Extraction of metals

Types of extraction

Methods of extraction

Many metals are found in the Earth's crust as ores. An ore is usually a compound of the metal mixed with impurities. When the metal is dug up, a method must be used to separate the metal from the rest of the ore. This is called extracting the metal.

The method of extraction depends on how reactive the metal is. The more reactive the metal, the more difficult it is to extract from its compound.

Electrolysis

Electrolysis is the most powerful extraction method. But it takes a lot of electricity and that makes it expensive. Hence, electrolysis is only used for the most reactive metals.

Examples of the different methods of extraction

Electrolysis: Used in extracting aluminium and extracting sodium from rock salt.

In the case of the rock salt, it is first melted in giant steel tanks:

The extraction of aluminium is dealt with in a separate quick learn within this topic.

Heating with Carbon monoxide: Used for extracting iron from iron ore using the blast furnace.

Roasting in Air: Used in extracting copper from copper (I) sulphide (copper pyrites).

The copper is extracted by roasting the ore in air.

Recycling metals

Metals are non-renewable resources. This means once dug up it cannot be replaced. Hence, the supply will eventually run out.

For example: it is expected that tin will run out within the next 15 years and copper in the next 40 years!

Therefore, the recycling of these two useful metals and others such as iron and aluminium is most important.

In recycling, metals are melted down before reshaping into their new use. However, this can be costly. Recycling companies will only recycle if it is economical!

The blast furnace

How to extract iron from its ore

The method

Three substances are needed to enable to extraction of iron from its ore. The combined mixture is called the charge:

Iron ore, haematite - often contains sand with iron oxide, Fe2O3.

Limestone (calcium carbonate).

Coke - mainly carbon.

The charge is placed a giant chimney called a blast furnace. The blast furnace is around 30 metres high and lined with fireproof bricks. Hot air is blasted through the bottom.

Several reactions take place before the iron is finally produced.

Oxygen in the air reacts with coke to give carbon dioxide:

The limestone breaks down to form carbon dioxide:

Carbon dioxide produced in 1 + 2 react with more coke to produce carbon monoxide:

The carbon monoxide reduces the iron in the ore to give molten iron:

The limestone from 2, reacts with the sand to form slag (calcium silicate):

Both the slag and iron are drained from the bottom of the furnace.

The slag is mainly used to build roads.

The iron whilst molten is poured into moulds and left to solidify - this is called cast iron and is used to make railings and storage tanks.

The rest of the iron is used to make steel.

The electrolysis of bauxite

How to extract aluminium from its ore

The method

The bauxite (red-brown solid) - aluminium oxide mixed with impurities - is extracted from the earth.

The extracted aluminium oxide is then treated with alkali, to remove the impurities. This results in a white solid called aluminium oxide or alumina.

The alumina is then transported to huge tanks. The tanks are lined with graphite, this acts as the cathode. Also blocks of graphite hang in the middle of the tank, and acts as anodes.

The alumina is then dissolved in molten cryolite - this lowers the melting point - saves money!

Electricity is passed and electrolysis begins. Electrolysis is the decomposition of a compound using electricity.

When dissolved, the aluminium ions and oxide ions in the alumina can move.

At the cathode:

Here the aluminium ions receive electrons to become atoms again:

At the anode:
The oxide ions lose electrons to become oxygen molecules, O:

Uses of aluminium:

1. Shiny metal - used as jewellery.

2. Low density - used to make aeroplanes and trains.

3. Non-toxic - used in drink cans.

Writing Formulae and Balancing Equations

The Mole

The masses of atoms

Relative Atomic Mass

The mass of an atom is tiny. A single hydrogen atom is only about 0.000 000 000 000 000 000 000 002 grams!

Dealing with such small numbers is difficult. Therefore, scientists found a simpler way of comparing the mass of different atoms.

They chose the carbon atom and compared all the other atoms with it. Since a carbon atom consists of 6 protons and 6 neutrons, they gave it a mass of 12 units (they ignored the electrons).

