4a - obtaining and using metals

Question 1

Oxidation

Answer 1

Oxidation can be defined as the gain of oxygen by an element or compound.
Examples
* Magnesium is oxidised to make magnesium oxide.
* 2Mg + O₂ -> 2MgO
* Magnesium + oxygen -> magnesium oxide

Question 2

Reduction

Answer 2

Reduction can be defined as the loss of oxygen from a compound.
Examples
* Zinc oxide is reduced to make zinc.
* 2ZnO + C -> 2Zn + CO₂
* zinc oxide + carbon + zinc carbon dioxide

Question 3

Reduction and oxidation

Answer 3

Reduction and oxidation happen simultaneously. Oxygen is removed from one compound and added to something else.
Example
Both reduction and oxidation occur when iron oxide reacts with carbon monoxide.
* Fe203 + 3CO → 2Fe + 3CO2
* iron oxide + carbon monoxide → iron carbon dioxide
* Iron oxide is reduced to iron (as oxygen is removed).
* Carbon monoxide is oxidised to carbon dioxide (as oxygen is added).

Question 4

Combustion reactions

Answer 4

Combustion reactions involve oxidation and reduction. They’re always exothermic.
Example
* The reaction of methane with oxygen is a combustion reaction.
* CH4 + 20₂ -> CO₂ + 2H₂O
* Methane + oxygen -> carbon dioxide + water
* Both the carbon and hydrogen are oxidised - they gain oxygen.
* The oxygen molecules are reduced as the oxygen atoms get split up by the reaction.

Question 5

The reactivity series

Answer 5

• The reactivity of a metal is derived from how easily it forms cations (positive ions). The metals at the top of the reactivity series are the most reactive- they easily lose their electrons to form cations. Reactive metals also gain oxygen (are oxidised) more easily.
• The metals at the bottom of the reactivity series are less reactive - they don’t give up their electrons to form cations as easily. They’re more resistant to oxidation than the metals higher up the reactivity series.
• As well as the metals, carbon is often included in reactivity series - a metal’s position in the reactivity series compared to carbon dictates how it’s extracted from its ore.
• Hydrogen is included in the reactivity series too - this shows the reactivity of metals with dilute acids.

Question 6

Finding an order of reactivity from experiments

Answer 6

If you compare the relative reactivity of different metals with either an acid or water and put them in order from most reactive to the least reactive, the order you get is a reactivity series. The higher a metal is in the reactivity series, the more easily it reacts with the water or acid.

You can also investigate the reactivity of metals by measuring the temperature change of the reaction with an acid or water over a set time period. If you use the same mass and surface area of metal each time, then the more reactive the metal, the greater the temperature change should be.

Question 7

Reactions of metals with acids

Answer 7

Acid + metal -> salt + hydrogen

You can see how reactive different metals are by monitoring the rate of hydrogen production when they react with an acid. The more reactive the metal, the faster the reaction will go. The speed of the reaction is indicated by the rate at which bubbles of hydrogen are given off - a speedy reaction is shown by bubbles being produced rapidly.

You could measure the production of hydrogen more precisely by attaching a gas syringe to the test tube at the beginning of the reaction and measuring the volume of gas given off at regular time intervals.

Question 8

test for hydrogen

Answer 8

The production of hydrogen can be detected using the burning splint test. This involves putting a lit splint at the mouth of the tube containing the metal and the acid. If hydrogen is there, you’ll hear a ‘squeaky pop’. The more reactive the metal, the more hydrogen is produced in a certain amount of time and the louder the ‘squeaky pop’.

Question 9

Reactions of metals with water

Answer 9

Metal + water → metal hydroxide + hydrogen
Metal + water vapour → metal oxide + hydrogen

Metals that aren’t very reactive, such as magnesium, zinc and iron won’t react much with cold water. They will however react with steam.

Question 10

Reactions of metals with salt solutions

Answer 10

As well as using water and acids to determine the reactivity of metals, you can use salt solutions (solutions containing a dissolved metal compound). The advantage of this is it allows you to directly compare the reactivity of one metal with another.

If you put a reactive metal into a solution of a less reactive metal salt, the reactive metal will replace the less reactive metal in the salt.

Example - If you put an iron nail in a solution of copper sulphate, the more reactive iron will displace the less reactive copper from the salt.
Fe + CuSO4 -> FeSO4 + Cu
Iron + copper sulphate -> iron sulphate + copper

If you put a less reactive metal into a solution of a more reactive metal salt, nothing will happen.

Question 11

Redox reactions

Answer 11

Redox reactions occur when electrons are transferred between substances.
* Oxidation is a loss of electrons. OIL
* Reduction is a gain of electrons RIG.
Both oxidation and reduction happen at the same time, hence the term redox. Oxidation and reduction can also be defined in terms of loss or gain of oxygen but on this page, they’re referring to the transfer of electrons.

