Most lost marks were due to misreading the question, not fully interpreting the question or through not giving enough critical Chemistry details.
Tips on how to improve your answers are below as well as details about some misconceptions (critical that these are overcome ASAP).
2002
Q16c) Equations for alkene reactions MUST be full structural equations (see here). BUT – be careful – you can use the completely summarised structure without drawing in all the C’s and H’s BUT if you draw in the carbons you must also draw in all the hydrogen. In addition it is very important to get states correct. Just ONE state missing or incorrect can lose you a mark. HINT: the cyclohexene is liquid, the bromine is aqueous, if you have 1,2-dibromocyclohexane as a product it is liquid (it cannot form H-bonds thus will not readily dissolve in water) BUT if you have 2-bromocyclohexanol as a product then it will be aqueous.
17) Be very careful to fully answer the question. Because it asked you to explain why the physical properties were similar but the chemical properties were different you had to do both. Both alkenes and alkanes are covalent molecular hydrocarbons thus they both have similar physical properties are non-conductors with low mpt and bpt. However, alkenes C=C double bond allows it to undergo addition reactions making them more chemically reactive then alkanes which only have single C-C bonds. NB the emphasis on the key chemistry facts: alkenes, C=C, reactive (addition) & alkanes, C-C, less reactive
2003
17b) Describe means you need to give some chemical detail. Identifying it is addition polymerisation is one mark. For the full marks you need to specify the process of how addition polymerisation works, give an equation for the reaction and or the specific details of LDPE vs HDPE manufacture
17c) We went over the phrasing of this in our notes and Experiment #2 “The chaotic natur of addition polymerisation means activated chains of different length collide and terminate, thus resulting in a polymer with a distribution of molecular weights”
2004
17a) If the question asks for ONE answer give one answer. Also if it asks for the common name and you give BOTH the common name and systematic name you will get zero as it shows that you do not know which is which. For the record (common/systematic): vinyl chloride / chloroethene & styrene / ethenylbenzene
17b) Because it said ‘discuss’ you need to give a little more then just a link between property and use. For the three marks: name the polymer (eg polystyrene), identify a property (when expanded it is tough and lightweight), link it to a use (thus used for packaging to protect goods), contrast to a different application (but when non –expanded it is clear and rigid so used for CD cases).
17c) i) READ THE QUESTION – draw the POLYMER (ie at least two repeating units). ii) polymers are so long that they form tangled chains with LOTS of interchain bonds. They are solids, not gases.
2005
16c) Be specific when referring to the method. We used 2ml of each chemical ONLY for safety reasons. You must say that we used identical volumes (for validity) so say the exact amount “we used 2ml each of cyclohexene and cyclohexane”, is better and quicker than writing “we used equal volumes of of cyclohexene and cyclohexane”.
2006
20) A 7 mark question will likely be the biggest question in the paper. This is the sort of Q where the band 5’s are separated from the band 6’s. So in these Q you have to do whatever you can to distinguish your answer from the rest of the state. The Q had four main parts
i) Why ethylene is important – ie telling you must have an intro describing what ethylene is (structure) noting the double bond, noting it is reactive and can undergo addition reactions
ii) Describe new materials – ie NB that addition reactions can make a range of chemicals eg halogenated hydrocarbonds (refrigeration), but the big focus should be on addition polymerisation (ie making the monomers styrene, vinyl chloride) and the polymers PS, PVC , LDPE and HDPE
iii) Making fuels – we know that ethanol for fuels comes from fermentation BUT - do what the question asks – details how ethylene can be turned into ethanol
iv) Use equations (and because the Q specified equations they must be balanced HAVE states and there must be more then one)– eg hydration of ethylene to ethanol (with states and catalyst), addition polymerisation of ethylene into polyethylene.
2007
26) polyethylene and polystyrene come in two different structural types with different properties and applications. Thus link the structure to properties to uses for EACH of LDPE, HDPE, rigid PS and expanded PS = 4 marks. Misconception alert. The benzene ring makes the polystyrene chains lock together making non-expanded PS quite rigid. BUT when expanded the properties are dominated by the air bubbles in the structure (it is effectively a composite of air and PS). You do not need to know the explaantion for why the properties change so dramatically when expanded (beyond having the air make it less dense) but it is important that you understand that the air does NOT act as a plasticiser and make the chains slide easier.
2008
16b) You had to mention the three chemicals you used and the alkene and alkane had to be corresponding (eg cyclohexene and cyclohexane), you had to say what you would OBSERVE to distinguish them (ie in cyclohexene the colour of the bromine water went brown to clear) and you had to include a fully balanced, structural equation with correct states (see 2002 Q16c above).
2009
6) Read the question carefully. Unsaturated means it has double (or triple bonds) and thus there will be an ADDITION reaction!
19) Read the question i) ‘physical and chemical processes’ & ii) ‘from a raw material’. Raw material means something found in nature. Some interpreted this to mean cellulose and they did the whole cellulose to glucose to ethanol to ethylene description – this is fine but the problem is that we don’t make ethylene that way (yet). So you had to start from CRUDE OIL. From there you had to describe each of these processes i) fractional distillation (P), ii) cracking (C), iii) addition polymerisation (C), iv) shaping/moulding into a bottle (P). NB LDPE & HDPE are thermoplastics so they can be molten and shaped. BIG MARKS = USE AT LEAST ONE CHEMICAL EQUATION
2010
Q24b) Whenever you get a question about distinguishing alkenes vs. alkanes you should ALWAYS use the Br water experiment. Why not bubble the gas through Br water and if it turns clear that means some ethylene is still present. If it stays brown that means only ethane is present….
2011
1) READ THE QUESTION CAREFULLY!
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