Crystals, Metals and Addition Polymers

CrystalsCrystals are formed by ions, atoms or

molecules arranged in a regular geometric arrangement called a lattice.

Diamond Lattace

William and Lawrence Bragg

William and Lawrence Bragg used x-ray crystallography to determine the arrangement of particles within crystals.

The arrangement of particles inside a crystal is often referred to as the crystal lattice.

The unit cell is a structure that repeats throughout the unit lattice.

Dorothy HodgkinDorothy Hodgkin worked out the

structure of Vitamin B12 using x-ray crystallography and a computer.

Hodgkin determined the three-dimensional structures of the following biomolecules:

cholesterol in 1937

penicillin in 1945

vitamin B12 in 1954

insulin in 1969

AmorphousA solid that does not have a crystal structure

is said to be Amorphous.

Wax and Paraffin are amorphous

AllotropesAllotropes are different physical forms of the

same element. Carbon has 3 different allotropes, Diamond Graphite and Buckminsterfullerene.

Diamond Graphite Buckminsterfullerene

Types of Crystals

Crystals are classified into 4 types according to the type of particle that makes up the crystal.

1. Ionic Crystals2. Molecular crystals3. Covalent macromolecular crystal4. Metallic Crystals

Ionic Crystals Ionic crystals are hard and brittle solids.

They possess high melting points.

They are poor conductors of electricity, but their ability to conduct increases drastically in melt.

Most ionic crystals dissolve in water since there is an attraction between the positive and negative ions and the polar water molecules

Molecular CrystalsMolecular crystals consist of such substances as

N2, CCI4, I2 and benzene.

Molecular crystals are very soft solids that possess low melting points.

They are poor conductors of electricity. Generally, the molecules are packed together as

closely as their size and shape will allow. The attractive forces are mainly van der Waals interactions

Covalent Macromolecular CrystalsCovalent crystals are hard solids that posess very

high melting points. They are poor conductors of electricity. In covalent crystals, atoms are held together by

covalent bonds.Well-known examples are two allotropic forms of

carbon, diamond and graphite

BuckminsterfullereneThe other allotrope of carbon

is buckminsterfullerene, named after the architect and inventor Richard Buckminster Fuller who created the geodesic domes

Discovered in 1985Has 60 carbon atons and

was nicknamed the “bucky ball”

Metallic CrystalsIndividual metal atoms sit on lattice sites

while the outer electrons from these atoms are able to flow freely around the lattice.

Metallic crystals normally have high melting points and densities.

Conduct ElectricityMalleable and Ductile

Type Structural Unit

Bonding Properties Examples

Ionic Positive and Negative ions

Electrostatic Hard BrittleHigh mp.Conduct when solid or molten

NaClMgONaN03

Non PolarMolecules

Non PolarMolecules

Weak Van der Waals

Soft Low mpNon-conductors

I2

CO2

Polar Molecules Polar Molecules Dipole –DipoleOr Hydrogen

Soft Low mpPoor conductors

H2ONH3

Covalent macromolecular

Atoms Covalent bonds Very hardHigh mpNon-conductors

DiamondSiO2

Metallic Metallic ions Metallic High mpGood Conductors

CuFeNa

Summary of Crystals

PolymersAddition PolymersAn Addition Polymer is made by addition

reactions between very many monomers (with double bonds) to form long molecules.

Monomers are small molecules with double bonds that join together in large numbers by addition reactions to make large molecules.

