Illustrated Guide to Home Chemistry Experiments

Chapter 16 Laboratory: Electrochemistry 295

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ digital multimeter (dmm)

£ sharp knife

£ steel wool or sandpaper

£ lemon (1)

£ Electrodes of as many metals as possible, such as

magnesium, aluminum, zinc, iron, nickel, tin, lead,

and copper. Ideally, these electrodes should be in the

form of metal strips, but if strips are not available,

you may use wires or rods. If you cannot obtain

electrodes made from all of these metals, get as

many as you can. most laboratory supplies vendors

sell inexpensive sets of electrodes that contain most

or all of these metals.

SBSTITUTIU oNS ANd modIfICATIoNS

• You may substitute another citrus fruit, such as an
  orange or a lime, for the lemon. Dr. Mary Chervenak
  suggests trying a potato.

zinc is oxidized to Zn2+ ions. With this combination, iron serves
as the oxidizer rather than the reducer.

Standard reduction potential values can also be used to predict
the voltage that will be produced by a cell that uses two different
metals as electrodes, simply by calculating the difference in
reduction potential between the two metals. For example, if
you build a cell that uses electrodes of copper (+0.34V) and
zinc (–0.76V), that cell will, in theory, provide current at 1.10V,
because:

• 0.76v – (+0.34v) = –1.10v

In practice, the actual voltage produced by such cells is always
somewhat lower than the theoretical voltage produced by an
ideal cell, because real-world physical cells have inefficiencies
in ion migration and other issues that reduce the voltage
somewhat. Still, using reduction potential values makes it easy
to estimate the approximate voltage that will be produced by
a cell that uses any two arbitrarily selected metals as electrodes.

But what exactly is a cell? When we drop an iron nail into a
solution of copper ions, the iron is oxidized and the copper
reduced, but these two half-reactions occur with the reactants
in physical contact. Electrons are exchanged, but we have no
way to access that electron flow and use it to do useful work.
If we separate the reactants physically, the two half-reactions
cease—unless, that is, we join them electrically by using a
conductor. An arrangement in which the two reactants are
physically separated but electrically joined is called a cell, and
each of the separate reactants is called a half-cell.

In a cell, electrons released by the reducing agent in its half-
cell travel from its electrode as an electric current through
a conductor (such as a wire) to the electrode of the half-cell
that contains the oxidizing agent, which is reduced by those
electrons. That electric current can be used to do useful work,
such as lighting a bulb or running a motor. The electrode at
which oxidation occurs is called the anode, and the electrode at
which reduction occurs is called the cathode.

In this laboratory session, we’ll build such an electrochemical
cell, also called a galvanic cell or voltaic cell, using electrodes
of various metals. We’ll embed those electrodes in a lemon.
The semipermeable membranes in the lemon will function as
physical barriers to isolate the two half-cells from each other,
while the hydronium and citrate ions produced by the citric acid
in the lemon juice provide internal electric connectivity between
the half-cells.

If we stopped at that point, no reactions would occur and
no current would be produced, because the semipermeable

CUTIOA nS

Although none of the materials used in this lab session is

particularly hazardous, make sure to dispose of the food

items after use. DO NOT EAT ANY FOOD ITEM used in

this session. The food items will be contaminated with

metal ions, including toxic heavy-metal ions. Wear splash

goggles, gloves, and protective clothing.

z

membranes in the lemon prevent internal migration of metal

ions between the electrodes. In other words, no return path for

the current exists. But if we provide a return path by connecting

the two electrodes directly together with a copper wire, the

reaction proceeds and current is produced. We’ll measure

the voltage of that current to establish that an electrochemical

cell exists, and compare our actual measured voltages with the

voltage that an ideal cell using those electrodes would produce.