Changing Materials

Contents

• 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

• What is crude oil?

• Fractional distillation

• Cracking and its products

• Fuels

• Plastics

What is crude oil?

• 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 DistillationThe 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• 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

– Alkanes with only single covalent bonds– Alkenes with one or more double covalent bonds

•

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• 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

• 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!

• 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.

Reacting Masses

Changes to the Earth and Atmosphere

• 4.5 billion years ago– 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.• 3 billion years ago

– 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– The first plant produced oxygen, this killed off many other life forms.

• 1 billion years ago– 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– 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• Useful Products from Oil

– Crude oil is a mixture of hydrocarbons. The can be separated and processed by cracking and polymerisation to make a variety of useful materials

• Useful Products from Metal Ores– how we obtain metals depend on how reactive the are

• Useful Products from Rocks– Limestone can be used as building material or as a raw material in cement, concrete and

glass.• Useful Products from Air

– Nitrogen from the atmosphere can be turned into synthetic fertiliser. Overuse cause eutrophication

• Representing Reactions– practice balancing equations

• Quantitative Chemistry– Practice lots of these problems.

• Changes to the Earth and Atmosphere– 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

– Rocks contain evidence of how they were formed. Different types of rock are linked in the rock cycle.