The mass of an atom relative to that of carbon-12 is called the Relative Atomic Mass.

If we compare the mass of a hydrogen atom with that of the carbon-12, we find its mass is a 1/12th of the carbon-12 atom. Therefore, a hydrogen atom is assigned the mass unit 1.

When a magnesium atom was compared, its mass was found to be twice that of carbon-12, therefore magnesium was assigned the mass unit 24.

Mass and Isotopes

Many elements possess atoms with differing masses due to them having different numbers of neutrons.

The atoms shown above belong to the isotopes of chlorine. They have different masses because one has two more neutrons than the other. Also it was found that chlorine-35 is more abundant than chlorine-37. For every four chlorine atoms, one will be a chlorine-37 the other three will be chlorine-35. Therefore, the average mass of a chlorine atom is 35.5.

The presence of isotopes and their abundances must be taken into account when calculating Relative Atomic Mass (RAM).

The RAM of an element is the average mass of its isotopes relative to an atom of carbon-12.

Avogadro’s Number

The Magic Number!

If you calculate the RAM of a substance, and then weigh out that number of grams of the substance you can calculate the number of atoms or molecules that it contains.

Carbon has a RAM of 12, if you were to weigh out exactly12 grams of carbon it would contain 602 000 000 000 000 000 000 000 carbon atoms.

This is called a mole of atoms. The number is called Avogadro's number, usually written as 6.02 x 10 .

Examples:
1. 24 grams of magnesium would contain 6.02 x 10 magnesium atoms.
2. 56 grams of iron would contain 6.02 x 10 iron atoms.

3. 18 grams of water would contain 6.02 x 10 water molecules.

One mole of a substance is 6.02 x 10particles of that substance. It is obtained by weighing out the RAM or the formula mass in grams.

Formulae of Compounds

What information can we get from a chemical formula

The formula of water is HO. This is because 1 atom of oxygen combines with 2 atoms of hydrogen.

Alternatively, we can say that 1 mole of oxygen atoms combines with 2 moles of hydrogen atoms to form 1 mole of water molecules.

Moles can be changed to grams; therefore we can say 16 grams of oxygen combine with 2 grams of hydrogen to form 18 grams of water.

The formula of carbon dioxide is CO .This is because 1 atom of carbon combines with 2 atoms of oxygen.

Therefore, 12 grams of carbon combine with 32 grams of oxygen to form 44 grams of carbon dioxide.

Alternatively, 1 mole of carbon atoms combine with 2 moles of oxygen atoms to produce 1 mole of carbon dioxide.

To find the formula of a compound

1. Start with the number of grams that combine

2. Change the grams to moles

3. This gives you the ratio which they combine

4. So now you know the formula

A formula obtained in this way is called the Empirical Formula.

Example:

1. 32 grams of sulphur react with 32 grams of oxygen

2. 1 mole of sulphur reacts with 2 moles of oxygen

3. Ratio of 1:2

4. Formula SO - sulphur dioxide

Finding mass by experiment

The Reaction between Magnesium and Oxygen

The apparatus below is used to calculate the mass of magnesium oxide, but firstly you must know the masses of the elements that combine.

Method:

1. Weigh the mass of the crucible and lid

2. Add a coil of magnesium ribbon and reweigh.

3. Heat crucible strongly, lifting lid occasionally to allow oxygen in.

4. When burning is complete allow the apparatus to cool.

5. With the lid on reweigh the crucible and its contents.

Results:

mass of magnesium oxide - mass of magnesium = mass of oxygen
4.0g - 2.4g = 1.6g

2.4g of magnesium = 0.1 moles

1.6g of oxygen = 0.1 moles

Therefore the ratio of magnesium to oxygen used is 1:1

Conclusion:
The formula of magnesium oxide is MgO

Writing Equations

How to write an equation

Four steps to writing equations:

1. Write the equation in words

2. Write the equation in symbols. Check that you are using the correct formulae.

3. Check that the equation is balanced. Balancing means that you have the same number of atom on one side as you do on the other. The reason for balancing is because atoms are not lost or created during a reaction. Remember when you balance you multiply the whole formulae whether its an element or molecule - you do not change its formulae.

