Materials (Chemistry)
From ZuluNotes - Free Leaving Cert Notes
This topic deals with the study of materials around us. In particular, you are required to study crystals, addition polymers and metals. [1] Chemistry is often regarded as the study of materials, [2] and this chapter should be of immense benefit to other aspects of your course.
Contents |
Crystals
- A crystal is a mineral, having a crystalline structure and an ordered arrangement of particles, often characterized by external planar faces intersecting at definite angles.
- William and Lawrence Bragg used X-ray crystallography to determine the structures of crystals.
- The crystal lattice is the geometric arrangement of the particles of a crystal.
- A unit cell is the basic building block of a crystal, which repeats itself throughout.
- Dorothy Hodgkin determined the structure of B12.
- Amorphous substances have no crystal structures.
- Allotropes are different physical forms of the same element.
- There are four types of crystal:
- Ionic
- Molecular
- Metallic
- Covalent macromolecular
| Ionic Crystals | Molecular Crystals | Metallic Crystals | Covalent Macromolecular Crystals | |
|---|---|---|---|---|
| Forces of Attraction | The force of attraction between ions hold the crystal strongly together | Molecules occupy the lattice points. The forces of attraction can be van der Waals (I2), DDA [3] (HCl) or hydrogen bonds (ice) | Positive metal ions and 'sea of electrons' | An interlocking system of covalent bonds makes diamond and quartz (SiO2) giant molecular crystals |
| Solubility | Dissolve readily due to attraction with water (like dissolve like) | Non-polar substances such as I2 are not soluble. In contrast, HCl and similar substances are completely miscible | Generally do not dissolve. Some metals react with water | Do not dissolve due to strong force of covalent bond links |
| Hardness | Generally hard and brittle | Tend to be weak due to weak van der Waals forces | Wide range of values of hardness between metals | Very hard because of the network of covalent bonds |
| Melting Point | Usually high (800 | Tend to be low due to weak van der Waals forces | Wide range of values of melting points between metals | Extremely high because of structure |
| Conductivity | No conductivity unless melted or dissolved | No conductivity due to lack of ions and free electrons | Very good conductance with 'sea of electrons' | No conductivity due to lack of ions and free electrons |
Addition Polymers
There are five polymerisation reactions to be studied on the course:
- poly(ethene)
- poly(chloroethene)
- poly(propene)
- poly(phenylethene)
- poly(tetrafluoroethene)
Poly(ethene)
- Also called polythene
- Polymerisation of ethene
- Low-density is used for shopping bags, milk bottles...
- High-density is used for buckets, bleach bottles...
Poly(chloroethene)
- Also called polyvinyl chloride (PVC)
- Polymerisation of chloroethene (ethene with a hydrogen replaced by a chlorine)
- Unplasticised PVC is used for gutters, drain pipes...
- PVC is made more flexible by the addition of a plasticiser
- Plasticised PVC is used for raincoats, shower curtains...
Poly(propene)
- Also called polypropylene
- Polymerisation of propene (ethene with a hydrogen replaced by a methyl group)
- Similar properties to high-density poly(ethene)
- Used to make laboratory beakers, ropes...
Poly(phenylethene)
- Also called polystyrene
- Polymerisation of phenylethene (ethene with a hydrogen replaced by a benzene ring)
- Rigid polystyrene is used for yoghurt pots, disposable cups...
- Expanded polystyrene is used for packaging for breakables, egg boxes...
Poly(Tetrafluoroethene)
- Also called Teflon or PTFE
- Polymerisation of tetrafluoroethene (ethene with hydrogens replaced by fluorines)
- Discovered by Roy Plunkett by accident
- Extremely inert, unaffected by acid, slippery, does not dissolve
- Used for non-stick pans, synthetic body parts...
Recycling of Plastics
Metals
The main differences between metals and non-metals are outlined in the following table: [4]
| Metals | Non-Metals | |
|---|---|---|
| Conductivity | Good conductivity due to 'sea of electrons' | Little conductivity due to lack of ions and free electrons |
| Malleability | Can easily be hammered into different shapes | Usually very brittle (in solid form) |
| Ductility | Can be drawn into wire form | Cannot be drawn into wire form |
| Lustre | Generally lustrous(shiny) | Generally dull |
| Hardness | Generally hard (although many exceptions) | Tend to be soft due to weak van der Waals forces |
- Graphite is the only non-metal with good conduction.
- An alloy is a mixture of metals. Alloying one metal with other metal(s) or non metal(s) often enhances its properties. For example, steel is stronger than iron, its primary element. [5]
References
- ↑ http://www.curriculumonline.ie/uploadedfiles/PDF/lc_chemistry_sy.pdf
- ↑ http://dictionary.reference.com/browse/chemistry
- ↑ http://www.answers.com/topic/dipole-dipole-attraction
- ↑ http://hyperphysics.phy-astr.gsu.edu/hbase/pertab/metal.html
- ↑ http://en.wikipedia.org/wiki/Alloy
