Loss of electrons
Oxidation
Gain of electrons
Reduction
1.
2.
3.
4.
Free elements = 0
Simple ions = charge
F always -1
O nearly always -2,
except when bonded to
F, or in a peroxide
5. H nearly always +1,
except when bonded to
a metal.
6. Sum of the oxidation
#’s in a neutral cmpd =
0
7. Sum of the oxidation
#’s = charge for a
polyatomic ion
8. For a covalent cmpd,
the more
electronegative
element is assigned the
negative oxidation #
and vice versa
Rules for Assigning Oxidation
Numbers
Involve the transfer of
electrons
Redox Reactions
Occur simultaneously. #
of electrons lost = # of
electrons gained.
Oxidation & Reduction
LOSS of ELECTRONS =
OXIDATION.
GAIN OF ELECTRONS =
REDUCTION
LEO goes GER
Single Replacement
Synthesis
Decomposition
Redox Reactions
Single Replacement: element +
compound  new element + new
compound
Synthesis: 1 product
Decomposition: 1 reactant
Recall Formats
Have to assign oxidation numbers to
everything in the equation. The
ones that change are redox.
Identifying Redox
Reactions
Shows either the oxidation or
reduction reaction, including the
electrons gained or lost.
Half-Reaction
1) Conservation of matter
2) Conservation of charge
Half-Reactions must obey
Total charge on LHS of equation
=
Total charge on RHS of equation
What does conservation
of charge mean?
Electron term is on the product
side.
Oxidation Half-Reation
Oxidation Half-Reaction
H2 
+
2H
+
2e
Electron term is on the reactant
side.
Reduction Half-Reation
Reduction Half-Reaction
2+
Zn
+
2e
 Zn
1) S  S2- + 2e2) Cl2  Cl- + e3) Mn7+ + 3e-  Mn4+
4) Ca2+  Ca + 2eWhich half-reaction shows
conservation of mass &
conservation of charge?
H2, N2, O2, F2, Cl2, Br2, I2
Pesky diatomics
H2, N2, O2, F2, Cl2, Br2, I2
You have to keep the subscript in
the half-reaction!
What’s the problem with
the Pesky Diatomics?
H2, N2, O2, F2, Cl2, Br2, I2
You have to keep the subscript in
the half-reaction!
+
2H
2e
H2 
+
O2 + 4e-  2O2-
a)
b)
c)
d)
Fe2+  Fe + 2eFe + 2e-  Fe2+
Fe  Fe2+ + 2e2+
Fe + 2e  Fe
Which half-reaction is
correct for reduction?
a)
b)
c)
d)
Ca2+ + 2e-  Ca
Ca2+  Ca + 2eCa + 2e-  Ca2+
2+
Ca  Ca + 2e
Which half-reaction is
correct for oxidation?
a)
b)
c)
d)
O2
O2
O2
O2
+ 2e-  O2 2O2- + 2e+ 4e-  2O22 2O + 4e
Which half-reaction is correct
for the reduction of O2?
Helps something else get
oxidized by itself being
reduced.
Oxidizing Agent
Helps something else get
reduced by itself being
oxidized.
Reducing Agent
4
3
2
1
0
-1
-2
-3
-4
2) And if
you’re lucky
you strike oil &
it shoots up
1) You dig down
with an oil rig
1. Assign all oxidation #’s.
2. Use oil rig to figure out what’s
oxidized & what’s reduced.
3. Write the half-reactions.
4. Add half-reactions, multiplying to
adjust electrons if necessary.
5. Transfer coefficients & balance
remaining elements.
Steps in Balancing Redox Eqs.
Use Table J! If the stand-alone
element is above the similar
element in Table J, the reaction
will occur.
How do you predict if a given
redox reaction will occur?
Compare Li with Al – both are
metals. Li > Al so reaction
occurs.
Li + AlCl3  ?
Compare I2 with Cl – both are
nonmetals. I2 < Cl2 so reaction
DOES NOT occur.
I2 + NaCl  ?
 Galvanic or Voltaic (NYS–electrochemical)
Spontaneous rxn  Electrical energy

Electrolytic
Electrical energy  Nonspontaneous rxn
2 kinds of cells in
electrochemistry?
