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Electrochemistry
Electrons in Chemical Reactions
REDOX REACTIONS
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Examples:
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Car Battery
Electrolysis of water
Rusting of an old Chevette
Cellular respiration
What do all these things have in common?
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Electrochemistry is the study of how electrons are transferred in
chemical reactions
The chemical changes that occur when electrons are transferred
between reactants are known as reduction-oxidation (REDOX)
reactions
The # of e- gained by one entity = the # of e- lost by another
Example: 2Na (s) + Cl2(g)
2NaCl (s)
Writing Half- Reactions
 Redox reactions can be separated into an
oxidation reaction and a reduction reaction
which are called half-reactions because
they involve only one-half of the redox
reaction taking place
 The half-reaction must be balanced by
adding electrons to the side with the more
positive charge
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Elements have a charge of zero
The charge of an ion is written as part of the
symbol ! eg. charge on N3- is -3
Reduction Half-Reactions
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Electrons are GAINED in a reduction
reaction and are written on the
REACTANT side of the equation
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Reduction:
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Fe 3+ + 3 e-
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Cl2
+ 2 e-
Fe (s)
2Cl-
REDUCTION is a GAIN in
electrons(RIG)
Oxidation Half-Reactions
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Electrons LOST in oxidation are placed on
the PRODUCT side of the equation
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Oxidation:
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Mg
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2F 1-
Mg 2+
+ 2e-
F2 (g) + 2e-
OXIDATION is a LOSS in electrons(OIL)
Remembering Half-Reactions
Oxidation = OIL
Reduction = RIG
So ... OIL RIG
RED (RIG)
O
+
OX (OIL)
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Now try these:
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Sulfur to sulphide ions
Sodium ion to sodium metal
Nickel to nickel ions (2 answers)
Reactivity of Metals and Their Ions
The more stable a metal atom is, the more reactive it is as an ion.
The more stable a metal ion is, the more reactive it is as a metal.
If you create a list that organizes metal ions from most reactive to
least reactive, you end up creating a list that organizes metal atoms
from least reactive to most reactive as shown in the table below. The
resulting table is a table of reduction half-reactions based on the
reactivity of atoms or ions, or an activity series.
Activity Series for Metals and Metal Ions
(page 80 in text, page 5 in data booklet)
Using the Activity Series
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The ranking of the reactivity of metal ions is
shown on the left-hand side of the table, with
the most reactive metal ions placed higher
than less reactive metal ions.
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Which is more reactive – tin ions or iron ions?
The right side of the table lists metals in order
of reactivity. Metals lower in the series are
more reactive than the metals higher in the
series.
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Which is more reactive – copper or zinc?
Using the Activity Series
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The table is most useful for determining if a
redox reaction is:
Spontaneous – a chemical reaction that occurs
without the addition of external energy
or
 Nonspontaneous – a chemical reaction that does
not occur without the addition of external energy
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How do we tell if a reaction is spontaneous or
nonspontaneous …
Using the Activity Series
Spontaneous Redox Reaction Example
Zinc – copper reaction in real life !
Potato
Clock
Two potatoes are pierced by zinc and
copper electrodes. A small clock runs
on the voltage produced.
Voltaic Cells
Applying our understanding of
Redox Reactions
Voltaic Cells
also called GALVANIC cells.
electrochemical cells that are used to convert
chemical energy into
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electrical energy
energy produced by SPONTANEOUS redox reactions
In a spontaneous redox reaction, electrons are
transferred from the substance oxidized to the substance
reduced
If reactants are arranged in a certain way, these electrons
can be made to move through a wire
The reactants must be separated and yet in contact with
each other so that the reaction will occur s l o w l y
They can be separated by a salt bridge or a porous cup
Salt Bridge containing
unreactive sodium sulfate
Porous Cup
Unglazed
porcelain
cup
Allows for ion transfer just like
a salt bridge does!
Parts of a Voltaic Cell
Half-cell - one part of a voltaic cell in which either
oxidation or reduction occurs – contains a solid
(electrode) and a solution (electrolyte)
Electrodes - solid conductors that connect the
electrolyte solution to the external circuit
Anode: oxidation occurs here, electrons are produced
negative (-) terminal
Cathode: reduction occurs here, electrons are used
positive (+) terminal
Electrolytes – solutions that conduct electricity – must
contain ions!
Salt bridge or porous cup - allows the passage of
electrons without contamination of the two half-cells
How does the electricity flow?
How does a voltaic cell work?
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The electrons move from the anode to the
cathode through a wire. This is the external
circuit.
Another circuit keeps the charges moving –
the internal circuit is the flow of ions in the
solutions through the salt bridge or porous
cup.
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Cations (positively-charged ions) move to the
cathode
Anions (negatively-charged ions) move to the
anode
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Voltaic cells consist of
 Anode: Zn(s)  Zn2+(aq) + 2e
(loses mass)
 Cathode: Cu2+(aq) + 2e-  Cu(s)
(gains mass)
As oxidation occurs, Zn is converted to Zn2+ and 2e-. The
electrons flow towards the cathode where they are used in
the reduction reaction.
Cell Notation
Identify the anode and cathode!
Batteries
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A BATTERY is a self-contained VOLTAIC CELL.
1. Primary Batteries
 converts stored chemical potential energy into electrical energy when the two
half cells within the battery are connected by an external circuit.
 usually the container is the anode and the graphite center is the cathode.
 the two are separated by a thick moist paste, which is the electrolyte.
 are not rechargeable
2. Secondary Batteries
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are rechargeable by
passing a direct
current through it.
Hg and Pb are often
used in these
batteries, are an
environmental
concern.
cathode grill is filled
with lead (IV) oxide,
the anode grill is filled
with spongy lead.
Electrolytic Cells
The Electrolytic Plant, which is the size of four
football fields, consumes the same amount of
power as a city of 250,000 people.
Copper is electroplated onto a
strip of silver.
Note the differences….