ch1 metals from rocks

7/30/2019 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).

ch1 metals from rocks

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