Transcript Changing Materials

Changing Materials
Contents
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Useful Products from Oil
Useful Products from Metal Ores
Useful Products from Rocks
Useful Products from Air
Representing Reactions
Quantitative Chemistry
Changes to the Earth and Atmosphere
The Rock Record
Useful Products From Oil
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What is crude oil?
Fractional distillation
Cracking and its products
Fuels
Plastics
What is crude oil?
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Crude oil is a mixture of a very large number of compounds.
It is formed from the remains of plants and animals which died millions of
years ago. This is why it is called a fossil fuel.
Most of the compounds in crude oil consist of molecules made up of
hydrogen and carbon atoms only, we call these type of compounds
hydrocarbons.
We represent hydrocarbons in the following ways:
Fractional Distillation
The many hydrocarbons in crude oil may be separated into
fractions, each of which contains molecules of a similar size, by
evaporating the oil and allowing it to condense at a number of
different temperatures. This process is called fractional distillation.
Cracking and its Products
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There is a much greater demand for shorter hydrocarbon than there is for the longer
hydrocarbons.
Long chain hydrocarbons can be broken into smaller hydrocarbons, by heating with a catalyst.
This is a thermal decomposition reaction known as cracking.
Cracking produces two types of hydrocarbon
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Alkanes with only single covalent bonds
Alkenes with one or more double covalent bonds
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Test for alkenes: alkenes turn bromine water from brown to colourless.
Fuels
• Most fuels contain carbon and/or hydrogen and may also
contain some sulphur. The gases released into the
atmosphere when a fuel burns may include:
• carbon dioxide;
• water (vapour), which is an oxide of hydrogen;
• sulphur dioxide
– This gas dissolves in rain and forms acid rain.
Plastics
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Alkenes are reactive and so are useful for making many other substances including
polymers. Polymers have very large molecules. They are formed when many small
molecules join together. This process is called polymerisation.
When alkenes join together to form a polymer with no other substance being
produced in the reaction, the process is called addition polymerisation.
Plastics are polymers and are made by polymerisation.
For example, poly(ethene) (often called polythene) is made by polymerising the
simplest alkene, ethene.
Useful Products from Rocks
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Metal Ores and Reactivity
The Blast Furnace
Extraction of Aluminium
Purification of Copper
Corrosion
Metals Ores and Reactivity
• Rocks from the Earth contain many useful metals.
• Most metals are combined with other elements in materials called
ores and have to be extracted using various methods.
• How each metal is extracted depends on how reactive it is.
• Gold is a very unreactive metal and is found as a pure metal,
because of this it has been in use for many thousands of years
despite of being a very rare metal.
• Iron and copper are more reactive than gold but less reactive than
carbon, these can be extracted from their ores by simply heating
with coke these have been known for several thousand years
• Aluminium is the most common metal in the Earth’s crust, however,
was only discover 200 years ago because it is a relatively reactive
metal which is hard to extract from its ore.
Extracting iron using the blast
furnace.
•A metal such as iron, which is less
reactive than carbon, can be extracted
from its ore using carbon
•Reactions in the blast furnace.
•C + O2  CO2
•The coke burns, to form carbon dioxide
and to produce heat
•CO2 + C  2CO
•The carbon dioxide reacts with more hot
coke to produce carbon monoxide gas.
•3CO + Fe2O3  2Fe + 3CO2
•The carbon monoxide removes the
oxygen from the iron ore this is called
reduction.
•The main impurity in the iron ore is silica
this reacts with the limestone to produce
slag (calcium silicate)
Extraction of Aluminium
•Aluminium is made by the
electrolysis of bauxite.
•Cryolite is added to lower the
melting point of bauxite.
•This is a very expensive process
and aluminium is only made in
this way because it cannot be
prepared by heating with carbon
because it is too reactive
•At the negative electrode
•Al3+ + 3e-  Al
(REDUCTION)
•At the positive electrode
•2O2-  O2 + 4e(OXIDATION)
Purification of Copper
• Copper can be extracted from its ore by reduction with
carbon, however, this is only 98% pure.
• Copper can be purified by electrolysis using a positive
electrode made of the impure copper and a negative
electrode of pure copper in a solution containing copper
ions.
• When the current is switched on copper ions in solution
are attracted to the negative and electrode and
deposited there. Copper atoms in the impure block lose
electrons and become positive ions and go into the
solution, replacing those which were deposited at the
negative electrode.
• Eventually the impure block disappears leaving behind
the impurities and the pure block becomes larger.
Corrosion
• Iron rusts in the presence of
oxygen and water.
• Barriers such as paint or
grease can be used to prevent
corrosion.
• Zinc blocks can be attached to
iron objects. As zinc is more
reactive than iron it will corrode
preferentially thus preventing
the iron from corroding. This is
called sacrificial protection
Aluminium is a very reactive metal but it can be used without
protection against corrosion. This is because it has a thin lay of
oxide which sticks very firmly to the aluminium and protects it
against further corrosion.
Limestone
Useful Products from Air
• Manufacture of ammonia from air
• Manufacture of fertiliser from ammonia
• Problems caused by the over use of
fertilisers.
Manufacture of ammonia from air
•Nitrogen and Hydrogen are needed to make Ammonia.