4. Add state symbols.

Example 1:

Magnesium burns in oxygen to produce magnesium oxide.

Because oxygen has two atoms on the left, we multiply by 2 the MgO so that we now have 2 oxygen atoms on the right.

However we now have two magnesium atoms on the right, so we need to multiply Mg by 2 on the left to balance.

Example 2:

Hydrogen gas reacts with oxygen gas to form water when a spark is placed in the mixture.

There are two oxygen atoms on the left but only one on the right. Hence we need to multiply the HO on the right by 2. This gives us two oxygen atoms on both sides but we are now left with only two hydrogens on the left and four hydrogen atoms on the right. Therefore, we multiply the H
by 2.

The Earth and the Atmosphere

Changes to the Earth

Layers in the atmosphere

The atmosphere is the layer of gas around the Earth.

The atmosphere can be divided into four parts:

Troposphere: Where we live.

Stratosphere: Some jet aircraft.

Mesosphere: Space shuttle orbits within.

Ionosphere: Mainly charged particles.

The gas is at its most dense at sea level but thins out rapidly as you rise through the troposphere.

What is air ?

How did the atmosphere evolve?

The life story so far...

The Earth formed around 4600 million years ago, when a hot, dense mass of gas and dust around the sun collapsed on itself - this was caused by gravity.

A mass of gas and dust got hotter and hotter as the particles were pulled in and squashed together. It then began to cool down, solidify and break up into chunks called planets - one of which was Earth.

Around 4500 million years ago, the hot gases that had built up inside the Earth burst out through volcanoes. Gradually, over millions of years our atmosphere developed from these gases.

As molten rock poured out of the Earth's crust it threw out water vapour, carbon dioxide, nitrogen, hydrogen chloride, hydrogen (so light it went straight into outer space!), and smaller amounts of argon and other noble gases.

The water vapour cooled and condensed and formed the oceans.

This is where life on Earth began 3500 million years ago.

All the hydrogen chloride and much of the carbon dioxide dissolved in rain and ocean water. This acidic solution attacked rock and wore it away.

The first green plants appeared about 2200 million years ago - this is when photosynthesis began. It used up carbon dioxide and produced oxygen.

Some of the oxygen reacted with other elements; the rest went into the atmosphere!

The ozone layer

The ozone layer is about 25 km above sea level, in the stratosphere. It has the formula O3. It is produced when ultra-violet light causes oxygen molecules to break into atoms.

Then:

The ozone layer protects us from the harmful sun's rays.The ozone layer protects us from the harmful sun's rays.

Cycles in Nature

All living things depend on nitrogen, oxygen and carbon dioxide in the air. We also depend on water. However, these substances do not just get used up and disappear, nature recycles them!

Four main cycles to consider are:

1. The nitrogen cycle
Nitrogen circulates between air, the soil and living things.

2. The carbon cycle
Carbon dioxide circulates between the air, soil, and living things.

3. Photosynthesis
This process followed by respiration recycles oxygen.

4. The water cycle
Water circulates between the air, oceans and living things.

Evidence for Rock Formation and Deformation

How the rocks of the Earth formed

Different Types of Rock

Many different processes have produced the characteristics of the Earth’s surface. These same processes ensure that the surface of the Earth is constantly changing!

Rock formation by heat - igneous rocks:

Deep inside the Earth is molten (liquid) rock. At times, this molten rock (magma) finds weaknesses in the Earth's crust and is thrown outside.

These 'gaps' in the crust are called volcanoes - they throw dust, fragments of rock and magma from a hole at the top. These fiery eruptions eventually solidify to form igneous rock. Around the volcano, new land develops as layer upon layer of rock form.

Examples of igneous rock are granite and basalt.

Igneous rocks tend to be hard with many containing interlocking crystals.

