Download CH152 Essay – Aluminium

Matt MacCourt
CH152 Essay – Aluminium
The element aluminium, can be found in group 3 of the periodic table, and has an
atomic number of 13. It is one of the ‘poor metals’, metals found in the p block of the
periodic table, and therefore not transition metals. It is the third most abundant
element in the earth’s crust, and the most abundant metallic element. However due to
its strong affinity to oxygen, it is not found in its pure form but as oxides and silicates.
Subsequently large scale manufacture of aluminium has only been possible for the
past couple of centuries as the discovery of electrolytic process was required to purify
large amounts of ore. It has only been in commercial production for 146 years and yet
more is produced annually than all other non-ferrous metals combined. It has many
uses in the modern world, due to its properties of being light and strong. It is therefore
used in products ranging from drinks cans to aeroplanes, and only iron is more
commonly used than aluminium.
In its appearance it is a dull silvery colour, attributed to oxidation that occurs quickly
when it is exposed to air. It has a light weight, 2.7 g/cm3, roughly a third of steel and
can be used for many of the applications of steel. Due to its naturally occurring
protective oxide coating it is also highly corrosion resistant, and this can be increased
further by different techniques. It is also a very effective thermal and electrical
conductor, relative to weight it is almost twice as good as copper, it displays good
reflectivity of light and heat which leads to further uses. Aluminium is found to be
completely impermeable, odourless and non-toxic which amongst other uses allows it
to be utilised with sensitive products, such as food. As a metal it is highly malleable
and ductile and can be easily machined. Most of its physical properties can be
improved by alloying it with other elements, such as copper or magnesium.
The chemical properties of aluminium are varied, it has an atomic mass of 26.98
g/mol, an electron configuration of [Ne] 3s2 3p1, and melting and boiling points of
933.47 K and 2792 K respectively. It also has 3 oxidation states, depending on the
compound it is found in, examples are given below:
Oxidation state 1 Al2O, made by heating Al2O3 with silicon at 1800ºC
Oxidation state 2 AlO, shown to be present when Al powder burns in oxygen
Oxidation state 3 Al2O3, found naturally as corundum or can be made by burning Al
in oxygen
Aluminium also has an electronegativity of 1.61 on the Pauling scale. Which is more
positive than transition elements but less so than the metals of groups 1 and 2. As an
element aluminium atoms are arranged in a crystal structure, which consists of a unit
cell which describes the position of the atoms in a repeating structure called a lattice.
The atoms in aluminium have a crystal system of cubic face-centered, which is shown
in the diagrams:
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Cubic Face-Centered
Crystal System
Aluminium is also paramagnetic, meaning it acts like a normal magnet when
subjected to a magnetic field. This is caused by the atoms having permanent dipole
moments even in the absence of a magnetic field, this is typically the result of a
partially filled electron shell, such as the one found in aluminium. There are nine
known isotopes for aluminium, with mass numbers from 23-30, however, only two
occur naturally. Al-27 is a stable isotope and is found with a natural abundance of
100%, Al-26 is a radioactive isotope with a half-life of 7.2x105 years, and it is only
produced in the atmosphere from argon. There are also many uses for the varying
isotopes of aluminium, adding to its usefulness.
The applications of aluminium are at least as widespread as virtually any other
element. In its purest form it has a low tensile strength but can be easily combined
with other elements to form stronger alloys, so much so that aluminium is readily used
in aircraft and rockets. In fact, it is used in so many industries that it would be tedious
to list them all, but some of the most common are in drinks cans and food packaging,
in almost all forms of transportation, such as the construction of cars, in machinery
and even as paint. Its properties of high strength and lightweight are one of the key
reasons to its continued usage, also, its natural resistance to corrosion, due to the
protective layer of oxidation that forms, leads to many uses in construction. Anodised
aluminium can also be made which has an even greater resistance to corrosion. There
are many uses for the various compounds of aluminium too, for example, aluminium
oxide can be found naturally as corundum, otherwise known as rubies and sapphires,
and is used in glass-making. Synthetic versions are also used in lasers.
The main use of aluminium, however, is in the engineering industry, whether it is
being used to build anything from space rockets to bicycles. Aluminium is an
attractive metal to the user as its use can reduce the weight of a component by two
thirds in comparison to iron or steel. But this also introduces a two thirds reduction in
rigidity, which can lead to problems as the part will be far more flexible under
pressure and this is undesirable if it is bending the wrong way. When machined into
tubing, it has the ability to withstand enormous weight or pressure along one axis, for
example, a soft drink can can be easily bent and crushed by putting pressure on the
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sides, but it is more difficult if pressure is applied from end to end. Obviously this has
the potential to cause problems with the chance of a component failing if the pressure
isn’t straight down the axis.
