Extraction and uses of metals

For whoever asked for stuff in section 5, see the Polymers post for part c, what else from section 5 do you need?

5.1 explain how the methods of extraction of the metals in this section are related to their positions in the reactivity series

Order of reactivity	Symbol	Method of Extraction
Potassium	K	Electrolysis The metal compound is:           Melted, then           Has electricity passed through it These metals are very reactive and are above carbon in the reactivity series, so they cannot be reduced by it. As they are very reactive, the make very stable compounds that requires a lot of energy to separate into its elements. So electrolysis is used.
Sodium	Na
Lithium	Li
Calcium	Ca
Magnesium	Mg
Aluminium	Al
Zinc	Zn	Reduction by carbon e.g. ZnO+Cà Zn + CO Or sometimes the carbon monoxide is the reducing agent-here think of reduction as ‘taking oxygen away’ to leave pure metal. Carbon is cheap and can also be used as the source of heat. If the ore is a sulphide, it is roasted first to get the oxide. Roasting is a process where is basically heating the ore in air.
Iron	Fe
Tin	Sn
Lead	Pb
Copper	Cu	These metals can be found uncombined, as the metal itself because they are very unreactive. We say they are found native. (Copper and silver are often found as ores but they are easy to extract by roasting the ore.)
Silver	Ag
Gold	Au
Platinium	Pt

5.2 describe and explain the extraction of aluminium from purified aluminium oxide by electrolysis, including:
i. the use of molten cryolite as a solvent and to decrease the required operating temperature
ii. the need to replace the positive electrodes
iii. the cost of the electricity as a major factor

!Remember: For electrolysis to work, the ions must be free to move. When an ionic compound is dissolved in water, or melts, the ions break free from the ionic lattice. These ions are then free to move.!

Aluminium is the most common metal in the Earth's crust, making up 7.5% by mass. Its main ore is bauxite-a clay mineral which you can think of as impure aluminium oxide. It is first treated to produce pure aluminium oxide-but you don't need to know how this is done for GCSE purposes.

Because aluminium is above carbon in the reactivity series, it has to extracted using electrolysis. Aluminium oxide however, has a very high melting point and it won't be practical to electrolyse molten aluminium oxide. Instead, it is dissolved in molten cryolite. Cryolite is another aluminium compound that melts at a more reasonable temperature. So the electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000°C.

The electrodes are made of carbon and the positive electrode--the anode--disintegrates. The hot oxygen produced here reacts with the hot carbon anode to give carbon dioxide. Hence it must be replaced regularly.
(You can also think of it as: due to the high temperatures, the carbon anodes burn in the oxygen to form carbon dioxide. The anodes have to be replaced regularly and add to the expense of the process.)

The extraction of aluminium is an expensive process because the large amount of electricity needed to keep the electrolytes molten is expensive. Hence using cryolite saves energy and money, as it acts as a solvent for the aluminium oxide and melts at a much lower temperature.

Remember graphite is carbon.

5.3 write ionic half-equations for the reactions at the electrodes in aluminium extraction

Aluminium ions are attracted to the cathode (the negative electrode) and are reduced to aluminium by gaining electrons.

Al³⁺ (l) + 3e^- à Al (l)

The molten aluminium produced sinks to the bottom of the cell. 

The oxide ions are attracted to the anode and lose electrons to form oxygen gas.

2O^2- (l) à O₂(g) + 4e^-

Remember: oxygen is diatomic!!

5.4 describe and explain the main reactions involved in the extraction of iron from iron ore (haematite), using coke, limestone and air in a blast furnace

Haematite is basically iron oxide, and the oxygen must be removed to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions. Since carbon is more reactive than iron, it can displace the iron from its oxide. Hence the method for extraction of iron is called 'reduction by carbon'. 

blast furnace diagram

The iron ore, coke and limestone enter the blast furnace at the top. The hot waste gases at the top of the furnace are piped away and used to heat the air blast at the bottom. 

Coke is impure carbon, and it burns in the hot air blast to form carbon dioxide. This is a strongly exothermic reaction which makes it an important reaction, as it helps heat up the blast furnace. 

C (s) + O₂ (g) à CO₂(g)

At high temperatures in the furnace, the carbon dioxide is reduced by more carbon to give carbon monoxide.

CO₂ (g) + C (s) à 2CO (g)

It is the carbon monoxide which is the main reducing agent in the furnace-especially in the cooler parts.

Assuming that the iron ore is haematite, Fe₂O₃:

Fe₂O₃ (s) + 3CO (g) à 2Fe (l) + 3CO₂ (g)

Due to the high temperatures, the iron produced melts and flows to the bottom of the furnace, where it can be tapped off.

In the hotter parts of the furnace, some of the iron oxide is also reduced by carbon itself. 

Fe₂O₃ (s) + 3C (s) à 2Fe (l) + 3CO (g)

Notice that carbon monoxide is formed, rather than carbon dioxide, at these temperatures.

However some use this equation instead: 

iron oxide + carbon    →    iron + carbon dioxide

2Fe₂O₃ + 3C    →    4Fe + 3CO₂

I'm not sure which of these equations you will need, or need at all, as the specification didn't exactly specify. 

-_- So I'll include it just in case. :)

The limestone is added to the furnace to remove impurities in the ore which would otherwise clog the furnace with solid material.

The furnace is hot enough for the limestone (calcium carbonate) to undergo thermal decomposition. It splits up into calcium oxide and carbon dioxide. This is an endothermic reaction (it absorbs heat) and it is important not to add too much limestone to avoid cooling the furnace.