•Nitrogen is obtained from the air.
•Hydrogen is obtained from water and natural gas.
•The Haber process is a reversible reaction
•This means that the reaction occurs in both directions
•High pressures favour the production of ammonia,
however it expensive to make industrial equipment to
cope with high pressures.
•Low temperatures favour the production of ammonia,
however at low temperatures the reaction would be too
slow to be commercially viable.
•The Haber process makes a compromise with these
two and recycles the unreacted hydrogen and nitrogen
N2(g) + 3H2(g) 2NH3(g)
Manufacture of fertiliser from
ammonia
Problems cause by the over use of
fertiliser
• Plants need several types of nutrients. Nitrogen based
nutrients are used to make proteins.
• Farmers can use natural sources of these nutrients or
synthetic nutrients.
• If fertilisers are over used the excess can wash into
streams. Plants and green algae grow out of control.
When they, die bacteria feed off of the dead plant
material The bacteria increase in number, they use up all
the oxygen in the water. Then the fish die. This process
is called eutrophication.
• Too many nitrates in the drinking water can also cause
problems. It can interfere with the blood’s ability to carry
oxygen. This can be especially severe in children and
babies, causing them to turn ble and even die.
Representing Equations
• When we can represent reactions by word and
symbol equations:
• methane + oxygen carbon dioxide + water
• or CH4 + O2  CO2 + H2O
• To this we can add state symbols to give more
information about the substances:
• (s) – solid, (l) – liquid, (g) – gas, (aq) – aqueous
• CH4(g) + O2(g)  CO2(g) + H2O(g)
• This doesn’t tell us the whole story we need to
balance the equation to show that we have not
destroyed or made new atoms.
Balancing Equations
• CH4(g) + O2(g)  CO2(g) + H2O (g)
• Remember that the formulae for each compound is
correct you cannot change CH4 to CH3 just to make the
atoms add up.
• Balance one type of atom at a time:
• There is one carbon atom on each side so we can leave
that alone, however there are 4 H atoms on the left hand
side and 2 on the right hand side we can correct this by
putting a 2 in front of the water.
• CH4 (g) + O2 (g)  CO2 (g) + 2H2O (g)
• Now both the carbon and the hydrogen balance, that just
leaves us with the oxygen. There are 2 O on the left
hand side and 4 on the right hand side. We can correct
this by putting a 2 in front of the oxygen on the left hand
side:
• CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g)
Quantitative Chemistry
• Calculating Masses
• The Mole
• Reacting Masses
Calculating Masses
• Relative Atomic Mass
– This is the mass number that you find on the
periodic table.
• Relative Molecular Mass
– This is the sum of all the
relative atomic masses of all of the atoms in a
compound e.g.
The Mole
• One mole of atoms or molecule of any substance will
have a mass in grams equal to the relative atomic mass
or relative molecular mass for that substance.
• The atomic mass of carbon is 12, therefore one mole of
carbon weighs 12g
• The relative molecular mass of oxygen (O2) is (2 x 16)
23, therefore one mole of oxygen weighs 32g.
• One mole of any substance contains 6.02 x 1023 atoms
or molecule.
• Remember the mole is just a number!
Reacting Masses
• By using the relative molecular masses in grams
we can deduce what masses of reactants to use
and what mass of products will be formed.
Changes to the Earth and
Atmosphere
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4.5 billion years ago
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3 billion years ago
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The first plant produced oxygen, this killed off many other life forms.
1 billion years ago
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The Earth cooled enough for the steam to condense and form the oceans. The main gas in
the atmosphere was carbon dioxide. Simple plants evolved.
2 billion years ago
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When the Earth first formed, its surface was molten. As it cooled surface rocks formed. An
atmosphere formed from volcanic gases: carbon dioxide, steam, ammonia and methane.
The excess oxygen in the atmosphere reacted in the presence of UV light to form ozone.
This ozone filtered out most of the damaging UV light and allowed more complicated life
forms to evolve.
The present time
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The atmosphere is approximately 1/5 oxygen and 4/5 nitrogen.
carbon dioxide is absorbed by plants, shellfish and dissolved in the oceans.
It is given out by burning fuels, the decay of organic waste and released from volcanoes.
ammonia in the air reacted with oxygen to form nitrogen. More nitrogen was formed by
bacteria in the soil.
The rock record
Summary
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Useful Products from Oil
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Useful Products from Metal Ores
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Practice lots of these problems.
Changes to the Earth and Atmosphere
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practice balancing equations
Quantitative Chemistry
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Nitrogen from the atmosphere can be turned into synthetic fertiliser. Overuse cause
eutrophication
Representing Reactions
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Limestone can be used as building material or as a raw material in cement, concrete and
glass.
Useful Products from Air
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how we obtain metals depend on how reactive the are
Useful Products from Rocks
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Crude oil is a mixture of hydrocarbons. The can be separated and processed by cracking
and polymerisation to make a variety of useful materials
The first atmosphere was mainly carbon dioxide. Plant produced all the oxygen in the
atmosphere. This oxygen formed ozone which protects us from uv light.
The Rock Record
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Rocks contain evidence of how they were formed. Different types of rock are linked in the
rock cycle.