Uses a spontaneous reaction to
produce a flow of electrons
(electricity). Exothermic.
Galvanic/Voltaic/Electrochemic
al (NYS) 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.
Galvanic Cell
Use a spontaneous single
replacement redox reaction to
produce a flow of electrons.
Electrons flow from oxidized
substance to reduced
substance.
Galvanic/Voltaic/Electrochemical
- 2 half-cells, each with a
container, an aqueous solution,
& an electrode connected by a
- Wire and a
- Salt Bridge
Parts of a Galvanic Cell
Surface at which oxidation or
reduction half-reaction occurs.
Electrode
The anode is the metal that’s
higher in table J. It’s more
easily oxidized.
Anode/Cathode in Galvanic Cell
Anode to Cathode.
Direction of electron flow (wire)
Galvanic Cell – remember
opposites attract.
Direction of electron flow (wire)
Anode to Cathode.
Direction of positive ion flow
(salt bridge)
Electrode where oxidation
occurs
Anode
Electrode where reduction
occurs
Cathode
AN Ox ate a RED CAT
Works for ALL cells
Memory Aid
Allows for migration of ions between
half-cells.
Necessary to maintain electrical
neutrality.
Reaction will not proceed without salt
bridge.
Salt Bridge
Electrode where electrons
originate. (higher in table J)
Negative Electrode / Galvanic Cell
Electrode that attracts
electrons. (lower in table J)
Positive Electrode / Galvanic Cell
Electron flow 
wire
Al =
anode
-
Al+3
Positive ion flow 
Salt bridge

Pb+2 & NO3-1
& NO3
Draw galvanic cell with Al & Pb
-1
Pb =
cathode
Oxidation: Al  Al3+ + 3eMetal electrode –
Loses mass
Aluminum ions in
solution –
concentration 
Reduction: Pb2+ + 2e-  Pb
Lead ions in solution –
Concentration 
Metal electrode
– gains mass
Write the half-reactions for the
previous cell
Overall Rxn
2(Al  Al+3 + 3e-)
3(Pb+2 + 2e-  Pb)
_____________________________
2Al + 3Pb+2  2Al+3 + 3Pb
Concentration of Zn2+ ions.
Generally [ ] means
concentration of whatever is
inside [ ].
What does [Zn2+] mean?
Galvanic Cells provide a voltage.
They are a type of battery.
Uses of Galvanic Cells
1. Plate metals on other metals
2. Prepare column 1 and column
17 elements from compounds
3. Recharge batteries
Uses of ELECTROLYTIC CELLS
Uses a flow of electrons
(electricity) to force a
nonspontaneous reaction to
occur. Endothermic.
Electrolytic Cell
1. Its got a power supply – a
battery!
2. You don’t have two separate
containers.
How do you identify an electrolytic
cell in a picture or diagram?
1) Fused salt cell
2) Plating Cell
What are the 2 kinds of electrolytic
cells you are responsible for?
An Ox ate a Red Cat.
Anode is still oxidation.
Cathode is still reduction
How do you label the anode &
cathode in an electrolytic cell?
Use the battery. The
electrode attached to the
positive pole of the battery is
positive & vice versa.
How do you label the positive &
negative electrodes in an electrolytic
cell?
A POX on electrolytic cells!
Anode
Oxidation
Positive
What’s the memory trick for
remembering the polarity of
electrolytic cells?
Putting a very thin layer of one
metal on top of another metal!
What is electroplating?
+
Battery
+
-
Loses
mass
3) Element
to be plated.
Cu+2 and SO4-2
1) See battery so
it’s electrolytic!
2) Trace the + & - signs
back to electrodes.
4) Anode = Oxidation
Cu  Cu+2 + 2e5) Cathode = Object to be
Plated = Reduction
Cu+2 + 2e-  Cu
Gains mass
Notice: Net reaction is just
moving Cu around.
Electrolytic cell used to prepare
group 1 & group 17 elements
from their compounds (salts).
What is a fused salt cell?
MOLTEN
What does fused mean?
+
-
Anode
Cathode
Na+
Anode = Oxidation.
2Cl-  Cl2 + 2e-
Cathode = Reduction.
Na+ + e-  Na
ClMolten NaCl
Fused Salt Cell: Opposites
Attract!