Oxidation

1
Oxidation

• Loss of electrons

2
Reduction

• Gain of electrons

3
Rules for Assigning Oxidation Numbers

1. Free elements 0
2. Simple ions charge
3. F always -1
4. O nearly always -2, except when bonded to F, or
   in a peroxide
5. H nearly always 1, except when bonded to a metal.

1. Sum of the oxidation s in a neutral cmpd 0
2. Sum of the oxidation s charge for a
   polyatomic ion
3. For a covalent cmpd, the more electronegative
   element is assigned the negative oxidation and
   vice versa

4
Redox Reactions

• Involve the transfer of electrons

5
Oxidation Reduction

• Occur simultaneously. of electrons lost of
  electrons gained.

6
LEO goes GER

• LOSS of ELECTRONS OXIDATION.
• GAIN OF ELECTRONS REDUCTION

7
Redox Reactions

• Single Replacement
• Synthesis
• Decomposition

8
Recall Formats

• Single Replacement element compound ? new
  element new compound
• Synthesis 1 product
• Decomposition 1 reactant

9
Identifying Redox Reactions

• Have to assign oxidation numbers to everything in
  the equation. The ones that change are redox.

10
Half-Reaction

• Shows either the oxidation or reduction reaction,
  including the electrons gained or lost.

11
Half-Reactions must obey

1. Conservation of matter
2. Conservation of charge

12
What does conservation of charge mean?

• Total charge on LHS of equation
• Total charge on RHS of equation

13
Oxidation Half-Reation

• Electron term is on the product side.

14
H2 ? 2H 2e-

• Oxidation Half-Reaction

15
Reduction Half-Reation

• Electron term is on the reactant side.

16
Zn2 2e- ? Zn

• Reduction Half-Reaction

17
Which half-reaction shows conservation of mass
conservation of charge?

1. S ? S2- 2e-
2. Cl2 ? Cl- e-
3. Mn7 3e- ? Mn4
4. Ca2 ? Ca 2e-

18
Pesky diatomics

• H2, N2, O2, F2, Cl2, Br2, I2

19
Whats the problem with the Pesky Diatomics?

• H2, N2, O2, F2, Cl2, Br2, I2
• You have to keep the subscript in the
  half-reaction!

20
H2 ? 2H 2e-O2 4e- ? 2O2-

• H2, N2, O2, F2, Cl2, Br2, I2
• You have to keep the subscript in the
  half-reaction!

21
Which half-reaction is correct for reduction?

1. Fe2 ? Fe 2e-
2. Fe 2e- ? Fe2
3. Fe ? Fe2 2e-
4. Fe2 2e- ? Fe

22
Which half-reaction is correct for oxidation?

1. Ca2 2e- ? Ca
2. Ca2 ? Ca 2e-
3. Ca 2e- ? Ca2
4. Ca ? Ca2 2e-

23
Which half-reaction is correct for the reduction
of O2?

1. O2 2e- ? O2-
2. O2 ? 2O2- 2e-
3. O2 4e- ? 2O2-
4. O2 ? 2O2- 4e-

24
Oxidizing Agent
Helps something else get oxidized by itself
being reduced.

25
Reducing Agent
Helps something else get reduced by itself
being oxidized.

26
4 3 2 1 0 -1 -2 -3 -4
2) And if youre lucky you strike oil it
shoots up
OIL RIG
1) You dig down with an oil rig
27
Steps in Balancing Redox Eqs.

1. Assign all oxidation s.
2. Use oil rig to figure out whats oxidized
   whats reduced.
3. Write the half-reactions.
4. Add half-reactions, multiplying to adjust
   electrons if necessary.
5. Transfer coefficients balance remaining
   elements.

28
How do you predict if a given redox reaction will
occur?
Use Table J! If the stand-alone element is above
the similar element in Table J, the reaction will
occur.

29
Li AlCl3 ? ?

Compare Li with Al both are metals. Li gt Al so
reaction occurs.
30
I2 NaCl ? ?

Compare I2 with Cl both are nonmetals. I2 lt
Cl2 so reaction DOES NOT occur.
31
2 kinds of cells in electrochemistry?

