Transcript Electrochemistry

Electrochemistry
Cells and Batteries
Galvanic Cells
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Electrochemistry-- the study of the processes involved
in converting chemical energy to electrical energy, and
vice versa.
Two concepts form the basis:
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Redox reactions involve the transfer of electrons from one
reactant to another.
An electric current is a flow of electrons in a circuit.
Galvanic cells
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Zinc metal reacts with a solution containing copper(II)
ions, forming zinc(II) ions and metallic copper.
The reaction is spontaneous (a spontaneous reaction
is a reaction that occurs by itself; without an external
energy source).
Galvanic Cells
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A galvanic cell, (or voltaic cell), converts
chemical energy to electrical energy.
For this to work we must prevent the reactants in
a redox reaction from coming into direct contact
with each other.
Instead, electrons flow from one reactant to the
other through an external circuit, which is a
circuit outside the reaction vessel.
This flow of electrons through the external circuit
is an electric current.
Galvanic Cells: The Daniell Cell
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One half of the cell consists of a piece of zinc
placed in a zinc sulfate solution.
The other half of the cell consists of a piece of
copper placed in a copper(II) sulfate solution.
A porous barrier, separates these two half-cells.
It stops the copper(II) ions from coming into
direct contact with the zinc electrode.
Galvanic Cells: The Daniell Cell
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The pieces of metallic zinc and copper act
as electrical conductors.
 The
conductors that carry electrons into and
out of a cell are named electrodes.
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The zinc sulfate and copper(II) sulfate act
as electrolytes.
 Electrolytes
are substances that conduct
electricity when dissolved in water.
Galvanic Cells: The Daniell Cell
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The redox reaction takes place in a galvanic cell
when an external circuit, such as a metal wire,
connects the electrodes.
The oxidation half-reaction occurs in one halfcell, and the reduction half-reaction occurs in the
other half-cell. For the Daniell cell:
 Oxidation:
Zn(s) → Zn2+(aq) + 2e−
 Reduction: Cu2+(aq) + 2e− → Cu(s)
Galvanic Cells: The Daniell Cell
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The electrode at which oxidation occurs is
named the anode.
 Zinc
atoms undergo oxidation at the zinc electrode.
The zinc electrode is the anode of the Daniell cell.
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The electrode at which reduction occurs is
named the cathode.
 Copper(II)
ions undergo reduction at the copper
electrode. Thus, the copper electrode is the cathode
of the Daniell cell.
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Free electrons cannot travel through the
solution. Instead, the external circuit conducts
electrons from the anode to the cathode of a
galvanic cell.
Galvanic Cells: The Daniell Cell
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At the anode of a galvanic cell, electrons are released
by oxidation.
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The zinc anode of the Daniell cell, zinc metal atoms release
electrons to become positive zinc ions. Making the anode of a
galvanic cell is negatively charged.
Relative to the anode, the cathode of a galvanic cell is
positively charged.
In galvanic cells, electrons flow through the external
circuit from the negative electrode to the positive
electrode.
Galvanic Cells: The Daniell Cell
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Each half-cell contains a solution of a neutral
compound. These solutions are aqueous zinc sulfate
and aqueous copper(II) sulfate.
To maintain electrical neutrality in each half-cell, ions
migrate through a porous barrier. Negative ions (anions)
migrate toward the anode, and positive ions (cations)
migrate toward the cathode.
The barrier is sometimes a salt bridge, which contains
an electrolyte solution. The open ends of the salt bridge
are plugged with a porous material. The plugs allow ion
migration to maintain electrical neutrality.
Galvanic Cell Notation
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The shorthand representation of a Daniell
cell is as follows.
 Zn
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| Zn2+ || Cu2+ | Cu
The phases or states may be included.
 Zn(s)
 the
| Zn2+(aq) || Cu2+(aq) | Cu(s)
anode is always shown on the left and
the cathode on the right
Batteries
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A dry cell is a galvanic cell with the electrolyte
contained in a paste thickened with starch.
