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Electrochemistry I: Batteries and Free Energy
The process of donating and accepting electrons is reversible. For example,
under one set of conditions metallic cadmium may donate electrons and become Cd
2+ ions, as it does when immersed in HC1,
Cd + 2H
+ -> Cd
2+ + H 2 t
or, under another set of conditions, the Cd
2+ ion may accept electrons and be
reduced to Cd metal, as it is when it comes in contact with metallic Zn:
Cd
2+ + Zn -> Zn
2+ + Cd
Reducing agents differ in their ability to donate electrons. For example, metallic Zn can donate electrons to Cd
2+ to produce metallic Cd and Zn
2+ , but
metallic Cd is unable to donate electrons to Zn
2+ to produce metallic Zn and
Cd
2+ . This illustrates the fact that metallic Zn is a stronger reducing agent than
metallic Cd.
A reversible electron-transfer reaction written in the form
ne~ + oxidizing agent *=* reducing agent
is called a half-reaction, because it cannot occur unless it is coupled with another half-reaction going in the opposite direction. If we use half-reactions in
the manner described in the next few paragraphs, we can assign a number to
each reducing agent to describe its strength, or ability to donate electrons.
Galvanic Cells
An electric current is a flow of electrons through a conductor. Many electrontransfer reactions can be arranged so that the electrons donated by the reducing
agent are forced to flow through a conducting wire to reach the oxidizing agent.
Such an arrangement is called a battery, or electrochemical (galvanic) cell: a
simple form is shown in Figure 17-1. The electron-transfer reaction that produces the current is
Zn + Cu
2 + -> Zn
2+ + Cu
Note that all the components of one half-reaction are placed in one beaker,
and all the components of the second half-reaction are placed in the other
beaker. One must not put the materials of the lefthand side of the equation in
one beaker, and those of the righthand side in the other! If one did, electron
transfer could occur on contact of the two reactants, and there would be no
flow of current from one electrode to the other. The negative electrode always
involves the half-reaction with the greatest reducing strength; in this case it is a
strip of zinc dipping into a solution of Zn
2+ (any soluble zinc salt). The positive
Electrochemistry I: Batteries and Free Energy
The process of donating and accepting electrons is reversible. For example,
under one set of conditions metallic cadmium may donate electrons and become Cd
2+ ions, as it does when immersed in HC1,
Cd + 2H
+ -> Cd
2+ + H 2 t
or, under another set of conditions, the Cd
2+ ion may accept electrons and be
reduced to Cd metal, as it is when it comes in contact with metallic Zn:
Cd
2+ + Zn -> Zn
2+ + Cd
Reducing agents differ in their ability to donate electrons. For example, metallic Zn can donate electrons to Cd
2+ to produce metallic Cd and Zn
2+ , but
metallic Cd is unable to donate electrons to Zn
2+ to produce metallic Zn and
Cd
2+ . This illustrates the fact that metallic Zn is a stronger reducing agent than
metallic Cd.
A reversible electron-transfer reaction written in the form
ne~ + oxidizing agent *=* reducing agent
is called a half-reaction, because it cannot occur unless it is coupled with another half-reaction going in the opposite direction. If we use half-reactions in
the manner described in the next few paragraphs, we can assign a number to
each reducing agent to describe its strength, or ability to donate electrons.
Galvanic Cells
An electric current is a flow of electrons through a conductor. Many electrontransfer reactions can be arranged so that the electrons donated by the reducing
agent are forced to flow through a conducting wire to reach the oxidizing agent.
Such an arrangement is called a battery, or electrochemical (galvanic) cell: a
simple form is shown in Figure 17-1. The electron-transfer reaction that produces the current is
Zn + Cu
2 + -> Zn
2+ + Cu
Note that all the components of one half-reaction are placed in one beaker,
and all the components of the second half-reaction are placed in the other
beaker. One must not put the materials of the lefthand side of the equation in
one beaker, and those of the righthand side in the other! If one did, electron
transfer could occur on contact of the two reactants, and there would be no
flow of current from one electrode to the other. The negative electrode always
involves the half-reaction with the greatest reducing strength; in this case it is a
strip of zinc dipping into a solution of Zn
2+ (any soluble zinc salt). The positive