Another property that can cause problems is the relatively low melting point, when
compared to other metals. This makes welding a more difficult procedure as
aluminium will melt before it becomes red hot, so the welder has no visual indication
of when it is going to melt. The result is that mistakes can happen which is quite
inconvenient if the part you are welding suddenly becomes a pool of molten metal.
In the past aluminium has seen usage as electrical wire, replacing copper. This has
become unpopular though as connections became loosened due to the greater degree
of expansion of aluminium under heat, also galvanic corrosion between dissimilar
metals caused higher resistance and this sometimes lead to fires. However, modern
connections have been designed to prevent loosening and aluminium can be used
safely.
The first recorded mention of aluminium is suspected to have been made in Pliny the
Elder’s Naturalis Historia:
"One day a goldsmith in Rome was allowed to show the Emperor Tiberius a dinner plate of a
new metal. The plate was very light, and almost as bright as silver. The goldsmith told the
Emperor that he had made the metal from plain clay. He also assured the Emperor that only
he, himself, and the Gods knew how to produce this metal from clay. The Emperor became
very interested, and as a financial expert he was also a little concerned. The Emperor felt
immediately, however, that all his treasures of gold and silver would decline in value if
people started to produce this bright metal of clay. Therefore, instead of giving the goldsmith
the regard expected, he ordered him to be beheaded." 1
Although it obviously cannot be proven that this metal was in fact, aluminium, it
seems reasonably likely that it was as aluminium has a history of being treated as a
valuable metal. In fact, Napoleon was known to allow his most honoured guests the
use of his best cutlery, made of aluminium, whereas the rest had to make do with gold
and silver. This is strange that the third most abundant element in the earth’s crust was
once treated as the most valuable, but it is because the technology to extract
aluminium on a large scale has only existed for a relatively short period of time. It had
been discovered that the substance alumine was the oxide of an undiscovered metal by
the time Sir Humphrey Davy proposed the name ‘aluminum’ in 1808. This was later
changed to aluminium, still, it was not until 1825 that an impure form of aluminium
was produced by the Danish physicist and chemist, Hans Christian Orsted. The major
breakthrough in terms of aluminium’s place in industry occurred in 1886, when two
scientists simultaneously discovered an electrolytic process to extract pure aluminium
from the bauxite ore it is found in. It was named the Hall-Héroult process, after
Charles Martin Hall and Paul Héroult, and is still the most common method of
producing commercial quantities of aluminium. Below is a simple diagram of the
apparatus used in the process:
1 http://www.world-aluminium.org/history/antiquity.html
02/12/2005
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Alumina (Al2O3) is dissolved in a bath of molten cryolite, this greatly reduces the
melting point and allows the process to take place at much lower temperatures. As the
mixture is electrolysed, aluminium forms at the carbon cathode and as it has greater
density than the cryolite it sinks to the bottom and can be run off. The carbon anodes
are oxidised and slowly erode as carbon dioxide is given off, thus the chemical
equation is:
2 Al2O3 + 3 C → 4 Al + 3 CO2
The electrical resistance that occurs in the bath is found to cause enough heat to keep
the cryolite molten.
Another important part of the aluminium industry is recycling. It actually requires less
energy to recycle aluminium than to extract it in the first place, this is because the
electrolytic process needs huge amounts of electricity, and recycling simply requires
melting. It is 100% recyclable and uses 5% of the original energy to purify it. This is
one of the main reasons why aluminium is one of the most useful elements around,
and why it is one of the most commonly produced. Which is why, of course, it will be
one of the most important elements in the near future.
References (all accessed on 02/12/2005):
http://www.world-aluminium.org/history/index.html
http://en.wikipedia.org/wiki/Aluminium
http://en.wikipedia.org/wiki/Hall-Heroult_process
http://www.alunet.net/ShowCats.asp?bhist=0&catID=X&type=3
http://www.c-a-b.org.uk/public/alumin.htm
http://www.physchem.co.za/Redox/Graphics/GRDE0050.gif
http://www.webelements.com/webelements/elements/text/Al/xtal.html
http://www.world-aluminium.org/production/mining/index.html
http://www.world-aluminium.org/production/recycling/index.html
Catherine E Housecroft & Edwin C Constable, Chemistry, Pearson Education Ltd,
2006
Matt MacCourt
1 http://www.world-aluminium.org/history/antiquity.html
02/12/2005