Form 4 / Chemistry / Reading Assignment 2 / 1
Heep Woh College
F.4 Chemistry Reading Assignment 2
Group Name: ______________________________
Class: 4 (
)
Group Member: 1. _______________________ (
) 2. _______________________ (
)
3. _______________________ (
) 4. _______________________ (
)
Part I:
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.
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.
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 2 / 2
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 / Chemistry / Reading Assignment 2 / 3
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.
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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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End of Part I
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 2 / 4
Part II:
Read the following passage and answer the questions below.
Why are Alkaline Cells Better than the Zinc-carbon Cells?
Alkaline cells are also called alkaline manganese cells. Compared with
zinc-carbon cells (i.e. ordinary dry cells), alkaline cells have larger current and
longer shelf life. They are durable and their case resists corrosive. All these
advantages are due to the special structure and materials used in alkaline cells.
In a zinc-carbon cell, the conducting material of the positive electrode is a
carbon-rod while that of the negative electrode is a zinc case. The zinc case is a
cylinder which acts as the container of the cell. The space between the case and
the carbon rod is filled with ammonium chloride, manganese(IV) oxide, etc. In an
alkaline cell, the positive electrode is manganese(IV) oxide. Although steel acts as
the container of the cell, it does not take part in chemical reactions. Therefore, the
outer case of alkaline cells is very hard and resistant to corrosion. It prevents
leakage and has a longer shelf life. The negative electrode in alkaline cells is also
zinc. It is in powdered form, which is different from the zinc cylinder in ordinary dry
cells. The zinc powder is wrapped by cellulose paper. It is soaked in the electrolyte,
potassium hydroxide. A metal rod is located at the center of the zinc powder (also
the center of the cell). Its end is connected to a steel disc.
Since the potassium hydroxide used in alkaline cells is in a liquid form, which is
different from the paste in zinc-carbon cells, their resistance is smaller. Apart from
that, zinc takes part in the reaction in powdered form, so the current produced is 3
to 5 times greater than that produced by zinc-carbon cells of the same size.
Moreover, upon discharging, zinc-carbon cells produce gases inside the cell but
alkaline cells do not. So the voltage of alkaline cells is more stable. In alkaline cells,
most fillings are involved in chemical reactions, so it can be made smaller in size. In
other words, when comparing alkaline cells with zinc-carbon cells of the same size,
the former has greater charge capacity and longer life expectancy.
Alkaline cells are better than zinc-carbon cells in different aspects. Nevertheless,
the requirement regarding techniques and materials used in manufacturing
alkaline cells are stricter. Hence, they are more expensive. According to the
properties of alkaline cells, we have to pay attention to the following when using
them. First, since alkaline cells are not secondary cells, they are neither
rechargeable after use nor heated. Second, their case can be neither broken
down nor punched. This is because the alkaline solution inside is highly corrosive to
skin and clothes.
S.Mo & C.K.Lau
Form 4 / Chemistry / Reading Assignment 2 / 5
Questions
1.
What are the conducting materials of the electrodes of zinc-carbon cells and
alkaline cells?
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2.
Why is the outer case of an alkaline cell harder than that of a zinc-carbon cell?
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3.
Why is the current produced by alkaline cells 3 to 5 times greater than that
produced by zinc-carbon cells of the same size?
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4.
Why is the voltage produced by alkaline cells more stable than that produced
by zinc-carbon cells?
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5.
What is the advantage of zinc-carbon cells over alkaline cells?
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The End
S.Mo & C.K.Lau