http://www.ck12.org Chapter 23. Electrochemistry
electrons than magnesium, so we are left with K+cations instead of Mg^2 + cations at the end of the reaction.
Similarly, we would predict that the following reaction wouldnotoccur spontaneously as written:
Sn + Fe^2 +→Sn^2 ++ FeIron is more active than tin, so neutral iron could be used to reduce tin cations, but not the other way around. The
activity series (sometimes referred to as the electromotive series) is a very useful tool as we explore how to generate
electric currents.
Galvanic Cells
One of the many useful applications of electrochemistry is in the generation of electric current. We saw earlier
how electrons can be transferred from one atom to another in an oxidation-reduction reaction. If we place a strip
of metallic zinc in a solution of copper sulfate, we will soon see a single-displacement reaction, in which zinc is
oxidized to its corresponding cations, and the copper cations will plate out on the strip as neutral copper. The
electron transfer from the zinc to the copper is direct, because the metal surface is in direct contact with the ions in
the solution.
Alternatively, we can channel the electron flow from zinc to copper through a wire by using a voltaic cell. Forcing
the electrons to travel through a wire in order to get from one species to another allows us to harness that energy
and use it to perform mechanical work, something that is not possible for the situation described above in which the
oxidizing and reducing agents were in direct contact. The first such cell was developed by Alessandro Volta in 1800.
Similar concepts were explored by Luigi Galvani in the late 1700s, so these cells are often referred to asgalvanic
cells. A typical cell is illustrated inFigure23.1.
There are two major components to this cell. A solution of Zn(NO 3 ) 2 (on the left) has a zinc strip immersed in it.
On the right, we see a copper strip in a solution of Cu(NO 3 ) 2. A porous barrier separates the two solutions. The
solutions are not in direct contact with one another, but ions can flow freely through the barrier. The two metal strips
are connected by a wire that allows electrons to move from one strip to the other.
Referring to our activity series, we see that Zn is more active than Cu. As a result, Zn will spontaneously give up
its electrons to reduce copper cations, but Cu will not spontaneously reduce zinc cations. In theFigure23.1, Zn
can undergo a reaction that produces Zn^2 +and two electrons. The two electrons flow through the wire (powering a
meter or device if one is present) towards the copper electrode. Once the electrons reach the copper sulfate solution,
they are used to reduce Cu^2 +ions to metallic copper, which plates out on the copper electrode. When a metal "plates
out," it is transferred from an aqueous ion in solution to a neutral metal atom in its solid form.
The porous disk allows for the transfer of anions from the copper side to the zinc side. During the reaction, more
Zn^2 +accumulates on the left side of the cell, and Cu^2 +is being depleted from the right side of the cell. Without
the porous disc, this would cause an unstable buildup of positive charge on the zinc side and negative charge on the
copper side. To relieve this imbalance, anions essentially move through the porous disk from right to left, balancing
the positive charge of the newly formed Zn^2 +ions and moving away from the side with a decreasing concentration
of positively charged Cu^2 +ions.
Batteries
Records from ancient Greece mention that Thales of Miletus (a Greek philosopher, mathematician, and astronomer
who lived from 620-546 BC) rubbed fur with amber and generated a strange phenomenon, which we now call static
electricity. Although the ancient Greeks did not have a good understanding of what was happening, we now have
well-developed theories about how to generate and use electric currents.