Galvanic Cells
11.1
You know that redox reactions involve the transfer of electrons from
one reactant to another. You may also recall that an electric current is
a flow of electrons in a circuit. These two concepts form the basis of
electrochemistry, which is the study of the processes involved in
converting chemical energy to electrical energy, and in converting
electrical energy to chemical energy.
As you learned in Chapter 10, a zinc strip reacts with a solution
containing copper(II) ions, forming zinc ions and metallic copper. The
reaction is spontaneous. It releases energy in the form of heat; in other
words, this reaction is exothermic.
Section Preview/
Specific Expectations
In this section, you will
■
identify the components in
galvanic cells and describe
how they work
■
describe the oxidation and
reduction half-cells for some
galvanic cells
■
determine half-cell
reactions, the direction
of current flow, electrode
polarity, cell potential,
and ion movement in some
galvanic cells
■
build galvanic cells in the
laboratory and investigate
galvanic cell potentials
■
communicate your understanding of the following
terms: electric current,
electrochemistry, galvanic
cell, voltaic cell, external
circuit, electrodes,
electrolytes, anode, cathode,
salt bridge, inert electrode,
electric potential, cell
voltage, cell potential, dry
cell, battery, primary battery,
secondary battery
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
This reaction occurs on the surface of the zinc strip, where electrons
are transferred from zinc atoms to copper(II) ions when these atoms and
ions are in direct contact. A common technological invention called a
galvanic cell uses redox reactions, such as the one described above, to
release energy in the form of electricity.
The Galvanic Cell
A galvanic cell, also called a voltaic cell, is a device that converts
chemical energy to electrical energy. The key to this invention is to
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.
An Example of a Galvanic Cell: The Daniell Cell
Figure 11.1 shows one example of a galvanic cell, called the Daniell cell.
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, sometimes called a
semi-permeable membrane, separates these two half-cells. It stops the
copper(II) ions from coming into direct contact with the zinc electrode.
Figure 11.1 The Daniell cell
is named after its inventor, the
English chemist John Frederic
Daniell (1790–1845). In the photograph shown here, the zinc sulfate
solution is placed inside a porous
cup, which is placed in a larger
container of copper sulfate
solution. The cup acts as the
porous barrier.
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Web
LINK
www.mcgrawhill.ca/links/
chemistry12
Galvanic cells are named after
the Italian doctor Luigi Galvani
(1737–1798), who generated
electricity using two metals.
These cells are also called
voltaic cells, after the Italian
physicist Count Alessandro
Volta (1745–1827), who built
the first chemical batteries.
To learn more about scientists
who made important discoveries in electrochemistry,
such as Galvani, Volta, and
Faraday, go to the web site
above. Click on Web Links
to find out where to go next.
In a Daniell cell, 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. The zinc sulfate and copper(II) sulfate act as
electrolytes. Electrolytes are substances that conduct electricity when
dissolved in water. (The fact that a solution of an electrolyte conducts
electricity does not mean that free electrons travel through the solution.
An electrolyte solution conducts electricity because of ion movements,
and the loss and gain of electrons at the electrodes.) The terms electrode
and electrolyte were invented by the leading pioneer of electrochemistry,
Michael Faraday (1791–1867).
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 half-cell, and the reduction half-reaction
occurs in the other half-cell. For the Daniell cell:
Oxidation (loss of electrons): Zn(s) → Zn2+(aq) + 2e−
Reduction (gain of electrons): Cu2+(aq) + 2e− → Cu(s)
The electrode at which oxidation occurs is named the anode. In this
example, zinc atoms undergo oxidation at the zinc electrode. Thus, the
zinc electrode is the anode of the Daniell cell. The electrode at which
reduction occurs is named the cathode. Here, copper(II) ions undergo
reduction at the copper electrode. Thus, the copper electrode is the
cathode of the Daniell cell.
Free electrons cannot travel through the solution. Instead, the external
circuit conducts electrons from the anode to the cathode of a galvanic
cell. Figure 11.2 gives a diagram of a typical galvanic cell.
e−
anode
A typical galvanic
cell, such as the Daniell cell shown
here, includes two electrodes,
electrolyte solutions, a porous
barrier, and an external circuit.
Electrons flow through the external
circuit from the negative anode to
the positive cathode.
