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CONCENTRATION CELLS 333

7.5 Concentration cells

Why does steel rust fast while iron is more passive?

Concentration cells

Steel is an impure form of iron, the most common contaminants
Many iron ores also
being carbon (from the coke that fuels the smelting process) and containironsulphide,
sulphur from the iron oxide ore. which is commonly
Pure iron is relatively reactive, so, given time and suitable con- called fool’s gold.
ditions of water and oxygen, it forms a layer of red hydrated iron
oxide (‘rust’):

4Fe (s) + 3O 2 + nH 2 O −−→ 2Fe 2 O 3 · (H 2 O) n(s) (7.47)

By contrast, steel is considerably more reactive, and rusts faster and to a greater
extent.
The mole fraction x of Fe in pure iron is unity, so the activity of
the metallic iron is also unity. The mole fraction x of iron in steel We define a concen-
tration cell as a cell in
will be less than unity because it is impure. The carbon is evenly which the two half-cells
distributed throughout the steel, so its mole fraction x (C) is con-
are identical except for
stant, itself ensuring that the activity is also constant. Conversely, their relative concen-
the sulphur in steel is not evenly distributed, but resides in small trations.
(microscopic) ‘pockets’. In consequence, the mole fraction of the
iron host x (Fe) fluctuates, with x being higher where the steel is more pure, and lower
in those pockets having a high sulphur content. To summarize, there are differences
in the activity of the iron, so a concentration cell forms.
The emf of a concentration cell (in this case, on the surface of
the steel where the rusting reaction actually occurs) is given by The electrolyte on the
surface of the iron com-
RT a 2 prises water containing
emf = ln (7.48) dissolved oxygen (e.g.
nF a 1
rain water).
Notice how this emf has no standard electrode potential E terms
O
(unlike the Nernst equation from which it derives; see Justification
Box 7.2).
A voltage forms between regions of higher irons activity a 1 and The commas in this
regions of lower iron activity a 2 (i.e. between regions of high purity schematic indicate that
and low iron purity); see Figure 7.14. We can write a schematic the carbon and sulphur
for a microscopic portion of the iron surface as: impurities reside within
the same phase as the
iron.
Fe(a 2 ), S,C|O 2 , H 2 O|Fe(a 1 ), S,C