1. Limestone - a very useful material
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GCSE Multiple choice QUIZ
on Limestone and its uses:
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Limestone, is a
sedimentary rock formed by the mineral and 'shelly' remains of
marine organisms, including coral, in warm shallow fertile seas. It is chemically mainly calcium
carbonate and is a useful material that is quarried and used directly as a building material.
It reacts with acids - 'fizzing' due to carbon dioxide formation -
test with 'limewater' - milky white precipitate.
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Chemically,
limestone mainly
consists of calcium carbonate, CaCO3, and is a
valuable natural
mineral resource, quarried in large
quantities in many countries (see
environmental impact at the end of the metal extraction
web page).
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Other uses of limestone
rock are
outlined below and it is an important raw material in the
manufacture
of cement and glass and iron.
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Powdered limestone can be
used to neutralise acidity in
lakes and soils. (neutralisation
chemistry). Like lime, it is a safe
agri-chemical to use on the land and does produce the controversial
side effects of artificial fertilisers, herbicides and pesticides etc.
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When limestone is heated
in a kiln at over 900oC, it breaks down into quicklime
(calcium
oxide) and carbon dioxide. Both are useful products. This type of reaction is endothermic
(heat absorbing)
and an example of thermal
decomposition (and other carbonates behave in a similar way).
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calcium carbonate
(limestone) ==> calcium oxide (quicklime) + carbon dioxide
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CaCO3(s)
CaO(s) + CO2(g)
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This is a reversible
endothermic reaction. To ensure the change is to favour the
right hand side, a high
temperature of over 900oC is needed as well as the
continual removal of the carbon dioxide.
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The high temperature
needed is produced by mixing the limestone with coal/coke (a fuel of
mainly carbon) and blowing hot air into the ignited mixture in
a rotating kiln for a continuous production line (raw materials in
at one end, lime out the other!) ....
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Note
on heating other carbonates - more
thermal decomposition.
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These also show a similar
thermal
decomposition to calcium e.g.
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copper(II)
carbonate(s, green) ==> copper(II)
oxide(s, black) + carbon dioxide
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CuCO3(s)
==> CuO(s) + CO2(g)
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zinc carbonate(s,
white)
==> zinc oxide(s, yellow hot, white cold)
+ carbon dioxide
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ZnCO3(s)
==> ZnO(s) + CO2(g)
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Zinc
carbonate occurs as the mineral ores calamine/Smithsonite and
the resulting zinc oxide can be used to
extract zinc metal
and zinc oxide itself is used as a whitening agent' in cosmetics and
in 'calamine lotion' a mild antiseptic and antipruritic
(anti-itching agent) for treating skin irritations.
-
FeCO3
and MnCO3 behave in a similar way
(so just swap
Zn/Ca/Cu with Fe or Mn)
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Sodium
hydrogen carbonate is used in baking powder because on heating
it thermally decomposes releasing carbon dioxide gas that gives the
'rising' action in baking.
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sodium
hydrogencarbonate ==> sodium carbonate + water + carbon dioxide
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2NaHCO3(s)
==> Na2CO3(s) + H2O(l/g)
+ CO2(g)
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This is just
one of many chemical process that occur when food is cooked.
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Quicklime
reacts very exothermically with water to produce slaked lime
(solid
calcium hydroxide).
 
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Lime
(calcium
oxide) and slaked lime (calcium hydroxide) are both used to reduce the acidity of soil
on land,
they are both faster and stronger
acting than limestone powder. They are also used to reduce acidity in
lakes and rivers due to acid rain. They are also used to
neutralise potentially harmful industrial acid waste
including sulphur dioxide in the flue gases of power stations.
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In the test for
carbon dioxide, calcium hydroxide solution (limewater) forms a
white milky precipitate of calcium carbonate (back to
where you started!).
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Formulae of
magnesium and calcium compounds (M = metal = Mg or Ca, same
group 2, same formula!)