Figure 11.2
Figure 11.3 Batteries contain
galvanic cells. The + mark labels
the positive cathode. If there is a −
mark, it labels the negative anode.
voltmeter
e−
cathode
porous barrier
−
+
Zn
Cu
ZnSO4(aq)
CuSO4(aq)
At the anode of a galvanic cell, electrons are released by oxidation.
For example, at the zinc anode of the Daniell cell, zinc atoms release
electrons to become positive zinc ions. Thus, 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.
These electrode polarities may already be familiar to you. An example
is shown in Figure 11.3.
Each half-cell contains a solution of a neutral compound. In a
Daniell cell, these solutions are aqueous zinc sulfate and aqueous
copper(II) sulfate. How can these electrolyte solutions remain neutral
when electrons are leaving the anode of one half-cell and arriving at the
cathode of the other half-cell? To maintain electrical neutrality in each
half-cell, some ions migrate through the porous barrier, as shown in
Figure 11.4, on the next page. Negative ions (anions) migrate toward
the anode, and positive ions (cations) migrate toward the cathode.
506 MHR • Unit 5 Electrochemistry
e−
anode
cathode
porous barrier
Zn
Figure 11.4 Ion migration in
a Daniell cell. Some sulfate ions
migrate from the copper(II) sulfate
solution into the zinc sulfate
solution. Some zinc ions migrate
from the zinc sulfate solution into
the copper(II) sulfate solution.
Cu
Zn2+
Cu2+
SO42−
SO42−
Zn2+
Cu2+
SO42−
SO42−
The separator between the half-cells does not need to be a porous barrier.
Figure 11.5 shows an alternative device. This device, called a salt bridge,
contains an electrolyte solution that does not interfere in the reaction.
The open ends of the salt bridge are plugged with a porous material, such
as glass wool, to stop the electrolyte from leaking out quickly. The plugs
allow ion migration to maintain electrical neutrality.
e−
voltmeter
NH4+
cathode
e−
Cl−
anode
salt
bridge
Cu
Figure 11.5 This Daniell cell
includes a salt bridge instead of
a porous barrier. In a diagram, the
anode may appear on the left or on
the right.
Zn
porous
plug
CuSO4(aq)
ZnSO4(aq)
Suppose the salt bridge of a Daniell cell contains ammonium chloride
solution, NH4Cl(aq) . As positive zinc ions are produced at the anode,
negative chloride ions migrate from the salt bridge into the half-cell that
contains the anode. As positive copper(II) ions are removed from solution
at the cathode, positive ammonium ions migrate from the salt bridge into
the half-cell that contains the cathode.
Other electrolytes, such as sodium sulfate or potassium nitrate, could
be chosen for the salt bridge. Neither of these electrolytes interferes in the
cell reaction. Silver nitrate, AgNO3(aq) , would be a poor choice for the salt
bridge, however. Positive silver ions would migrate into the half-cell that
contains the cathode. Zinc displaces both copper and silver from solution,
so both copper(II) ions and silver ions would be reduced at the cathode.
The copper produced would be contaminated with silver.
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Galvanic Cell Notation
A convenient shorthand method exists for representing galvanic cells. The
shorthand representation of a Daniell cell is as follows.
Zn | Zn2+ || Cu2+ | Cu
The phases or states may be included.
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
As you saw in Figure 11.4 and Figure 11.5, the anode may appear on the
left or on the right of a diagram. In the shorthand representation, however,
the anode is always shown on the left and the cathode on the right. Each
single vertical line, |, represents a phase boundary between the electrode
and the solution in a half-cell. For example, the first single vertical line
shows that the solid zinc and aqueous zinc ions are in different phases or
states. The double vertical line, ||, represents the porous barrier or salt
bridge between the half-cells. Spectator ions are usually omitted.
Inert Electrodes
The zinc anode and copper cathode of a Daniell cell are both metals, and
can act as electrical conductors. However, some redox reactions involve
substances that cannot act as electrodes, such as gases or dissolved
electrolytes. Galvanic cells that involve such redox reactions use inert
electrodes. An inert electrode is an electrode made from a material that
is neither a reactant nor a product of the cell reaction. Figure 11.6 shows
a cell that contains one inert electrode. The chemical equation, net ionic
equation, and half-reactions for this cell are given below.
