Page 319 - Physical chemistry understanding our chemical world

286 ELECTROCHEMISTRY

The layer is deposited with the saucepan immersed in a vat of
Al 2 O 3 is also called dye solution (usually acidified to pH 1 or 2), and made the positive
alumina.
terminal of a cell. As the electrolysis proceeds, so the aluminium
on the surface of the saucepan is oxidized:

+
2Al (s) + 3H 2 O −−→ Al 2 O 3(s) + 6H (aq) (7.8)
The aluminium is white and shiny before applying the potential. A critical potential
exists below which no electro-oxidation will commence. At more extreme potentials,
the surface atoms of the aluminium oxidize to form Al 3+ ions, which combine with
oxide ions from the water to form Al 2 O 3 .This electro-precipitation of solid alu-
minium oxide is so rapid that molecules of dye get trapped within it, and hence its
coloured aspect.
The dye resides inside the layer of alumina. Its colour persists
We say the dye occlu- because it is protected from harmful UV light, as well as mechan-
des within a matrix of ical abrasion and chemical attack.
solid aluminium oxide. But the chemical reaction forming this coloured layer of oxide
represents only one part of the cell. A cell contains a minimum of
Acellmust comprise a two electrodes, so a cell comprises two reactions – we call them
minimum of two elec- half-reactions: one describes the chemical changes at the positive
trodes. electrode (the anode) and the other describes the changes that occur
at the negative cathode.
The same number of electrons conducts through (i.e. are conducted by) each of the
two electrodes. If we think in terms of charge flowing per unit time, we would say
the same ‘current’ I flows through each electrode. The electrons travel in opposite
directions, insofar as they leave or enter an electrode, which explains why the current
through the anode is oxidative and the current through the cathode is reductive.
We say
(7.9)
I (anode) =−I (cathode)
where the minus sign reminds us that the electrons either move in or out of the
electrode.
Because these two currents are equal (and opposite), the same amount of reaction
will occur at either electrode. We see how an electrode reaction must also occur at
the cathode as well as the desired oxidative formation of alumina at the anode. (The
exact nature of the reaction at the anode will depend on factors such as the choice of
electrode material.)

How do we prevent the corrosion of an oil rig?

Introduction to electrochemical equilibrium

Oil rigs are often built to survive in some of the most inhospitable climates in the
world. For example, the oil rigs in the North Sea between the UK and Scandinavia