CH1 Metals From Rocks
Transcript of CH1 Metals From Rocks
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All the metals we see in use around us came originally from the Earths crust.
Most metals are present in the crust as compounds e.g. metal oxides. The metalhas to be extracted and refined.
The least reactive metals (Platinum, Gold, Silver) can be found as the nativemetal, not chemically combined with anything else.
Definition: Rocks which containenoughof a metal compound to beworthextractingthe metal, are calledores.
Gold as a native metal
Malachite is an orecontaining copper
Bauxite is an ore fromwhich aluminium can
be extracted
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Getting the metal out of the ore requires some chemistry. If the metal is lessreactive than carbon, then carbon can be used to displace the metal.
e.g. lead oxide + carbon lead + carbon dioxide
2 PbO + C 2 Pb + CO2
Reduction is a reaction in which oxygen is removed.
We say that the lead oxide has been reduced by the
carbon. (We can also say that the carbon has beenoxidised, since it gained the oxygen)
Carbon is used to do these reactions because it is cheapand readily available coke (from coal) is mostlycarbon, and charcoal (from wood) can also be used.
In general, any more reactive metal (or carbon) can
displace a less reactive metal from its compounds
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Iron is a key metal in our society. Its most common use is asthe main component in steel. It is produced using a blast furnace.
Iron ore (haematite) contains ironoxide, from which the iron is extracted.
Coke is used as a source of carbon to
reduce the iron ore, and limestone isadded to remove impurities.
A blast ofhot air provides the oxygenrequired.
The iron which is made is called castiron. It is molten, so it can be removedfrom the bottom of the blast furnace and
poured into moulds to solidify.
haematite
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Molten iron from the blast furnace can be cooled in moulds
and solidified. The pig iron is very brittle and not muchuse. It contains about 4% carbon and 96% iron.
Pig iron can be purified to make pure iron. The atoms in
pure iron are arranged in regular layers, which can easilyslide over each other. This makes the pure iron very softand easily shaped too soft for most uses. Molten iron poured
into moulds
Pig iron is very brittle
To make iron more useful, we add small amounts of otherelements. This has the effect of disrupting the regularstructure of iron atoms, so the layers cant slide over each
other. The resulting material is much stronger.
We call a metal that has had other elements added an alloyIron that has been alloyed with other elements is called steel.
atoms inan alloy
atoms in
pure iron
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Type of steel Properties Uses
Carbon Steels
(0.03 1.5% carbon)
Low Carbon - Soft, not easilyshattered, malleable
High Carbon Very strongbut brittle
Knives, machinery, ships,
containers, structural steel for
buildings
- Cheap
Mild steel
(one type of low carbon steel
with less than 0.1% carbon)
very easily pressed into shape Mass production, e.g. making
car bodies
Cheap
Low alloy nickel
(1-5% nickel)
Very resistant to stretching
(high tensile strength)
Long span bridges, bicycle
chains, armour plating- Moderate cost
Low alloy tungsten
(1-5% tungsten)
Hard, works well at high
temperatures
High-speed tools
- Moderate cost
High alloy chromium(12-15% chromium)
Strong and chemically stable - Expensive
Stainless steels
(chromium nickel steels)
Hard, strong, corrosion
resistant
Cooking utensils, cutlery,
chemical reaction vessels
- Expensive
- Dont rust
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Rusting occurs when iron reacts with oxygen in the air,in the presence ofwater. Rust is hydrated iron oxide.
Many steels (except the most expensive ones) will rust
because they contain a high proportion of iron, forexample the mild steel used in car bodies.
Rusting can be prevented in two ways:
- Providing a protective layer e.g. of paint, or oil overthe metal to keep it apart from air, water etc.
- Using alloys containing a high proportion of chromium(stainless steels) which makes them resistant torusting.
Metals or alloys which dont contain irondont rust.
Instead if they are attacked by substances such as
oxygen in the air we call this CORROSION.
Once the protectivepaint layer is removed,mild steel rusts readilyThis spanner wont rust
because the steel used isan alloy containing a
high proportion ofchromium and vanadium
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Iron is not the only metal which is alloyed to give
it more useful properties:
Gold can be bought as 24-carat (pure gold)
which is very soft and easily bent. To make itharder it is alloyed with copper or silver. Most
jewellery uses gold alloys rather than pure gold.
Aluminium is a metal with low density usefulfor things which need to be light. It can be
alloyed with a wide range of other elements,giving it different properties. Some are used to
build aircraft, while others can be used as
armour plating.
Pure copper is a very soft metal which
conducts heat and electricity well. Copper isalloyed to make harder materials such as
bronze (an alloy of copper and tin) and brass(an alloy of copper with zinc). Brass is much
harder than copper, and easier to hammer into
sheets and bend into shapes.
