ELECTRIC CURRENT
With time, Cu plates out onto Zn metal strip, and Zn strip “disappears.”
Zn is oxidized and is the reducing agent Zn(s) ---> Zn2+(aq) + 2e-
Cu2+ is reduced and is the oxidizing agent Cu2+(aq) + 2e- ---> Cu(s)

17. GOOD AFTERNOON!  Please turn in your redox problems to the bin.
PRE-AP:
Please turn in your redox problems to the bin.
Grab the sheets from the corner.
Homework DUE: FRIDAY
REGULAR
Please grab a worksheet from the corner
Take out the homework from last night – on desk
REMINDERS:
Project Description and Outline DUE MAY 2nd.
Project DUE May 23rd (or by May 9th for +5 pts)

18. •Electrons travel thru external wire.
Zn --> Zn2+ + 2e-
Cu2+ + 2e- --> Cu
Oxidation
Anode
Negative
Reduction
Cathode
Positive
<--Anions
Cations-->
RED CAT
•Electrons travel thru external wire.
Salt bridge allows anions and cations to move between electrode compartments.

19. Standard reduction Potential
E° = standard potential of the cell.
Represents the voltage of the cell when the electrons create a current by passing through the wire.
E°= E (reduction) + E (oxidation)

20. Electrolysis of Water
Electrolysis of water is the breaking apart of water from H20 into its ions by running an electrical charge through it.
Example of the first use and type of a electrochemical cell

21. CHEMICAL CHANGE ---> ELECTRIC CURRENT
To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire.
This is accomplished in a GALVANIC or VOLTAIC cell.
A group of such cells is called a battery.

22. Terms Used for Voltaic Cells

23. Balancing Equations for Redox Reactions
Some redox reactions have equations that must be balanced by special techniques.
MnO Fe H > Mn Fe H2O
Mn = +7
Fe = +2
Mn = +2
Fe = +3

24. Balancing Equations
Consider the reduction of Ag+ ions with copper metal.
Cu + Ag give--> Cu Ag

25. Step 1: Divide the reaction into half-reactions, one for oxidation and the other for reduction.
Ox Cu ---> Cu2+
Red Ag+ ---> Ag
Step 2: Balance each element for mass. Already done in this case.
Step 3: Balance each half-reaction for charge by adding electrons.
Ox Cu ---> Cu e-
Red Ag+ + e- ---> Ag

26. Step 4: Multiply each half-reaction by a factor so that the reducing agent supplies as many electrons as the oxidizing agent requires.
Reducing agent Cu ---> Cu e-
Oxidizing agent 2 Ag e- ---> 2 Ag
Step 5: Add half-reactions to give the overall equation.
Cu Ag > Cu Ag
The equation is now balanced for both charge and mass.

27. Add H2O on O-deficient side and add H+ on other side for H-balance.
Balancing Equations
Balance the following in acid solution—
VO Zn ---> VO Zn2+
Step 1: Write the half-reactions
Ox Zn ---> Zn2+
Red VO > VO2+
Step 2: Balance each half-reaction for mass.
Red
2 H+ +
VO > VO H2O
Add H2O on O-deficient side and add H+ on other side for H-balance.

28. Balancing Equations Step 3: Balance half-reactions for charge.
Ox Zn ---> Zn e-
Red e H+ + VO > VO H2O
Step 4: Multiply by an appropriate factor.
Red 2e H VO > 2 VO H2O
Step 5: Add balanced half-reactions
Zn H VO > Zn VO H2O

29. Tips on Balancing Equations
Never add O2, O atoms, or O2- to balance oxygen.
Never add H2 or H atoms to balance hydrogen.
Be sure to write the correct charges on all the ions.
Check your work at the end to make sure mass and charge are balanced.
PRACTICE!