light-dependent reaction See
photosynthesis.
light-independent reaction See
photosynthesis.
lignin A complex organic polymer
that is deposited within the cellulose
of plant cell walls during secondary
thickening. LigniÜcation makes the
walls woody and therefore rigid.
lignite See coal.
lime See calcium oxide.
limestone A sedimentary rock that
is composed largely of carbonate
minerals, especially carbonates of
calcium and magnesium. *Calcite
and *aragonite are the chief minerals; *dolomite is also present in the
dolomitic limestones. There are
many varieties of limestones but
most are deposited in shallow water.
Organic limestones (e.g. *chalk) are
formed from the calcareous skeletons of organisms; precipitated limestones include oolite, which is
composed of ooliths – spherical bodies formed by the precipitation of
carbonate around a nucleus; and clastic limestones are derived from fragments of pre-existing calcareous
rocks.
limewater A saturated solution of
*calcium hydroxide in water. When
carbon dioxide gas is bubbled
through limewater, a ‘milky’ precipitate of calcium carbonate is formed:
Ca(OH) 2 (aq) + CO 2 (g) → CaCO 3 (s) +
H 2 O(l)
If the carbon dioxide continues to be
bubbled through, the calcium carbonate eventually redissolves to form
a clear solution of calcium hydrogencarbonate:
CaCO 3 (s) + CO 2 (g) + H 2 O(g) →
Ca(HCO 3 ) 2 (aq)
If cold limewater is used the original
calcium carbonate precipitated has a
calcite structure; hot limewater
yields an aragonite structure.
limit cycle See attractor.
limonite A generic term for a
group of hydrous iron oxides,
mostly amorphous. *Goethite and
*haematite are important constituents, together with colloidal
silica, clays, and manganese oxides.
Limonite is formed by direct precipitation from marine or fresh water in
shallow seas, lagoons, and bogs (thus
it is often called bog iron ore) and by
oxidation of iron-rich minerals. It is
used as an ore of iron and as a pigment.
Lindemann–Hinshelwood mechanism A mechanism for unimolecular chemical reactions put forward by
the British physicist Frederick Lindermann (1886–1957) in 1921 and examined in more detail by the British
chemist Sir Cyril Hinshelwood
(1897–1967) in 1927. The mechanism
postulates that a molecule of A becomes excited by colliding with another molecule of A, and that having
been excited there is a possibility
that it undergoes unimolecular
decay. If the process of unimolecular
decay is sufÜciently slow, the reaction has a Ürst-order rate law, in
agreement with experiment. The Lindemann–Hinshelwood mechanism
predicts that if the concentration of
A is reduced, the reaction kinetics become second order. This change from
Ürst to second order agrees with experiment qualitatively, although it
does not do so quantitatively. The
mechanism fails quantitatively because the molecule has to be excited
in a speciÜc way for a reaction to
take place. The RRK and RRKM theories improve on this deÜciency of the
Lindemann–Hinshelwood mechanism.
light-dependent reaction
326
l
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photosynthesis.
light-independent reaction See
photosynthesis.
lignin A complex organic polymer
that is deposited within the cellulose
of plant cell walls during secondary
thickening. LigniÜcation makes the
walls woody and therefore rigid.
lignite See coal.
lime See calcium oxide.
limestone A sedimentary rock that
is composed largely of carbonate
minerals, especially carbonates of
calcium and magnesium. *Calcite
and *aragonite are the chief minerals; *dolomite is also present in the
dolomitic limestones. There are
many varieties of limestones but
most are deposited in shallow water.
Organic limestones (e.g. *chalk) are
formed from the calcareous skeletons of organisms; precipitated limestones include oolite, which is
composed of ooliths – spherical bodies formed by the precipitation of
carbonate around a nucleus; and clastic limestones are derived from fragments of pre-existing calcareous
rocks.
limewater A saturated solution of
*calcium hydroxide in water. When
carbon dioxide gas is bubbled
through limewater, a ‘milky’ precipitate of calcium carbonate is formed:
Ca(OH) 2 (aq) + CO 2 (g) → CaCO 3 (s) +
H 2 O(l)
If the carbon dioxide continues to be
bubbled through, the calcium carbonate eventually redissolves to form
a clear solution of calcium hydrogencarbonate:
CaCO 3 (s) + CO 2 (g) + H 2 O(g) →
Ca(HCO 3 ) 2 (aq)
If cold limewater is used the original
calcium carbonate precipitated has a
calcite structure; hot limewater
yields an aragonite structure.
limit cycle See attractor.
limonite A generic term for a
group of hydrous iron oxides,
mostly amorphous. *Goethite and
*haematite are important constituents, together with colloidal
silica, clays, and manganese oxides.
Limonite is formed by direct precipitation from marine or fresh water in
shallow seas, lagoons, and bogs (thus
it is often called bog iron ore) and by
oxidation of iron-rich minerals. It is
used as an ore of iron and as a pigment.
Lindemann–Hinshelwood mechanism A mechanism for unimolecular chemical reactions put forward by
the British physicist Frederick Lindermann (1886–1957) in 1921 and examined in more detail by the British
chemist Sir Cyril Hinshelwood
(1897–1967) in 1927. The mechanism
postulates that a molecule of A becomes excited by colliding with another molecule of A, and that having
been excited there is a possibility
that it undergoes unimolecular
decay. If the process of unimolecular
decay is sufÜciently slow, the reaction has a Ürst-order rate law, in
agreement with experiment. The Lindemann–Hinshelwood mechanism
predicts that if the concentration of
A is reduced, the reaction kinetics become second order. This change from
Ürst to second order agrees with experiment qualitatively, although it
does not do so quantitatively. The
mechanism fails quantitatively because the molecule has to be excited
in a speciÜc way for a reaction to
take place. The RRK and RRKM theories improve on this deÜciency of the
Lindemann–Hinshelwood mechanism.
light-dependent reaction
326
l
www.AzShimi.ir www.AzShimi.com