• Galvanic or Voltaic (NYSelectrochemical)
• Spontaneous rxn ? Electrical energy
• Electrolytic
• Electrical energy ? Nonspontaneous rxn

32
Galvanic/Voltaic/Electrochemical (NYS) Cell

Uses a spontaneous reaction to produce a flow of
electrons (electricity). Exothermic.
33
Galvanic Cell
Redox reaction is arranged so the electrons are
forced to flow through a wire. When the
electrons travel through a wire, we can make them
do work, like light a bulb or ring a buzzer. So
the oxidation reduction reactions have to be
separated physically.

34
Galvanic/Voltaic/Electrochemical
Use a spontaneous single replacement redox
reaction to produce a flow of electrons. Electron
s flow from oxidized substance to reduced
substance.

35
Parts of a Galvanic Cell
- 2 half-cells, each with a container, an
aqueous solution, an electrode connected by a
- Wire and a - Salt Bridge

36
Electrode
Surface at which oxidation or reduction
half-reaction occurs.

37
Anode/Cathode in Galvanic Cell
The anode is the metal thats higher in table J.
Its more easily oxidized.

38
Direction of electron flow (wire)

Anode to Cathode.
39
Direction of electron flow (wire)

Galvanic Cell remember opposites attract.
40
Direction of positive ion flow(salt bridge)

Anode to Cathode.
41
Anode

Electrode where oxidation occurs
42
Cathode

Electrode where reduction occurs
43
Memory Aid

AN Ox ate a RED CAT Works for ALL cells
44
Salt Bridge

Allows for migration of ions between half-cells.
Necessary to maintain electrical
neutrality. Reaction will not proceed without
salt bridge.
45
Negative Electrode / Galvanic Cell

Electrode where electrons originate. (higher in
table J)
46
Positive Electrode / Galvanic Cell

Electrode that attracts electrons. (lower in
table J)
47
Electron flow ?
wire
Pb cathode
Positive ion flow ?
Al anode
Salt bridge
-
?
Pb2 NO3-1
Al3 NO3-1
Draw galvanic cell with Al Pb
48
Write the half-reactions for the previous cell

Oxidation Al ? Al3 3e- Reduction
Pb2 2e- ? Pb

Aluminum ions in solution concentration ?
Metal electrode Loses mass
Lead ions in solution Concentration ?
Metal electrode gains mass
49
Overall Rxn
2(Al ? Al3 3e-) 3(Pb2 2e- ? Pb)
_____________________________
2Al 3Pb2 ? 2Al3 3Pb
50
What does Zn2 mean?

Concentration of Zn2 ions. Generally
means concentration of whatever is inside .
51
Uses of Galvanic Cells

Galvanic Cells provide a voltage. They are a
type of battery.
52
Uses of ELECTROLYTIC CELLS

1. Plate metals on other metals
2. Prepare column 1 and column 17 elements from
   compounds
3. Recharge batteries

53
Electrolytic Cell

Uses a flow of electrons (electricity) to force a
nonspontaneous reaction to occur. Endothermic.
54
How do you identify an electrolytic cell in a
picture or diagram?

1. Its got a power supply a battery!
2. You dont have two separate containers.

55
What are the 2 kinds of electrolytic cells you
are responsible for?

1. Fused salt cell
2. Plating Cell

56
How do you label the anode cathode in an
electrolytic cell?

An Ox ate a Red Cat. Anode is still
oxidation. Cathode is still reduction
57
How do you label the positive negative
electrodes in an electrolytic cell?

Use the battery. The electrode attached to the
positive pole of the battery is positive vice
versa.
58
Whats the memory trick for remembering the
polarity of electrolytic cells?

A POX on electrolytic cells!
Oxidation
Anode
Positive
59
What is electroplating?

Putting a very thin layer of one metal on top of
another metal!

60
1) See battery so its electrolytic!

-
Battery
2) Trace the - signs back to electrodes.

-
4) Anode Oxidation Cu ? Cu2 2e-
Loses mass
5) Cathode Object to be Plated
Reduction Cu2 2e- ? Cu
3) Element to be plated.
Gains mass
Notice Net reaction is just moving Cu around.
Cu2 and SO4-2
61
What is a fused salt cell?

Electrolytic cell used to prepare group 1 group
17 elements from their compounds (salts).

62
What does fused mean?

MOLTEN

63
Fused Salt Cell Opposites Attract!

-
Anode Oxidation. 2Cl- ? Cl2 2e-

Anode
Cathode
Cathode Reduction. Na e- ? Na
Na

Cl-
Molten NaCl