CaCO₃ (s) à CaO (s) + CO₂ (g)

Calcium oxide is a basic oxide, and its function is to react with acidic oxides such as silicon dioxide, SiO₂. Silicon dioxide is the main constituent of sand, and is typical of the sort of impurities that need to be removed from the furnace.

CaO (s) + SiO₂ (s) à CaSiO₃ (l)

The product is calcium silicate. This melts and trickles to the bottom of the furnace as a molten slag, which floats on top of the molten iron as it is less dense, and can be tapped off separately. Slag is used to make roads. 

Note:Thank you to the anonymous person who pointed out an error about the decomposition of limestone. It decomposes to calcium oxide (not the prev. carbon oxide typo) and carbon dioxide :)

5.5 explain the uses of aluminium and iron, in terms of their properties

Uses of aluminium

Pure aluminium isn't very strong, so alloys of aluminium are normally used instead. The aluminium can be strengthened by adding other elements such as silicon, copper or magnesium. 

Aluminium resists corrosion because it has a thin layer of its oxide on the surface, preventing air and water getting to its surface and reacting with it. It has a low density too, so is often used for aircraft bodies. 

As it has a shiny appearance, and resists corrosion and has a low density + is a good conductor of heat, it can be used for cooking equipment like saucepans. 

Due to its low density and ability to conduct electricity, it can also be used for cable wires-it is ductile so this is possible. The aluminium in the cables is strengthened by a core of steel. 

Uses of iron

(See bottom of this part for a simple table to summarise everything, since it's a bit lengthy..)

Cast iron

Molten iron straight from the furnace can be cooled rapidly and solidified by running it into sand moulds. This is known as pig iron. If the pig iron is remelted and cooled under controlled conditions, cast iron is formed. This is very impure iron, containing about 4% carbon as its main impurity. Although cast iron is very hard, it is also very brittle and tends to shatter if it is hit hard. It is used for things like manhole covers, gutterings and drainpipes, and cylinder blocks in car engines. 

Mild steel

Mild steel is iron containing up to about 0.25% carbon. This small amount of carbon increases the hardness and strength of the iron. It is used for (among other things) wire, nails, car bodies, ship building, girders and bridges. 

Wrought iron

This is pure iron. It was once used to make decorative gates and railings but has now been largely replaced by mild steel. The purity of the iron makes it very easy to work because it is fairly soft, but the softness and lack of strength mean that it isn't useful for structural purposes.

High-carbon steel

High carbon steel is iron containing up to 1.5% carbon. Increases the carbon content makes the iron harder, but at the same time it gets more brittle. High-carbon steel is used for cutting tools and masonry nails. Masonry nails are designed to be hammered into concrete blocks or brickwork where a mild steel nail would bend. If you miss-hit a masonry nail, it tends to break into two bits because of its increased brittleness.

Stainless steel

Stainless steel is an alloy of iron with chromium and nickel. Chromium and nickel form strong oxide layers in the same way as aluminium, and these oxide layers protect the iron as well. Stainless steel is therefore very resistance to corrosion. 

Obvious uses include kitchen sinks, saucepans, knives and forks, and gardening tools. But there are also major uses for it in the brewing, dairy and chemical industries where corrosion-resistant vessels are essential. 

Types of iron	Iron mixed with	Some uses
Wrought iron	(pure iron)	Decorative work such as gates and railings
Mild steel	Up to 0.25% carbon	Nails, car bodies, ship building, girders
High-carbon steel	0.25-1.5% carbon	Cutting tools, masonry nails
Cast iron	About 4% carbon	Manhole covers, guttering, engine blocks
Stainless steel	Chromium and nickel	Cutlery, cooking utensils, kitchen sinks

http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/metals/obtaining_using_metalsrev4.shtml

Example question:

Q1. Aluminium is described as ductile because it can easily be pulled into a wire. Explain, in terms of its structure, why it is ductile. (2m)

My answer: It is a lattice structure where the layers can slide over each other easily to be drawn out into a wire. The atoms are all the same unlike in an alloy so it is easy to draw out. 

Markscheme:

1. layers of ions/particles (1m)

Accept planes/sheets/row

Do not penalise atoms instead of ions here

Reject molecules/protons/electrons

2. Slide over each other (2nd mark)

Accept explanation in terms of non-directional bonding

Do not award mark if wrong particles named, eg protons/electrons

Q2. Explain, in terms of its structure, why aluminium is a good conductor of electricity. (2m)

My answer: It is a lattice of positive ions in a sea of delocalised electrons. The delocalised electrons are free to move about and carry charges, so it can conduct electricity. 

Markscheme:

1. delocalised/sea of electrons (1m)

Accept free

2. move (through structure)/mobile (2nd mark)

"ions free to move" scores 0

Q3. State a property that makes aluminium suitable for manufacturing aircraft bodies. (1m)

My answer: Low density

Markscheme:

1. low density/high strength to weight ratio

ignore light

accept lightweight/not dense

Q4. Cast iron from the furnace contains up to 4% carbon, which causes the iron to be brittle. Most of the iron from the blast furnace is converted to steel. Mild steel, which is used in the construction industry, contains only 0.15-0.25% carbon.

Explain how the carbon content of the iron from the blast furnace is lowered to produce mild steel. (2m)

My answer: Oxyen is passed through the molten iron so the carbon reacts with it to from carbon dioxide and escapes. 

Sorry I don't have the markscheme for this, but you'd get a mark for 'oxygen is passed through' and 'forms carbon dioxide' I believe. :) 

All the model answers I give got full marks, so I hope they help. :) Ciao!