Rock formation from sediments:

These are called sedimentary rocks.

Erosion:

Wind, rain, snow, sea, rivers, oceans and glaciers can cause land to be worn away.

This process of wearing away is called 'erosion'.

Rocks that have been weathered and eroded into small fragments are often found in rivers or in the sea being transported. Eventually, these fragments settle at the bottom of river or sea beds.

In time, this sediment will eventually harden and become new layers of rock.

These sedimentary rocks are made from recycled material from older rocks.

Dead animals and plants are often trapped in the layers of sedimentary rock - they can form fossils over many years.

Examples of sedimentary rock are limestone, chalk and sandstone. Sedimentary rock tends to be crumbly, often contain a layered effect and may contain fossils.Examples of sedimentary rock are limestone, chalk and sandstone. Sedimentary rock tends to be crumbly, often contain a layered effect and may contain fossils.

Rock formation from rocks that change with heat and pressure:

In many places on Earth, layers of sedimentary rock have been squashed, buckled or broken. Inside the Earth great forces act to cause these effects.

Mountains are made from rocks that have buckled up and heated while deep under other rock. These new rocks which have formed from sedimentary rock been subjected to heat and/or pressure is called metamorphic rock.

Examples of metamorphic rock are marble and slate. Metamorphic rock is very hard.

Rates of reaction

The Rate of a Chemical Reaction

What is rate?

Rate is a measure of how fast or slow something is. In chemistry, we speak of a rate of reaction, this tells us how fast or slow a reaction is.

Why do chemists want to know the rate of a reaction?

If you are making a product, it is important to know how long the reaction takes to complete, before the product is produced.

Rate is a measure of a change that happens over a single unit time. That unit time is most often a second, a minute, or an hour.

How to measure rate

Using the reaction between zinc and hydrochloric acid as an example, the following are methods by which you could measure the rate of that reaction.

1. Measure that amount of zinc used up per minute

2. Measure the amount of hydrochloric acid used up per minute

3. Measure the amount of zinc chloride been formed per minute

4. Measure the amount of hydrogen been produced per minute

When choosing which method to measure rate always choose the most straightforward.

In the example above, by far the easiest would be to collect the bubbles of hydrogen and measure its volume.

To measure the hydrogen gas released in the reaction we use the apparatus as shown. As the bubbles of gas are given off, the plunger in the syringe moves out as hydrogen gas fills it. After, say every 20 seconds we read the volume of gas in the syringe. The reaction is complete when the syringe no longer moves.

Products from crude oil

Chemicals from oil

How oil is formed

Oil is thought to have formed over millions of years from the break down of tiny dead creatures. Natural gas is formed alongside oil.

The dead organisms sank to the bottom of lakes or seas and became trapped in muddy sediments. As the sediments built up, the lower layers were under pressure. They eventually turned to rock. If there was no oxygen in the sediments, heat and pressure turned the remains of the organisms into oil and natural gas.

Some rocks are porous - they have a network of tiny holes in them. Sandstone and limestone are examples. Oil is a liquid so it seeps into porous rocks. Gas also diffuses into these rocks.

Porous rocks may also contain water. Gas and oil do not mix with water. They are less dense than water. This means they form layers above the water.

Sometimes the rock layers form so that the oil and gas are trapped under the rock such as shale that is not porous. Large amounts of oil and gas may collect in a porous rock. The pressure on the oil may build up so much that when a hole is drilled through the rock cap, oil gushes out.

Fractional distillation of crude oil

Crude oil is a mixture of many thousands of different compounds with different properties. They are called hydrocarbons because they only contain the elements hydrogen and carbon.

To make crude oil useful, batches of similar compounds with similar properties need to be sorted. These batches are called fractions and they are separated by fractional distillation.

The theory behind this technique is that some of the compounds in crude oil are easily vaporised, for example, they are volatile due to their low boiling points. Others are less volatile and have higher boiling points.

In fractional distillation, the crude oil is heated to make it vaporise. The vapour is then cooled. Different fractions of the oil are collected at different temperatures.