Question 12

Displacement reactions and redox

Answer 12

Displacement reactions are a type of redox reaction. In displacement reactions, a more reactive element reacts to take the place of a less reactive element in a compound.

In displacement reactions involving metals and salts, a more reactive metal will displace a less reactive metal in a salt solution. The more reactive metal loses electrons and the less reactive metal gains electrons. So, during a displacement reaction, the more reactive metal is oxidised and the less reactive metal is reduced.

If you place zinc in a solution of copper sulphate (CuSO4), the more reactive zinc will displace the less reactive copper from the solution. You end up with zinc sulphate solution and copper metal.

Question 13

Extracting metals from their ores

Answer 13

Some unreactive metals, such as gold and platinum, are present in the Earth’s crust as uncombined elements, rather than as a compound. These metals can be mined straight out of the ground, but they usually need to be refined before they can be used.
The rest of the metals we get by extracting them from metal ores, which are mined from the ground. A metal ore is a rock which contains enough metal to make it profitable to extract the metal from it. In many cases the ore is an oxide of the metal.

e.g., The main aluminium ore is called bauxite. it’s aluminium oxide (Al2O3).

Question 14

Extraction of metals by reduction with carbon

Answer 14

A metal below carbon in the reactivity series can be extracted from its ore by reducing it in a reaction with carbon. In this reaction, the ore is reduced as oxygen is removed from it and carbon gains oxygen so is oxidised.

Question 15

Extraction of metals by electrolysis

Answer 15

Metals higher than carbon in the reactivity series, or that react in different ways with carbon, have to be extracted using electrolysis, which is expensive. The metal ore is melted, then an electric current is passed through it. The metal is discharged at the cathode and the non-metal at the anode.

Question 16

Extraction of metal ores and sustainability

Answer 16

• Extraction of metal ores from the ground is only economically viable when the ore contains sufficiently high proportions of the useful metal, such as iron ores and aluminium ores
• For low grade ores (ores with lower quantities of metals) other techniques are being developed to meet global demand
• This is happening in particular with nickel and copper as their ores are becoming more and more scarce
• Phytoextraction/phytomining and bioleaching (bacterial) are two relatively new methods of extracting metals that rely on biological processes

Question 17

Bioleaching & Phytomining

Answer 17

• Phytoextraction/phytomining and bioleaching (bacterial) are two relatively new methods of extracting metals that rely on biological processes
• Both of these methods avoid the significant environmental damage caused by the more traditional methods of mining
• Traditional mining involves a great deal of digging, moving and disposing of large amounts of rock
• Biological methods are, however, very slow and also require either displacement or electrolysis to purify the extracted metal
• Both techniques are also used to extract metals from mining wastes, which may contain small quantities of metals or toxic metals that need to be removed from that environment

Question 18

Phytomining

Answer 18

• This process takes advantage of how some plants absorb metals through their roots
• A plant is chosen that will absorb the desired metal from the soil without dying - these plants are called hyper accumulatans
• The plants are grown in areas known to contain metals of interest in the soil
• As the plants grow the metals are taken up through the plants vascular system and become concentrated in specific parts such as their shoots and leaves
• These parts of the plant are harvested, dried and burned
• The resulting ash contains metal compounds from which the useful metals can be extracted by displacement reactions or electrolysis

Question 19

Bioleaching

Answer 19

• Bioleaching is a technique that makes use of bacteria to extract metals from metal ores
• Some strains of bacteria are capable of breaking down ores to form acidic solutions containing metals ions such as copper(II)
• The solution is called a leachate which contains significant quantities of metal ions
• The ions can then be reduced to the solid metal form by displacement reactions or electrolysis
• This method is often used to extract metals from sulfides e.g. CuS or Fe2S
• Although bioleaching does not require high temperatures, it does produce toxic substances which need to be treated so they don’t contaminate the environment
• Bioleaching is not only used for the primary extraction of metals, but it is also used in mining waste clean up operations

Question 20

What is recycling?

Answer 20

Recycling involves using waste materials to make new products. For example, yoghurt pots can be melted down and the plastic used to make other products.

Question 21

Recycling – pros:

Answer 21

• Conserving energy and resources
• Environmental benefits
• Economic benefits

Question 22

Recycling – pros - Conserving energy and resources

Answer 22

Extracting raw materials can take large amounts of energy, lots of which comes from burning fossil fuels. Fossil fuels are running out (they’re a non-renewable resource) so it’s important to conserve them. Not only this but burning them contributes to acid rain and climate change.

