Q1)
*You do not need to know the details of the reactions to make styrene and vinyl chloride but you do need to know the reactants – they are in our notes.
*Styrene is NOT used for insulation (or esky’s, CD cases etc…). However polystyrene is. So you need to say that styrene is used as the monomer for polystyrene and that polystyrene is used for insulation… i.e., is near enough good enough for Chemistry? – see below
Q3)
*It is good that we understand the connection between
Production conditions (which determines)
=> structure (which determines)
=> properties (which determines)
=> uses.
But when explaining one of these features - e.g. properties – you must primarily relate it to the factor that directly influences it. Eg properties are determined by structure; uses are determined by properties. SO - when explaining why LDPE is non-crystalline, explain it in terms of its branched structure (if required you can then go and relate this back to production conditions, but the main focus of your explanation should be the connection between structure and properties).
* Cellulose is not water soluble. Paper does break up in water but the cellulose itself is not dissolving. The term that people were looking for is biodegradable (basically able to be eaten by bacteria) cellulose (paper) is but polyethylene is not.
Q4)
Similar to Q3, these are questions about properties so you relate the answers to their structure. It is important that you understand that the further apart chains are, the weaker their inter-chain bonding (intermolecular weak dispersion forces) is. Thus strength and melting points are lower
Q5)
For the last two columns it is important that you link an appropriate PROPERTY to the use. It is no point saying that polystyrene is used for packaging because it is a thermal insulator. Say it is used for thermos insulation because it is a thermal insulator OR say it is low density and tough so used for packaging. It is also important to distinguish between non-expanded and expanded polystyrene when discussing its properties and used (ditto for plasticised and non-plasticised PVC).
PORTFOLIO #2 Feedback on Summaries
Again – very well done – just that I think everyone is still trying to use too many words. I’ll try and give some tips below to help.
NOTE – please use the term ethylene NOT ethene (ethene is the systematic name but the IUPAC name is ethylene)
(A) With the safety you need to say the danger AND specifically how to avoid the danger – ie things you should and shouldn’t do.
(B) In the method the dice rolling modelled the chaotic nature of the addition polymerisation process. You do not need to record the dice roll numbers used. You do need to say that ‘different’ dice rolls resulted in initiation, propagation, termination and backbiting (for LDPE models only).
(C) A free radical is a chemical species that has an unpaired electron (and is thus very reactive). Technically a free radical is a chemical species with an unpaired electron that could be formed by splitting a molecule.
(D) It is important to note that initiation in LDPE is started when the organic peroxide REACTS with the ethylene monomer by breaking the double bond and forming a free radical. HDPE is initiated by the ethylene monomer forming a temporary bond with the catalyst (which could be metallocene or Ziegler Natta), which breaks the double bond and forms a free radical.
(E) Diagrams have to at least be big enough to see. If you got an (E) code they need to increase in size.
(F) To explain the distribution of chain lengths the best phrase I have come across is “The chaotic nature of the polymerisation process means activated chains of different length collide and terminate. Thus producing a polymer with a range of chain lengths”.
(G) Question 6 & 7 need you to EXPLAIN the differences and similarities – I think this is where you can express your understanding more succinctly. Eg Q6 “LDPE and HDPE are both covalent molecular and thus they have no free electrons to conduct electricity. HDPE has linear chains allowing for close crystalline packing and high density. The close packing means there is strong inter-chain bonding and thus HDPE is strong and rigid. LDPE…”
For Q7 you can choose your words carefully to be thorough yet concise. Eg Q7 “LDPE’s branched chains and non-crystalline low density packing mean it is flexible and transparent, making it suitable for cling wrap and plastic bags…”
Remember you DON’T need to detail EVERY application, just a few illustrative examples.
(H) When assessing whether model kits can effectively illustrate formation you need to consider whether they were able to show initiation, propagation and termination. It does not matter whether you think they can or can’t do this effectively as long as you say which and provide a reason.
(I) In general, pictures are useful in chemistry for effective communication. When dealing with complex formulae or concepts, diagram are quicker and easier to prepare (presenter) and understand (audience).
(J) When assessing whether model kits can effectively illustrate properties you need to consider whether they were able to show the (similar and different) physical and chemical properties of the polymers (strength, flexibility, mpt, bpt, conductivity etc).
(K) Not only did you have to say how pictures are useful for communicating chemistry concepts in general – but you also needed to give an example of where they were useful in this experiment (including the summary – perhaps read (L) below for an idea).
(L) You do not need to spend time (and use valuable space) describing initiation, propagation and termination in the text as long as you have properly labelled diagrams (ie see (I) and (K) above).
(M) You cannot explain backbiting properly unless you have four pictures.
(N) Remember – the rules of the assessment are size 10 Arial narrow, single spacing with 1cm margins. You would not want to have a summary inadmissible because of a formatting ‘error’.
(O) You must be specific about which properties are controlled by which bonding. You are not demonstrating understanding if you list a whole lot of properties: eg "They are insulating, insoluble, low mpt, .... this is because they are covalent molecular and have weak dispersion bonds". in such an answer it is not clear which type of bonding controls which property and why. Also NB they are insoluble because they are non polar CM but also because the long chains are tangled together (hard to separate) - this (and the large molecule size) also keeps their mpt and bpt higher than other CM substances as well.