As the hydrocarbon molecule chain increases its boiling point increases, it becomes more viscous, becomes more difficult to light, the flame becomes sootier and it develops a stronger smell.

Products from crude oil

Alkanes

Physical properties:

The chemistry of carbon compounds is called organic chemistry. There are millions of organic chemicals, but they can be divided into groups called homologous series.

All members of a particular series will have similar chemical properties and can be represented by a general formula.

The alkane series is the simplest homologous series. The main source of alkanes is from crude oil.

Alkanes are covalent compounds. They are hydrocarbons, which means they contain hydrogen and carbon. The general formula for an alkane is .

Properties and uses of alkanes:

The first four alkanes are gases at room temperature.

Alkanes with 5-17 carbon atoms are liquids.

Alkanes with 18 or more carbon atoms are solids.

As the number of carbon atoms increases, the melting points, boiling points and densities increases.

They are insoluble in water but dissolve in organic solvents such as benzene.

Their chemical reactivity is poor. The C-C bond and C-H bond are very strong so alkanes are not very reactive.

They will carry out combustion. Burning alkanes in air (oxygen) produces water and carbon dioxide. The reactions are very exothermic (give out heat energy), so alkanes in crude oil and natural gas are widely used as heating fuels.

For example:

If alkanes combust in too little air, carbon monoxide may form. This is dangerous and can cause death.

Cracking alkanes

The lighter fractions (for example, petrol) are in large demand. The heavier fractions are not so useful but unfortunately chemists have to be able to convert these heavier fractions into petrol and other useful products, due to supply and demand, by a method known as cracking.

Cracking breaks down molecules into smaller ones.

Catalysts or heat may be used to crack the alkane chain into smaller ones.

Note, that one of the products that is formed when we crack naphtha contains a double bond between two carbon atoms. A hydrocarbon that possesses one double bond belongs to the next homologous series called alkenes.

Another reaction that often occurs after fractional distillation is reforming.

Hydrocarbons of the same formula have different boiling points.

Straight-chained alkanes have greater boiling points than the branched version. This means they catch light more easily - but this can be too much for the hot cylinder of the car engine. Reforming converts straight-chained alkanes to branched.

Alkenes

The members of this series contain a double bond. They are hydrocarbons.

The general formula of the alkenes is CnH2n Most alkenes are formed when fractions from the fractional distillation of crude oil are cracked.

Properties of alkenes:

Like alkanes, the boiling point, melting point and densities increase with larger size molecules.
They are insoluble in water.

They combust like alkanes to produce carbon dioxide and water. However, they burn with sootier flames due to their higher percentage of carbon content to hydrogen.

Chemically, alkenes are more reactive than alkanes. This is because they possess a double bond that can be broken open and added to in a reaction.

For example:

These reactions are called addition reactions.

Saturated and unsaturated:

Organic compounds, like alkanes, which have four single covalent bonds to all their carbon atoms are described as saturated.

Alkenes are hydrocarbons with a double bond between two carbon atoms and are described as unsaturated.

This is because they do not have the maximum number of atoms attached to their four bonds, as one is double!

Polyunsaturated margarines and vegetable oils contain many C=C bonds.

Polymerisation

Making plastics

Facts about plastics:

Polythene (polyethene) is made by forming a long chain of ethene molecules. Many other compounds are made in a similar way. A compound made like this is called a polymer.

Polymers are long chains of monomers. A monomer is the building block or in other words the repeating unit that is used to make the polymer. In the above example, ethene is the monomer and polythene the polymer.

Polystyrene (many styrene molecules) is another well-known polymer.

Many polymers can be easily moulded into many shapes - these are called plastics.

Polymerisation is the name given to the reaction that produces polymers.

Remember: alkenes can become polymers but alkanes cannot. This is because alkanes are saturated whereas alkenes are unsaturated which means that they can carry out addition reactions, required for polymerisation.
This type of polymerisation is called addition polymerisation.