Recycling materials saves energy as this process often only uses a small fraction of the energy needed to extract and refine the material from scratch.

As there’s a finite amount of many raw materials, e.g., metals, on Earth, recycling conserves these resources too. Metals, like fossil fuels, are non-renewable. It’s particularly important to recycle materials that are rare.

Question 23

Recycling – pros - Environmental benefits

Answer 23

Extracting metals also impacts on the environment. Mines are damaging to the environment and destroy habitats - not to mention the fact that they’re a bit of an eyesore. Recycling more metals means that we don’t need so many mines.

Recycling materials also cuts down on the amount of rubbish that gets sent to landfill. Landfill takes up space and pollutes the surroundings.

Question 24

Recycling – pros - Economic benefits

Answer 24

As you saw above, extracting materials often requires more energy than just recycling them, and energy doesn’t come cheap. So, recycling saves money. It is particularly beneficial to the economy to recycle metals that are expensive to extract or buy.

Recycling is also a massive industry and creates lots of jobs. The materials to be recycled have to be transported to recycling centres, where they then need be sold. Jobs are created at every stage of this process - be processed. They are then reprocessed into new products which can created by simply disposing of waste by dumping it into landfill.

Question 25

Stages in a life cycle assessment

Answer 25

Life cycle assessments (LCAs) assess the environmental impact of the entire lifetime of a product. The 4stages of the lifetime of a product can be seen as:
* Getting the raw materials
* Manufacturing and packaging
* Using the product
* Product disposal
At each stage certain factors need to be considered, including the amount of energy that is needed, how much water and other resources are used, the amount of pollution produced, how much waste is formed and how this waste is disposed of.

Question 26

Getting the raw materials

Answer 26

Extracting raw materials needed for a product can damage the local environment, e.g. mining metals. Extraction can also result in pollution due to the amount of energy needed.
The transportation of raw materials to where they are used in manufacturing can result in greenhouse gas emissions from the combustion of fossil fuels. Raw materials often need to be processed to extract the desired materials and this often needs large amounts of energy. E.g. extracting metals from ores or fractional distillation of crude oil. There are often large amounts of waste associated with these processes which need to be disposed of.
Raw materials for chemical manufacture often come from crude oil. Crude oil is a non-renewable resource, and supplies are decreasing.

Question 27

Manufacturing and packaging

Answer 27

Manufacturing products and their packaging can use a lot of energy and other resources. It can also cause a lot of pollution, e.g. harmful fumes such as carbon monoxide or hydrogen chloride.
The chemical reactions used to make compounds from their raw materials can produce waste products. Some waste can be recycled and turned into other useful chemicals, reducing the amount that ends up polluting the environment.
Most chemical manufacture needs water. Businesses have to make sure they don’t put polluted water back into the environment at the end of the process.

Question 28

Using the product

Answer 28

The use of a product can damage the environment.
e.g. - Paint gives off toxic fumes & Burning fuels releases greenhouse gases and other harmful substances. Fertilisers can leach into streams and rivers causing damage to ecosystems,
How long a product is used for or how many uses it gets is a factor considered by LCAS-products that need lots of energy to produce, but are used for ages, may mean less waste and raw materials are needed in the long run.

Question 29

Product disposal

Answer 29

Energy is used to transport waste to landfill, which causes pollutants to be may released into the atmosphere. The waste kept in landfill takes up space and can pollute land and water, e.g. if paint peels off a product and gets into rivers. If the material is biodegradable, the space taken up by landfill only be temporary. However, non-biodegradable materials, such as many plastics, may take up to a thousand years to degrade.
Another way to dispose of products is incineration. This is when waste is burnt at very high temperatures. This cuts down on waste going to landfill and can be used to generate electricity but can cause air pollution.
If all or part of the product can be recycled or reused, it will reduce the amount of waste going to landfill.

Question 30

electrolysis of bauxite

Answer 30

The main ore of aluminium is bauxite, which can be mined and purified to give aluminium oxide, Al2O3.

Aluminium is then extracted by electrolysis. Aluminium oxide has a very high melting point of over 2000 ° - so melting it would be very expensive.

Instead, mix the aluminium oxide with a mineral called cryolite which lowers the melting point to about 900 °C, which saves energy, making the process cheaper and easier.

The positive Al3+ ions are attracted to the negative electrode (cathode), where they each pick up three electrons and turn into neutral aluminium atoms. These sink to the bottom of the electrolysis tank - reduction Al3+ + 3e → Al

The negative O²- ions are attracted to the positive electrode (anion) where they each lose two electrons. The neutral oxygen atoms combine to form O² molecules - oxidation 2O²- → O² +4e

The overall equation for the reaction is:** 2Al²O3→ 4AI + 3O²**