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IONS:
The metal ion in aqueous solution or solid compounds is M2+,
which combines with other
ions such as: oxide O2-, hydroxide OH-,
carbonate CO32-, hydrogencarbonate HCO3-,
chloride Cl-, sulphate SO42-,
nitrate NO3- to form the calcium or
magnesium compounds.
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COMPOUND FORMULAE:
oxide MO, hydroxide M(OH)2, carbonate MCO3,
hydrogencarbonate M(HCO3)2, chloride
MCl2,
sulphate MSO4, nitrate M(NO3)2
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The oxides and
hydroxides readily react with acids.
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general word equation:
oxide or hydroxide + acid ==> salt
+ water
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examples ...
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calcium oxide +
hydrochloric acid ==> calcium chloride + water
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magnesium
hydroxide + nitric acid ==> magnesium nitrate + water
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calcium hydroxide
+ sulphuric acid ==> calcium sulphate + water
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since hydrochloric
acid gives a chloride salt, nitric acid gives a nitrate
salt, sulphuric acid a sulphate salt ... the symbol equations are
... where M = Mg or Ca (or any other Group 2 metal)
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MO(s)
+ 2HCl(aq)
==> MCl2(aq) + H2O(l)
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MO(s)
+ 2HNO3(aq)
==> M(NO3)2(aq) + H2O(l)
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MO(s)
+ H2SO4(aq) ==> MSO4(aq/s*)
+ H2O(l)
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if M(OH)2
involved, there is a 2H2O at the end NOT a single H2O
to balance the symbol equation
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M(OH)2(s)
+ 2HCl(aq)
==> MCl2(aq) + 2H2O(l)
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M(OH)2(s)
+ 2HNO3(aq)
==> M(NO3)2(aq) + 2H2O(l)
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M(OH)2(s)
+ H2SO4(aq)
==> MSO4(aq/s*)
+ 2H2O(l)
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*
the
sulphates of e.g. calcium and barium are not very soluble and this
slows the reaction down!
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Solubility of calcium
compounds (and the chemically similar magnesium):
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Magnesium
and calcium oxides or hydroxides are slightly soluble in water
forming alkaline solutions. They readily react and
dissolve in most acids (see above).
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Magnesium and calcium
carbonate are insoluble in water
but readily dissolve in most
dilute acids
like hydrochloric, nitric and sulphuric.
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Equation examples for calcium carbonate (similar for magnesium carbonate)
...
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calcium carbonate
+ hydrochloric acid ==> calcium chloride + water + carbon
dioxide
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calcium carbonate + nitric acid ==>
calcium nitrate + water + carbon dioxide
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calcium carbonate + sulphuric acid ==>
calcium sulphate + water + carbon dioxide
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CaCO3(s)
+ H2SO4(aq)
==> CaSO4(aq,s)
+ H2O(l)
+ CO2(g)
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Calcium
carbonate reacts slowly in dilute sulphuric acid because
calcium sulphate is not very soluble and coats the limestone.
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Magnesium and calcium
hydrogencarbonate are soluble in water
and cause 'hardness' i.e. scum
with 'traditional' non-detergent soaps. Formulae are Mg(HCO3)2
and Ca(HCO3)2
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Cement
is produced by roasting a mixture of powdered limestone with powdered clay*
in a rotary kiln. When cement is mixed with water, sand and crushed
rock, a slow chemical reaction produces a hard, stone-like building
material called concrete.
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Glass
is
made by heating together a mixture of limestone (CaCO3), sand (mainly
silica = silicon dioxide = SiO2) and 'soda' (sodium carbonate, Na2CO3).
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Limestone
is used
to remove acidic oxide impurities in the extraction of
iron and in making steel.
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Calcium oxide and calcium hydroxide
also react with acids to form
salts. You will find details of this kind of reaction on the Acids
and Bases page.
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Limestone and hard/soft
water are covered on the Extra
Aqueous Chemistry page.
-
 multiple
choice test on limestone etc.
|
Other thermal decompositions
(not
needed by some syllabus's)
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