Examples of addition polymers include: Poly(ethene) Low density(LDPE) and high

density(HDPE) Poly(chloroethene) Poly(phenylethene) Poly(tetrafluorethene) Poly(propylene)

POLYMERISATION OF ALKENESPOLYMERISATION OF ALKENES

ETHENE

EXAMPLES OF ADDITION POLYMERISATION

PROPENE

TETRAFLUOROETHENE

CHLOROETHENE

POLY(ETHENE)

POLY(PROPENE)

POLY(CHLOROETHENE)

POLYVINYLCHLORIDE PVC

POLY(TETRAFLUOROETHENE)

PTFE “Teflon”

POLYMERISATION OF ALKENESPOLYMERISATION OF ALKENES

Process • during polymerisation, an alkene undergoes an addition reaction with itself

• all the atoms in the original alkenes are used to form the polymer

• long hydrocarbon chains are formed

ADDITION POLYMERISATION

the equation shows the original monomer and the repeating unit in the polymer

ethene poly(ethene)

MONOMER POLYMER

n represents a large number

Polyethene

Polyethene can be made in two ways, with branches (low density) and in straight chains (high density). Fawcett and Gibson discovered it in 1933.LDPE consists of branched chains that do not pack closely together. Thus they are soft and flexible and used in bags and cling-film.HDPE is produced when ionic catalysts, called Ziegler-Natta catalysts are used at low pressure. The polyethene formed has no branching along the polymer chain. This allows the chains to be packed neatly as in a crystal. Compared with LDPE, HDPE is hard and less flexible and has a higher melting point. It is suitable for making bowls, buckets and baskets.

Poly(chloroethane) (Polyvinyl Chloride PVC)Rigid plastic used to make window frames,

gutters etc. (uPVC)

The addition of plasticisers can turn PVC into a softer and more flexible product (pPVC). Used for raincoats, floor tiles, cable insulation.

Polypropene (polypropylene)Polypropene has a similar structure to that of polyethene

except that methyl groups are attached to every second carbon atom (head to head and head to tail polymerisation is possible).

The presence of the methyl groups means that the groups of atoms arrange themselves in a kind of helical screw pattern along the length of the chain to cause the minimum interference with each other. This regular pattern means that polypropene is less flexible than polyethene and this also causes the polypropene to be harder and slightly stronger than polyethene. Polypropene is used in the manufacture of buckets, bowls, toys, laboratory sinks, etc.

Polyphenylethene (polystyrene)Polyphenylethene is similar in structure to polyethene

and polypropene except that phenyl groups are attached to every second carbon atom in the carbon chain.

There are two types Rigid polystyrene is used to manufacture items like

yoghurt pots, food containers, disposable drinking cups, flower pots, etc.

Expanded polystyrene is used in insulating houses, ceiling tiles, egg boxes, packing for computers, etc.

Polytetrafluoroethene (Teflon)Teflon is the trade name of the chemical polytetrafluoroethene discovered by Roy Plunkett of the Du Pont company in the USA. It is used in non-stick frying pans, space suits, artificial heart valves, etc. It was discovered in 1938.

He found that this greasy white solid had remarkable properties: it was extremely inert and was not affected by strong acids, bases or heat. He tried to dissolve it in various solvents and failed. A really unusual property of it was that it was very slippery. (It is listed in the Guinness Book of World Records as having the ‘lowest coefficient of static and] dynamic friction of any solid’.)

Recycling of PlasticsThe world’s production and use of plastic material has

increased from less than 5 million tonnes in the 1950s to about 80 million tonnes today. Annual consumption of plastics in Western Europe is 28 million tonnes. In total, Western Europe produces some 11.5 million tonnes of plastics waste each year. An average European family of 4 throws away around 40 kg of plastics each year. There are a number of reasons why it is necessary to recycle plastics.

(i) Plastic recycling saves natural resources. (ii) Plastic recycling makes economic sense. (iii) Recycling of plastics cuts waste disposal costs. (iv) Plastic recycling reduces litter. (v) Plastics recycling creates employment.

Thermoplastics and Thermosetting plastics

Not all plastics can be recycled

Thermoplastics or Thermosoftening plastics can be softened and remolded a number of times.

Thermosetting plastics cannot be remoulded-Bakelite

Recycling of polystyreneRecycling of polystyrene contains five stages.

1.Sorting-polystyrene separated from other plastics.2.Shedding-granulator3.Washing4.Drying-excess water removed.5.Re-extrusion-melting followed by remoulding.