| How does Corrosion work | 1) Metals can oxidise in air. Eg, sodium (very reactive) turns from shiny to dull, when cut or scratched: 4Na(s) + O2(g) → 2Na2O(s)
2) Other metals may oxidise more slowly. Gold and other very unreactive metals do not oxidise in air at all
3) Corrosion happens when a metal continues to oxidise. The metal becomes weaker over time and eventually all of it may become metal oxide
4) Rusting is an example of corrosion. It occurs when iron or steel reacts with oxygen and water |
| Preventing corrosion: Prevent Rusting, Physical Barriers, Electrotrapping & Sacrificial Protection | PREVENT RUSTING:
- oxygen can be excluded by storing the metal in an atmosphere of unreactive nitrogen or argon
- desiccant: a substance that absorbs water vapour, so it keeps the metal dry
PHYSICAL BARRIERS:
- painting
- oiling and greasing
- coating with plastic
ELECTROTRAPPING, using electrolysis to put a thin layer of a metal on the object:
- the cathode is the iron or steel object
- the anode is the plating metal
- the electrolyte contains ions of the plating metal
SACRIFICIAL PROTECTION:
- If a metal is in contact with more reactive metal (The more reactive metal oxidises more readily than , so it ‘sacrifices’ itself while so the less reactive metal does not rust. Once the sacrificial metal has corroded away, it can simply be replaced)
GALVANISING:
When iron is coated in zinc, the process is called galvanisation. The zinc layer stops oxygen and water reaching the iron. Zinc is more reactive than iron, so it also acts as a sacrificial metal. This protection works, even if the zinc layer is scratched |
| 4 Uses of Metals | COPPER ALLOYS:
Bronze - copper + tin (statues, medals, coins)
Brass - copper + zinc (music instruments, door knobs, locks, taps)
GOLD:
- Soft, malleable, unreactive ∴ corrosion resistant(stays shiny). Used on space helmet visors, layer of gold is thin enough to see through but thick enough to reflect sun rays
- Jewellery, alloyed with other metals (silver copper& zinc), making it stronger, but still shiny
ALLOY STEELS:
- High carbon steels are used for construction because of their strength
- Low carbon steels are used for making car body panels because they are malleable
- Stainless steel is used for cutlery because it does not rust
MAGNALIUM:
- Alloy of aluminium + magnesium. Stronger than aluminium but still has low density. Used to make aircraft parts |
| Creation of Glass & Clay Ceramics | GLASS CERAMICS:
1) Most the glass we use is soda-lime glass. made by melting a mixture of sand (silicon oxide), sodium carbonate and limestone, then allowing the molten liquid to cool and solidify
2) Borosilicate glass is made by heating sand with boron trioxide. Borosilicate glass has a much higher melting point than soda-lime glass
CLAY CERAMICS:
1) include brick, china and porcelain. They are made by shaping wet clay and then heating it to a high temperature in a furnace, which causes crystals to form and join together
2) often coated with a glaze, which hardens on heating to form a hard, smooth, opaque and waterproof layer. This explains why they are often used for dinner plates and bowls |
| Polymers: Properties, Thermosoftening & Thermosetting | PROPERTIES:
- Low density poly(ethene), LDPE. Flexible, unreactive, can be made into films. Used in Most carrier bags, bubble wrap (polymer chains are branched and this means that the molecules are arranged randomly)
- High density poly(ethene), HDPE. Strong, flexible, resists shattering, resists chemical attack. Used in plastic bottles, pipes, buckets (less branching of the polymer chains, so the molecules line up much more closely)
THERMOSOFTENING:
- melt when they are heated ∴ can be recycled
- do not have covalent bonds between neighbouring polymer molecules, so the molecules can move over each other when heated and the plastic melts.
THERMOSETTING:
- tend to char and burn when heated
- resistant to much higher temperatures than thermosoftening plastics
- used to make electrical plugs, which must not melt, even if there is a malfunction and the wiring inside gets hot (due to the covalent bonds holding the neighbouring polymer chains together) |
| Composite materials & 4 Examples | Consists of two or more materials with different properties, this combination improves:
- the reinforcement
- the matrix, which binds the reinforcement together
FIBRE GLASS & CARBON FIBRE REINFORCED POLYMER(CFRP):
- Fibres in these composite materials have low density, are strong in tension (not easily stretched but flexible)
- The polymer resin which binds the fibres together is not strong but it is stiff. The composite materials show a combination of these properties. They are strong, stiff and lightweight
CHIPBOARD:
- Wood itself is a natural composite material. It consists of a reinforcement of cellulose fibres bonded together by a matrix of lignin
- The fibres are aligned alongside each other, so wood is stronger in one direction than it is in the other
- Chipboard contains randomly arranged wood chips bonded together by a glue, so it is strong in all directions
REINFORCED CONCRETE:
- The properties of concrete can be improved by reinforcing it with steel rods or mesh
- Concrete's compressive strength is higher than its tensile strength but steels tensile strength is higher than its compressive
- This allows reinforced concrete to be strong and slightly flexible, which is important when constructing large buildings and structures |
