Transcript Topic 9.4 REDOX Voltaic Cells

Topic 9.4 Voltaic Cells
Assessment Statements
 9.4.1 Explain how a redox reaction is used to
produce electricity in a voltaic cell.
 (include diagrams)
 9.4.2 State that oxidation occurs at the negative
electrode (anode) and reduction occurs at the
positive electrode.
References
 Powerpoint
 Workbook exercises
 Moodle.
 Textbook on moodle (under topic sections)
Key Terms
Assessment Statement
 9.4.1 Explain how a redox reaction is used to
produce electricity in a voltaic cell.
 (include diagrams)
 9.4.2 State that oxidation occurs at the negative
electrode (anode) and reduction occurs at the
positive electrode (cathode).
Remember
 REDOX reactons are simply transfering electron from
one atom to another.
 When we make electricity, we are just moving electrons
from one place to another.
 SO……………………………
 What if we separate the half-equations and force
the electrons to travel a bit first…………………..
Voltaic Cells
In spontaneous
oxidation-reduction
(redox) reactions,
electrons are
transferred and
energy is released.
We can split these
into HALF CELLS.
Half Cells
 The half reaction is called a half cell
 The simplest half cell is made by putting a strip
of metal into a solution of its ions
Voltaic cells
In redox reactions there is a transfer of electrons
We can make use of it and build a battery providing us with electrons:
Voltaic cells
Two half reactions:
Zn (s)  Zn+2 (aq) + 2eElectron provider
Cu+2 (aq) + 2e-  Cu (s)
Copper is less reactive
Zinc reactive
so reaction takes place
Anode the site at
which the oxidation
of zinc
Cathode, is the site at
which reduction of copper
Voltaic Cells
 We can use that
energy to do work if
we make the
electrons flow
through an external
device.
 We call such a setup
a voltaic cell.
Voltaic Cells
 A typical cell looks
like this.
 The oxidation
always occurs at
the anode.
 The reduction
always occurs at
the cathode.
 By definition, the anode is where oxidation occurs
An Ox (ANode is OXidation)
 cathode is where reduction occurs
RedCat (REDuction is on the CAThode)
Voltaic Cells
Once even one
electron flows from
the anode to the
cathode, the
charges in each
beaker would not be
balanced and the
flow of electrons
would stop.
Parts of a Voltaic Cell
 Therefore, we use a
salt bridge, usually
a U-shaped tube
that contains a salt
solution, to keep
the charges
balanced.
 Cations move toward
the cathode.
 Anions move toward
the anode.
Parts of a Voltaic Cell
+ 1.56V
 Electrode:
 Conductor in a
circuit that carries
electrons to a metal
 Anode = oxidation
 Negatively
charged
 Cathode = reduction
 Positively charged
The different half cells make voltaic cells with different voltages
Voltaic Cells
 In the cell, then,
electrons leave the
anode and flow
through the wire to
the cathode.
 As the electrons
leave the anode,
the cations formed
dissolve into the
solution in the
anode compartment.
Voltaic Cells
 As the electrons
reach the cathode,
cations in the
cathode are
attracted to the
now negative
cathode.
 The electrons are
taken by the cation,
and the neutral
metal is deposited
on the cathode.
Remember for your Exam
 Sketch and label a




voltaic cell
Know which is the
cathode and anode
(labeled diagram).
Identify the solutions
and electrode
composition.
Direction of electron
flow.
Explain how it works.
Quiz
 Draw a cell schematic to show the reaction:
Cu  Cu2+ + 2eI2 + 2e-  2I-
(platinum electrode)
Cu + I2  Cu2+ + 2I-
 Label the anode, cathode, and direction of electron
flow
 Calculate the voltage (later in HL)
Current and Voltage
The rate at which electric charge flows through the circuit is known
as the current, which is measured in amperes (amps,A)
A large current could be produced by a large amount of charge
moving slowly or a small amount of charge moving quickly
 An electrical current flows through a circuit if there is a potential
difference between electrical potential in two points of the circuit
 Potential difference is measured in volts (V) and is termed
voltage One volt gives a charge of one coulomb one joule of
energy.
The following slides are ext information to help you understand
electricity better but is not assessed in IB Chemistry
Electromotive Force (emf)
 Water only
spontaneously flows
one way in a
waterfall.
 Likewise, electrons
only spontaneously
flow one way in a
redox reaction—
from higher to lower
potential energy.
Electromotive Force (emf)
 The potential difference between the anode and
cathode in a cell is called the electromotive force
(emf).
 It is also called the cell potential, and is
designated Ecell.
Cell Potential
Cell potential is measured in volts (V).
1V=1
J
C
 The volt reading is a measure of the cell potential
of the system
 The voltage depends on:
 Nature of the electrodes
 Nature of the ions in solution
 Concentrations of the solutions
 Temperature of the solutions
Videos
 Galvanic cells
 Animation
 Potentials
 Song (This guy is seriously prolific)
 Li Batteries
Design an experiment
 Design an experiment to look at the impact of a
variable on the production of voltage in a voltaic cell.
 What things MAY change the voltage produced?