lead(IV) tetraethyl and an increasing
use of lead-free petrol.
Leblanc process An obsolete
process for manufacturing sodium
carbonate. The raw materials were
sodium chloride, sulphuric acid,
coke, and limestone (calcium carbon-
ate), and the process involved two
stages. First the sodium chloride was
heated with sulphuric acid to give
sodium sulphate:
2NaCl(s) + H 2 SO 4 (l) →Na 2 SO 4 (s) +
2HCl(g)The sodium sulphate was then
heated with coke and limestone:
Na 2 SO 4 + 2C + CaCO 3 →Na 2 CO 3 +
CaS + 2CO 2Calcium sulphide was a by-product,
the sodium carbonate being ex-
tracted by crystallization. The
process, invented in 1783 by the
French chemist Nicolas Leblanc
(1742–1806), was theÜrst for produc-
ing sodium carbonate synthetically
(earlier methods were from wood ash
and other vegetable sources). By the
end of the 19th century it had been
largely replaced by the *Solvay
process.
lechatelieriteA mineral form of
*silicon(IV) oxide, SiO 2.
Le Chatelier’s principle If a sys-
tem is in equilibrium, any change
imposed on the system tends to shift
the equilibrium to nullify the effect
of the applied change. The principle,
which is a consequence of the law of
conservation of energy, wasÜrst
stated in 1888 by Henri Le Chatelier
(1850–1936). It is applied to chemical
equilibria. For example, in the gas re-
action
2SO 2 + O 2 ˆ2SO 3an increase in pressure on the reac-
tion mixture displaces the equilib-
rium to the right, since this reduces
the total number of molecules pre-
sent and thus decreases the pressure.
The standard enthalpy change for
the forward reaction is negative (i.e.
the reaction is exothermic). Thus, an
increase in temperature displaces the
equilibrium to the left since this
tends to reduce the temperature. The
*equilibrium constant thus falls with
increasing temperature.
A- Le Chatelier’s original paper
Leclanché cellA primary *voltaic
cell consisting of a carbon rod (the
anode) and a zinc rod (the cathode)
dipping into an electrolyte of a
10–20% solution of ammonium chlo-
ride. *Polarization is prevented by
using a mixture of manganese diox-
ide mixed with crushed carbon, held
in contact with the anode by means
of a porous bag or pot; this reacts
with the hydrogen produced. This
wet form of the cell, devised in 1867
by Georges Leclanché (1839–82), has
an e.m.f. of about 1.5 volts. The *dry
cell based on it is widely used in
torches, radios, and calculators.lectinAny of a group of proteins,
derived from plants, that can bind to
speciÜc oligosaccharides on the sur-
face of cells, causing the cells to
clump together. Lectins can be used
to identify mutant cells in cell cul-
tures and to determine blood groups
as they can cause the agglutination
of red blood cells. Lectins are found
in seeds of legumes and in other tis-
sues, in which they are thought to
act as a toxin.LEED(low-energy electron diffrac-
tion)A technique used to study the
structure of crystal surfaces and
processes taking place on these sur-
faces. The surface is bombarded with
a monochromatic electron beam 10–4
to 10–3m in diameter, with energies
between 6 and 600 V. The electrons
are diffracted by the surface atoms323 LEED
l