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Transition Metals
The Transition Metals are a fairlyunreactive block of elements in
the centre of the Periodic Table.
They have typical metal
properties, and have a widerange of uses that rely on these
properties.
Ni, Co andFe are also
able toformed into
magnets
Transition metals are: very good conductors of electricity
very good conductors of heat
hard, tough strong
easily bent or hammered into useful
shapes (malleable)
able to resist high temperatures (very high melting point)
(Mercury, Hg, is a liquid at room temperature)
Cobalthip joint
Chromiumplating
Vanadiumsteel alloy
Titaniumhip socket
Nickel 5-cent coin
Cast ironiron
Copperwiring
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Copper is used forelectrical wiring and forplumbing pipes.
The reason copper is used for electrical wiring
is not simply because it conducts electricityall metals conduct electricity. It is because it
can be drawn out into wires easily (it isductile), and the wires are fairly resistant to
corrosion. Copper has a high melting pointso the wires can stand getting hot without
melting. Copper does have very good
electrical conductivity. Silver or gold would be
even better, but they are not used for wires
because they are too expensive.
The reason copper is used for plumbing pipes
is because copper can be bent and shapedeasily (malleable). Copperdoesnt react withwater, and resists corrosion fairly well. It canbe used to carry hot liquids because its
melting point is high.
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Copper ores are mined so that the copper can beextracted. Copper is a very useful metal, but copper-rich
ores are now in very limited supply.
Two methods of extraction are used: Copper ore can be heated very strongly to produce crudecopper this is smelting. The copper is then purifiedusing electrolysis. 85% is made this way.
The other 15% of our copper is made by reacting copper
ore with sulphuric acid to make copper sulphate solution.The copper can then be extracted from the siolution by
electrolysis.
During electrolysis the positive
copper ions move toward the
negative electrode, and the puremetal is collected here.
Electrolysis uses huge amounts of electricity, and costs a lot.The heat required for smelting also requires a lot of energy and
this process is therefore expensive.
a copper mine
chalcopyrite,a copper ore
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PhytominingIn the future we will need to extract copper from very low-grade ores (containing only a little copper). Scientists are
developing ways of using plants to extract the copper.
This could be a lot cheaper and a lot greener. Plants takeup the copper from the soil which contains the metal ore,
through their roots. When fully grown, the plant material
containing the copper is burnt, and the ash produced is
smelted to yield the metal.
Bioleaching
Bacteria can also be used to extractmetals from low-grade ores. The
bacteria feed on the ore, releasing the
metal ions into a leachate solutionfrom which they can be collected and
extracted by electrolysis.
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electrolysis ofaluminium ore
Aluminium is a reactivemetal, so we cant extract it withcarbon. Instead we use electrolysis passing an electriccurrent through the molten aluminium oxide at high temperature.
Although aluminium is a reactive metal, it does not corrodeeasily. This is because the aluminium atoms at its surface react
with oxygen in the air to form
a thin, tough oxide layer.This stops further reactions
from taking place.
Titanium is not particularly reactive, so we could displace it from its ore usingcarbon. Howevercarbon reacts with the titanium, making it brittle. Instead weobtain titanium from its ore by displacing it using a more reactive metal, such as
sodium. We get the sodium metal using electrolysis.
Titanium also forms an oxide layer which protects it, like aluminium. This makes itresistant to corrosion.
Remember ! Electrolysis is expensive because we need to use high temperatures
to melt the ores, and a great deal ofelectricity.
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Aluminium and Titanium are used when we need metalsor alloys which are strong but light, and do not corrodeNote: Aluminium is NOT a transition metal !
Aluminium very light (low density)
excellent conductor
protective oxide layer
(resistant to corrosion)
not very strong BUT
alloys are rigid and strong
Uses:
Cans, cooking foil, pans
High voltage cables
Aeroplanes and space vehicles
Titanium very strong
very resistant to corrosion
protective oxide layer
denser than aluminium BUT
less dense than most transition metals
extremely high melting point
Uses:
High performance aircraft bodies
Racing bikes
Jet engine parts
High temp. pipes e.g. nuclear reactorsAlloys for artificial hip joints
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How expensive is metal ?
Metals and metal ores are finite resourcesthey will run out one day. Some metals will run
out sooner than others.
The cost of metal is affected by several things:
How scarce the metal/ore is, and how hard to find
The cost of getting it out of the ground
How expensive it is to extract the metal from its ore and purify it
Al
Recycling metals such as aluminium is important. It is nearlyalways cheaper for us to recycle metals than to extract new
metals from their ores especially when we need to use large
amounts of energy to extract them, such as for aluminium.
Recycling metal conserves resources of metal ores. Because less energy isrequired, it also saves on the use of fuels (conserving crude oil or coal reserves)and reduces the emission of carbon dioxide into the atmosphere (helping to
limit Global Warming).