Form 4 / Combined Science (Chemistry) / Reading Assignment 3 / 1
Heep Woh College
F.4 Combined Science (Chemistry Part) Reading Assignment 3
Group Name: ______________________________
Class: 4 (
)
Group Member: 1. _______________________ (
) 2. _______________________ (
)
3. _______________________ (
) 4. _______________________ (
)
Part I:
Read the following passage and answer the questions below.
Extraction of Iron
Iron is the second-most abundant metal in the Earth's crust after aluminium. It is one
of the most commonly used metals in the modern world. Iron as a metal in
elemental form is rarely used on its own. Most of the iron extracted today is
converted to steel, an alloy of iron and carbon, which proves to be more useful
than iron. Steel has good domestic as well as industrial use, mainly because it does
not corrode easily, and because of its high tensile strength. It is far less brittle than
iron.
The common ores of iron are hematite [Fe2O3], limonite [Fe2O3]•xH2O, magnetite
[Fe3O4] and siderite [FeCO3]. Iron from hematite is usually extracted through the
carbon reduction process.
The iron ore with carbon in the form of coke (once charcoal) and limestone are
added to a blast furnace (temperatures of at least 1300°C, but now usually 2000°C).
The product of the blast furnace process is not pure iron, but pig iron which
contains 4-5% carbon and silicon, which must be removed in further processes. An
earlier process (which did produce fairly pure wrought iron) used a bloomery,
where the iron was kept in the solid state throughout, but this was gradually
abandoned because it could not easily be scaled up.
The steps in the extraction of iron by carbon reduction method are:
1. Hot air is pumped into the blast furnace through the bottom. The carbon reacts
with the oxygen to produce carbon dioxide.
2.
After carbon dioxide is formed, it reacts with coke (contains mainly carbon) to
form carbon monoxide - the main reducing reagent in the furnace.
3.
The carbon monoxide in the blast furnace reacts with the hematite (iron(III)
oxide). This occurs since carbon monoxide reacts with the oxygen, and with this
compound forms carbon dioxide. This effectively reduces the iron oxide as the
iron gains three electrons in the process and becomes iron atoms.
4.
While the iron is being extracted, the limestone flux reacts with the impurities in
the ore and melts them to form slag, which effectively prevents the impurities
from affecting the reduction of the iron ore.
CaCO3 → CaO + CO2
CaO + SiO2 → CaSiO3
[slag]
S.Mo & C.K.Lau
Form 4 / Combined Science (Chemistry) / Reading Assignment 3 / 2
Questions
Write word equations and balanced chemical equations for the following
processes:
1.
Hot air is pumped into the blast furnace through the bottom. The carbon reacts
with the oxygen to produce carbon dioxide.
____________________________________________________________________________
____________________________________________________________________________
2.
After carbon dioxide is formed, it reacts with coke (contains mainly carbon) to
form carbon monoxide - the main reducing reagent in the furnace.
____________________________________________________________________________
____________________________________________________________________________
3.
The carbon monoxide in the blast furnace reacts with the hematite (iron(III)
oxide). This occurs since carbon monoxide reacts with the oxygen, and with this
compound forms carbon dioxide. This effectively reduces the iron oxide as the
iron gains three electrons in the process and becomes iron atoms.
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____________________________________________________________________________
End of Part I
Part II:
Read the following passage and answer the questions below.
Production of aluminium from aluminium ore
Aluminium is the most abundant metal in the Earth’s crust. Pure aluminium can be
produced from the chief ore of aluminium – bauxite. It is a mixture of hydrated
aluminium oxide and hydrated iron oxide.
Metallic aluminium has many properties that make it useful in a wide range of
applications. It is lightweight, strong and non-toxic. The surface of aluminium quickly
oxidizes to form an oxide layer that can resist corrosion. Furthermore, aluminium can
be recycled easily.
S.Mo & C.K.Lau
Form 4 / Combined Science (Chemistry) / Reading Assignment 3 / 3
History of aluminium extraction
In 1824, Hans Oersted, a Danish chemist, extracted aluminium using a three-step
process.
Step 1
Prepare aluminium chloride by passing chlorine gas over a very hot mixture
of aluminium oxide and carbon.
Step 2
Heat a mixture of aluminium chloride and potassium amalgam (an alloy of
potassium and mercury), which produces potassium chloride and
aluminium amalgam.
Step 3
The amalgam is distilled under vacuum. Aluminium metal is left as a residue
and liquid mercury is collected.
In 1827, Friedrick Wohler repeated Oersted’s work and produced aluminium
successfully when he replaced potassium amalgam with pure potassium metal.
In 1886, Charles Martin Hall from the United States and Paul Heroult from France
independently discovered and patented an aluminium production process. In the
process, aluminium oxide obtained from bauxite is dissolved in molten cryolite and
decomposed by electricity. The Hall-Heroult process remains the only method by
which aluminium is produced industrially.
Industrial production of aluminium
The first step in the industrial production of aluminium is the separation of aluminium
oxide from the iron oxide in bauxite. This is done by dissolving the mixture of
aluminium oxide and iron(II, III) oxide in a concentrated sodium hydroxide solution.
The aluminium ion forms a soluble complex ion with the hydroxide ion, while the iron
ion does not.
Al2O3(s) + 2NaOH(aq) + 3H2O(l) → 2NaAl(OH)4(aq)
After the insoluble iron(II, III) oxide is filtered from the solution, aluminium hydroxide is
precipitated from the solution by adding acid to lower the pH to about 6. Then the
precipitate is heated to produce dry aluminium oxide.
2Al(OH)3(s) → Al2O3(s) + 3H2O(l)
The extraction of aluminium from the oxide is done by electrolysis, but first the
aluminium oxide must be made molten so that electricity can pass through it.
Aluminium oxide has a very high melting point (over 2000oC). It would be expensive
to melt aluminium oxide. So, instead, aluminium oxide is dissolved in molten
cryolite – an aluminium compound with a lower melting point. As the melting point
of the mixture of aluminium oxide and cryolite is lower, less fuel is needed. Thus, the
use of cryolite reduces the energy cost of the extracting process.
S.Mo & C.K.Lau
Form 4 / Combined Science (Chemistry) / Reading Assignment 3 / 4
Questions:
1.
What type of reaction did Oersted carry out when he produced aluminium?
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2.
Why do you think Oersted decided to use a potassium amalgam rather than a
metal such as iron or copper?
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____________________________________________________________________________
3.
What environmental risks did Oersted’s experiment pose?
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4.
Do you think Wohler’s method was cleaner than Oersted’s method? Give a
reason for your answer.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
S.Mo & C.K.Lau
Form 4 / Combined Science (Chemistry) / Reading Assignment 3 / 5
5.
In 1886, using electricity to extract aluminium was welcomed because it was
cheaper and produced purer aluminium than previous methods. Do you think
this method was cleaner than previous ones? Give reasons for your answer.
____________________________________________________________________________
____________________________________________________________________________
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6.
Write an ionic half-equation for the reaction that occurs at the cathode in the
electrolytic cell.
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7.
Why does the anode sometimes have to be replaced?
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The End
S.Mo & C.K.Lau