hydrogen. In methane 12 g of carbon
react with 4 g of hydrogen. In hydro-
gen sulphide, 32 g of sulphur react
with 2 g of hydrogen (i.e. 64 g of S for
4 g of hydrogen). Sulphur and carbon
form a compound in which the C:S
ratio is 12:64 (i.e. CS 2 ). The law is
sometimes called the law of recipro-
cal proportions.
chemical dating An absolute *dat-
ing technique that depends on meas-
uring the chemical composition of a
specimen. Chemical dating can be
used when the specimen is known to
undergo slow chemical change at a
known rate. For instance, phosphate
in buried bones is slowly replaced by
Ûuoride ions from the ground water.
Measurement of the proportion of
Ûuorine present gives a rough esti-
mate of the time that the bones have
been in the ground. Another, more
accurate, method depends on the
fact that amino acids in living organ-
isms are l-optical isomers. After
death, these racemize and the age of
bones can be estimated by measuring
the relative amounts of d- and l-
amino acids present. See amino acid
racemization.
chemical engineeringThe study
of the design, manufacture, and oper-
ation of plant and machinery in in-
dustrial chemical processes.
chemical equationA way of de-
noting a chemical reaction using the
symbols for the participating parti-
cles (atoms, molecules, ions, etc.); for
example,
xA + yB →zC + wDThe single arrow is used for an irre-
versible reaction; double arrows (ˆ)
are used for reversible reactions.
When reactions involve different
phases it is usual to put the phase in
brackets after the symbol (s = solid; l
= liquid; g = gas; aq = aqueous). The
numbers x, y, z, and w, showing the
relative numbers of molecules react-
ing, are called the stoichiometric
coefÜcients. The sum of the coefÜ-
cients of the reactants minus the
sum of the coefÜcients of the prod-
ucts (x + y – z – w in the example) is
the stoichiometric sum. If this is zero
the equation is balanced. Sometimes
a generalized chemical equation is
considered
ν 1 A 1 + ν 2 A 2 + ... →... νnAn+
νn+1An+1...
In this case the reaction can be writ-
ten ΣνiAi= 0, where the convention is
that stoichiometric coefÜcients are
positive for reactants and negative
for products. The stoichiometric sum
is Σνi.chemical equilibriumA re-
versible chemical reaction in which
the concentrations of reactants and
products are not changing with time
because the system is in thermody-
namic equilibrium. For example, the
reversible reaction
3H 2 + N 2 ˆ2NH 3
is in chemical equilibrium when the
rate of the forward reaction
3H 2 + N 2 →2NH 3
is equal to the rate of the back reac-
tion
2NH 3 →3H 2 + N 2
See also equilibrium constant.
chemical equivalentSee equiva-
lent weight.chemical fossilAny of various or-
ganic compounds found in ancient
geological strata that appear to be
biological in origin and are assumed
to indicate that life existed when the
rocks were formed. The presence of
chemical fossils in Archaean strata
indicates that life existed over 3000
million years ago.
chemically induced dynamic nu-
clear polarizationSee cidnp.117 chemically induced dynamic nuclear polarization
c