Chemical equation: Pb(s) + 2FeCl3(aq) → 2FeCl2(aq) + PbCl2(aq)
Net ionic equation: Pb(s) + 2Fe3+(aq) → 2Fe2+(aq) + Pb2+(aq)
Oxidation half-reaction: Pb(s) → Pb2+(aq) + 2e−
Reduction half-reaction: Fe3+(aq) + e− → Fe2+(aq)
The reduction half-reaction does not include a solid conductor of
electrons, so an inert platinum electrode is used in this half-cell. The
platinum electrode is chemically unchanged, so it does not appear in
the chemical equation or half-reactions. However, it is included in the
shorthand representation of the cell.
Pb | Pb2+ || Fe3+ , Fe2+ | Pt
A comma separates the formulas Fe3+ and Fe2+ for the ions involved in the
reduction half-reaction. The formulas are not separated by a vertical line,
because there is no phase boundary between these ions. The Fe3+ and Fe2+
ions exist in the same aqueous solution.
e−
anode
This cell uses
an inert electrode to conduct
electrons. Why do you think that
platinum is often chosen as an inert
electrode? Another common choice
is graphite.
cathode
salt
bridge
Figure 11.6
PbCl2(aq)
508 MHR • Unit 5 Electrochemistry
Pt
Pb
FeCl2(aq)
FeCl3(aq)
Practice Problems
1. (a) If the reaction of zinc with copper(II) ions is carried out in a test
tube, what is the oxidizing agent and what is the reducing agent?
(b) In a Daniell cell, what is the oxidizing agent and what is the
reducing agent? Explain your answer.
2. Write the oxidation half-reaction, the reduction half-reaction, and the
overall cell reaction for each of the following galvanic cells. Identify
the anode and the cathode in each case. In part (b), platinum is
present as an inert electrode.
(a) Sn(s) | Sn2+(aq) || Tl+(aq) | Tl(s)
(b) Cd(s) | Cd2+(aq) || H+(aq) | H2(g) | Pt(s)
3. A galvanic cell involves the overall reaction of iodide ions with
acidified permanganate ions to form manganese(II) ions and iodine.
The salt bridge contains potassium nitrate.
(a) Write the half-reactions, and the overall cell reaction.
(b) Identify the oxidizing agent and the reducing agent.
(c) The inert anode and cathode are both made of graphite. Solid
iodine forms on one of them. Which one?
4. As you saw earlier, pushing a zinc electrode and a copper electrode
into a lemon makes a “lemon cell”. In the following representation
of the cell, C6H8O7 is the formula of citric acid. Explain why the
representation does not include a double vertical line.
Zn(s) | C6H8O7(aq) | Cu(s)
Introducing Cell Potentials
You know that water spontaneously flows from a higher position to
a lower position. In other words, water flows from a state of higher
gravitational potential energy to a state of lower gravitational potential
energy. As water flows downhill, it can do work, such as turning a water
wheel or a turbine. The chemical changes that take place in galvanic
cells are also accompanied by changes in potential energy. Electrons
spontaneously flow from a position of higher potential energy at the
anode to a position of lower potential energy at the cathode. The moving
electrons can do work, such as lighting a bulb or turning a motor.
The difference between the potential energy at the anode and the
potential energy at the cathode is the electric potential, E, of a cell. The
unit used to measure electric potential is called the volt, with symbol V.
Because of the name of this unit, electric potential is more commonly
known as cell voltage. Another name for it is cell potential. A cell
potential can be measured using an electrical device called a voltmeter.
A cell potential of 0 V means that the cell has no electric potential,
and no electrons will flow. You know that you can generate electricity
by connecting a zinc electrode and a copper electrode that have been
inserted into a lemon. However, you cannot generate electricity by
connecting two copper electrodes that have been inserted into the lemon.
The two copper electrodes are the same and are in contact with the same
electrolyte. There is no potential difference between the two electrodes.
Electric potentials vary from one cell to another, depending on various
factors. You will examine some of these factors in the next investigation.
The cell voltage is sometimes
called the electromotive force,
abbreviated emf. However,
this term can be misleading.
A cell voltage is a potential
difference, not a force. The
unit of cell voltage, the volt,
is not a unit of force.
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