Dry cells are inexpensive. The cheapest AAA-, AA-, C-,
and D-size 1.5-V batteries are dry cells.
A battery is defined as a set of galvanic cells connected
in series. The negative electrode of one cell is connected
to the positive electrode of the next cell in the set. The
voltage of a set of cells connected in series is the sum of
the voltages of the individual cells.
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A 9-V battery containssix 1.5-V dry cells connected in series.
http://www.youtube.com/watch?v=ksxSOwA933M&feature=kp
Cell Potentials
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The difference between the potential energy at
the anode and the potential energy at the
cathode is the electric potential, E, (cell
voltage or cell potential) of a cell.
The unit used to measure electric potential is
called the volt, with symbol V.
A cell potential can be measured using an
electrical device called a voltmeter.
Standard Cell Potentials
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The half-cell potential for a reduction half-reaction is
called a reduction potential.
The numerical values of cell potentials and half-cell
potentials depend on various conditions, so tables of
standard reduction potentials are true when ions and
molecules are in their standard states.
Each half-cell reduction potential is given relative to the
reduction potential of the standard hydrogen electrode,
which has been assigned a value of zero.
Calculating Standard Cell
Potentials
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Zn | Zn2+ (1 mol/L) || Cu2+ (1 mol/L) | Cu
Method 1: E°cell = E°cathode − E°anode
 Cu2+(aq) + 2e− Cu(s) E° = 0.342 V
 Zn2+(aq) + 2e− Zn(s) E° = −0.762 V
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= E°cathode − E°anode
= 0.342 V − (−0.762 V)
= 0.342 V + 0.762 V
= 1.104 V
The standard cell potential for a Daniell cell is
1.104 V.
E°cell
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The half-cell potential for an oxidation halfreaction is called an oxidation potential.
If the reduction half-reaction is as follows,
 Zn2+(aq)
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+ 2e−  Zn(s) E° = −0.762 V
Then the oxidation half-reaction is:
 Zn(s)
 Zn2+(aq) + 2e− E°ox = +0.762 V
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The standard cell potential can also be
calculated as the sum of a standard reduction
potential and a standard oxidation potential.
Method 2: E°cell = E°red + E°ox
+ 2e−  Cu(s) E°red = 0.342 V
 Zn(s) Zn2+(aq) + 2e− E°ox = +0.762 V
 Cu2+(aq)
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E°cell
= E°red + E°ox
= 0.342 V + 0.762 V
= 1.104 V
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Calculate the standard cell potential for the
galvanic cell in which the following
reaction occurs.
 2I−(aq)
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+ Br2(l) → I2(s) + 2Br−(aq)
Practice: page 773 #5-8
Electrolytic Cells
Electrolytic cells- a type of cell that uses
energy to move electrons from lower
potential energy to higher potential energy.
 They convert electrical energy to chemical
energy. They are non-spontaneous, and
require energy to occur.
 The process that takes place in an
electrolytic cell is called electrolysis.
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Electrolytic Cells
Electrolytic cells includes electrode, at
least one electrolyte, and an external
circuit.
 Electrolytic cells require an external
source of electricity, sometimes called the
external voltage.
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Electrolytic Cells
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The external source of electricity forces
electrons onto one electrode. This electrode
becomes negative relative to the other. Na+ ions
move toward the negative electrode, where
they gain electrons and are reduced to the
element sodium.
Electrolysis
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Electrolysis is the splitting of water.
 2H2O
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 2H2 + O2
This is a non-spontaneous reaction that
requires energy. Electrolytic cells can
perform electrolysis.
Electroplating
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In the galvanic cell, the zinc anode gradually
dissolves. The copper cathode grows as more
copper is deposited onto it.
In the electrolytic cell, the copper anode
gradually dissolves. The zinc cathode grows as
more zinc is deposited onto it.
The process in which a metal is deposited, or
plated, onto the cathode in an electrolytic cell is
known as electroplating.