Illustrated Guide to Home Chemistry Experiments

Chapter 16 Laboratory: Electrochemistry 301

If you immerse magnesium metal in a solution

of copper sulfate, a spontaneous redox reaction

occurs. An atom of magnesium releases two

electrons and is oxidized to an Mg2+ ion. Those

two electrons are captured by a Cu2+ ion,

reducing it to copper metal, which is deposited

on the magnesium electrode as a thin plating.

This reaction occurs spontaneously, because

magnesium ions are less attractive to electrons

than are copper ions.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ digital multimeter (dmm)

£ patch cables with alligator clips (2)

£ beaker, 150 mL (2)

£ graduated cylinder, 100 mL

£ Beral pipette or eyedropper

£ stirring rod

£ flexible plastic tubing (~15 cm; inside diameter 5 mm

or larger)

£ cotton ball (1)

£ steel wool or sandpaper

£ electrodes, magnesium and copper (one each)

£ sodium chloride, 1 m (a few mL)

£ magnesium sulfate, 1.0 m (50 mL)

£ copper sulfate, 1.0 m (50 mL)

We can write this redox reaction as two half-reactions, with the
standard reduction potentials shown in parentheses:

mg(s) → mg2+(aq) + 2e– (+2.37v)

Cu2+(aq) + 2e– → Cu(s) (+0.34v)

In the combined reaction, magnesium atoms serve as the
reducing agent, which means that they are oxidized to
magnesium ions. Conversely, copper ions serve as the oxidizing
agent, which means that they are reduced to metallic copper.
Adding up the reduction potentials tells us that this cell should
be able to provide an electron flow at 2.71V. There’s a problem
with that, though.

When magnesium metal is in direct contact with a solution
of copper ions, electrons are transferred directly from the
magnesium metal to the copper ions, so those electrons are
not accessible to do work. In effect, both half-reactions are
occurring in the same reaction vessel, so there is no externally
accessible flow of electrons that can be captured to do work.

But what if we could split the two half-reactions into separate
vessels and somehow intercept that electron flow? As it turns
out, we can do just that by putting the copper and magnesium
parts of the cell in separate containers. That physically isolates
the magnesium atoms and ions from the copper atoms and
ions, but we still somehow have to provide an external path for
the flow of electrons and an electrical connection between the
copper and magnesium segments of the cell.

The first part of that circuit is no problem. We can use ordinary
copper wire clipped to the electrodes as a way to route
electricity outside the cell, where it can do useful work such as
lighting a light bulb. But we still need the internal connection

LABORATORY 1 6.5:

BUILd A voLTAIC CELL wITH Two HALf-CELLS

between the two half-cells. The answer is a device called a salt

bridge, which is a tube filled with a conducting solution, with

one end immersed in each of the half-cells. The salt bridge

conducts electricity while preventing ions from migrating from

one half-cell to the other.

In this lab session, we’ll build a voltaic cell comprising a

magnesium half-cell and a copper half-cell, and measure the

voltage and current produced by the cell.

CUTIOA nS

Copper sulfate is moderately toxic. Wear splash goggles,

gloves, and protective clothing.

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