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Using materials (chemistry only)

10.3.1 Corrosion and its prevention

Corrosion

  • Destruction of materials by chemical reactions with substances in the environment
  • Eg rusting 4Fe + 3O2+ 6H2O → 4Fe(OH)3

How to prevent corrosion?

Surface coating Sacrificial protection
  • Apply a coating – acts as barrier
  • Eg greasing, painting or electroplating
  • Aluminium has an oxide coating → protects metal from further corrosion
  • Coatings are reactive
  • Contain a more reactive metal so more reactive metal corrodes instead
  • Zinc is used to galvanise iron

The corrosion of iron is called rusting. A student investigated the rusting of iron. This is the method used.

  1. Set up the test tubes as shown in the figure below.
  2. Leave the test tubes for 1 week.
  3. Examine the nails for signs of rust.

Explain what would happen to the nails in each of the test tubes. (5)

  • Tube 1: (nail) rusts because air / oxygen and water present
  • Tube 2: (nail) does not rust because no water / only air / oxygen
  • Tube 3: (nail) does not rust because no air / oxygen / only water
  • Tube 4: (nail) does not rust because paint is a barrier (to water / air / oxygen) or a protective layer / coating (against water / air / oxygen)
  • Tube 5: (nail) does not rust because stainless steel resistant to corrosion

Magnesium is fixed to some steel ships. Explain how this prevents the steel from rusting. (2)

  • Mg is more reactive than Fe
  • So Mg provides sacrificial protection

Explain why aluminium window frames do not corrode after they are made. (2)

  • (Aluminium has a coating of) aluminium oxide
  • (So the aluminium oxide) protects the metal (from further corrosion)

10.3.2 Alloys as useful materials

Alloys

  • Mixture of 2 or more different types of metals
  • Not malleable/ductile, hard – different size of atoms disrupt structure – harder to slide – harder than pure metal
Alloys Composition Notes
Bronze Copper & tin
Brass Copper & zinc
Gold (used as jewellery) Silver, copper & zine Iron – contain specific amounts of carbon & other metals
Steels Iron – contain specific amounts of carbon & other metals High carbon steel

  • Strong but brittle

Low carbon steel

  • Softer
  • More easily shaped

Stainless steels

  • Steels containing chromium & nickel
  • Hard

Resistant to corrosion
Aluminium Low density

Suggest two reasons why 9 carat gold is often used instead of pure gold to make jewellery. (2)

  • 9 carat is cheaper
  • Pure gold is soft
  • Can change the colour

The hip joint sometimes has to be replaced. Early replacement hip joints were made from stainless steel.

Stainless steel is an alloy of iron, chromium and nickel. The diagram below represents the particles in stainless steel.

Pure iron would not be suitable for a replacement hip joint. Suggest why. (1)

  • Not strong

The three metals in stainless steel have different sized atoms. Stainless steel is harder than pure iron.

Explain why. (2)

  • Structure is different / distorted / disrupted
  • So it is difficult for layers / atoms / particles to slip / slide (over each other)

10.3.3 Ceramics, polymers and composites

Glass

Hard, see-through, unreactive

Types Soda-lime glass Borosilicaate glass
How to make Heat a mixture of sand, NaCO3, limestone Melt sand & B2O3 at higher temp than soda-lime glass

Clay ceramic

  • Eg pottery & bricks
  • Made by shaping wet clay, then heating in a furnace

Polymers

  • Long chain molecule made from joining many short molecules (monomers) together
  • Strong intermolecular force – hard to break – solid at room temp
  • Different polymers, different properties, different uses

Poly(ethene) is not biodegradable. Give one problem caused by waste poly(ethene). (1)

Disposal / lack of space / does not decompose in landfill sites

Complete the equation below to show the formation of poly(propene) (3)

Properties of polymers

  • Depends on what monomers they are made from & conditions under which they are made
  • Eg Low density (LD) & high density (HD) poly(ethene) are produced from ethene
Thermosoftening polymer Thermosetting polymer
  • Contain long polymer chains
  • Chains are not joined tgt (but are tangled up with each other)
  • Low melting point – soften and then melt when heated
  • Contain long polymer chains
  • Chains are joined by covalent bonds
  • High melting point – do not soften or melt when heated

Melamine is a polymer used to make non-disposable cups. Melamine does not melt when it is heated.

Explain why. (2)

  • (Melamine is a) thermosetting polymer
  • Which contains crosslinks / bonds between polymer chains

Explain why thermosetting polymers are better than thermosoftening polymers for saucepan handles. (2)

  • Thermosetting polymers do not melt (when heated)
  • Due to cross-links (between chains)

Poly(ethene) is a thermosoftening polymer that melts when heated. The diagrams show a thermosetting polymer and a thermosoftening polymer.

The thermosetting polymer does not melt when heated. Use the diagrams and your knowledge of structure and bonding to explain why. (3)

  • There are (covalent) bonds / links between chains or crosslinks
  • Which are strong
  • So the bonds between chains cannot be (easily) overcome / broken (by heating)