S2.4 Models to materials Flashcards
what do bonding triangles show?
there’s a continuum from one type of bonding to the other 2 so idea of purely ionic or covalent is an oversimplification
general rules of the bonding triangle
small difference in electronegativity+low average negative electronegativity=metallic; small difference in electronegativity+high average electronegativity=covalent; large difference in electronegativity+mid range average electronegativity=ionic
disadvantage of using bonding triangles
don’t predict properties of transition metals or their compounds well
what is thermoplastic, what is their structure and give 2 ex
individual chains with intermolecular forces, it can be repeatedly heated and cooled and remoulded into different shapes; bc intermolecular forces are partially overcome when heated and form again when polymer cools eg polyethene and PVC
desirable properties of plastics(5)
strength and durability, flexibility, lack of reactivity, thermal insulation, electrical insulation
which plastics are strong, rigid and hardwearing?
HDPE(high density polyethene), PVC, nylon and perspex
which plastics are flexible?
LDPE, plasticised PVC are easily rolled into sheets/films and can be used as cling film
why are plastics unreactive?
addition polymers have unreactive, saturated C—C bonds and inert side groups, polyethene containers used for storage of corrosive substances eg hydrofluoric acid which reacts with glass
which plastics have good thermal insulation?
polypropene, polyethene, polystyrene have high SHCs and v low thermal conductivity so excellent thermal insulators
which plastics are good electrical insulators?
PVC and used as a covering for electrical wires
what is a thermosetting polymer, give structure and examples
polymers with extensive cross-linking(covalent bonds) cannot be remoulded eg Bakelite and polyurethanes or naturally=cross-link chains in collagen
what does it mean when plastics are amorphous?
completely random structure with chains arranged like in a plate of spaghetti
what does it mean when plastics are semi-crystalline?
contain some regions where the chains are more ordered and packed more closely together and some amorphous regions
which factors determine how strong a plastic is?
length of chain(relative molecular mass), degree of branching and arrangement of groups on chain
which structure of polymers tend to be stronger and why?
polymers with longer chains and less branching bc there are stronger London forces between chains
what does less branching in polymers lead to?
a more crystalline polymer and stronger and stiffer material
describe the structure in LDPE
contains high proportion of branching, highly branched polymer chains are less able to pack closely together so contact points between chains are reduced, therefore weaker London forces, weak intermolecular forces=more flexible and lower tensile strength
what is tensile strength?
how well a material resists a stretching force without breaking
describe bonding and structure in HDPE
virtually no branching, lack of branching=polymer chains pack more tightly in a more regular arrangement-structure is more crystalline and higher density, more efficient chain packing increases strength of London forces so chains held more tightly, makes polymer stiffer and increases its tensile strength
draw diagram of branching in LDPE and HDPE
what is both LDPE and HDPE usually used for?
LDPE=plastic bags bc more flexible; HDPE=packing crates bc more rigid
draw bonding in Kevlar(show H bonds)
what is nylon and kevlar used for and what type of bonding do they have?
both used for making ropes and kevlar used for body armour; both have strong intermolecular hydrogen bonds along whole chain
which polymers are generally transparent?
amorphous polymers but semi-crystalline usually translucent
