hydrogen. In methane 12 g of carbon
react with 4 g of hydrogen. In hydrogen 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 reciprocal proportions.
chemical dating An absolute *dating technique that depends on measuring 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 estimate of the time that the bones have
been in the ground. Another, more
accurate, method depends on the
fact that amino acids in living organisms are l-optical isomers. After
death, these racemize and the age of
bones can be estimated by measuring
the relative amounts of d- and lamino acids present. See amino acid
racemization.
chemical engineering The study
of the design, manufacture, and operation of plant and machinery in industrial chemical processes.
chemical equation A way of denoting a chemical reaction using the
symbols for the participating particles (atoms, molecules, ions, etc.); for
example,
xA + yB → zC + wD
The single arrow is used for an irreversible 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 reacting, are called the stoichiometric
coefÜcients. The sum of the coefÜcients of the reactants minus the
sum of the coefÜcients of the products (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 + … → … ν n A n +
ν n+1 A n+1 …
In this case the reaction can be written Σν i A i = 0, where the convention is
that stoichiometric coefÜcients are
positive for reactants and negative
for products. The stoichiometric sum
is Σν i .
chemical equilibrium A reversible chemical reaction in which
the concentrations of reactants and
products are not changing with time
because the system is in thermodynamic equilibrium. For example, the
reversible reaction
3H 2 + N 2 ˆ 2NH 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 reaction
2NH 3 → 3H 2 + N 2
See also equilibrium constant.
chemical equivalent See equivalent weight.
chemical fossil Any of various organic 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 nuclear polarization See cidnp.
117
chemically induced dynamic nuclear polarization
c
www.AzShimi.ir www.AzShimi.com
react with 4 g of hydrogen. In hydrogen 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 reciprocal proportions.
chemical dating An absolute *dating technique that depends on measuring 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 estimate of the time that the bones have
been in the ground. Another, more
accurate, method depends on the
fact that amino acids in living organisms are l-optical isomers. After
death, these racemize and the age of
bones can be estimated by measuring
the relative amounts of d- and lamino acids present. See amino acid
racemization.
chemical engineering The study
of the design, manufacture, and operation of plant and machinery in industrial chemical processes.
chemical equation A way of denoting a chemical reaction using the
symbols for the participating particles (atoms, molecules, ions, etc.); for
example,
xA + yB → zC + wD
The single arrow is used for an irreversible 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 reacting, are called the stoichiometric
coefÜcients. The sum of the coefÜcients of the reactants minus the
sum of the coefÜcients of the products (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 + … → … ν n A n +
ν n+1 A n+1 …
In this case the reaction can be written Σν i A i = 0, where the convention is
that stoichiometric coefÜcients are
positive for reactants and negative
for products. The stoichiometric sum
is Σν i .
chemical equilibrium A reversible chemical reaction in which
the concentrations of reactants and
products are not changing with time
because the system is in thermodynamic equilibrium. For example, the
reversible reaction
3H 2 + N 2 ˆ 2NH 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 reaction
2NH 3 → 3H 2 + N 2
See also equilibrium constant.
chemical equivalent See equivalent weight.
chemical fossil Any of various organic 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 nuclear polarization See cidnp.
117
chemically induced dynamic nuclear polarization
c
www.AzShimi.ir www.AzShimi.com