Showing posts with label 921. Show all posts
Showing posts with label 921. Show all posts
Tuesday, 5 November 2013
Wednesday, 16 October 2013
Tuesday, 15 October 2013
Friday, 11 October 2013
Monday, 4 February 2013
9.2.1 HSC Q Feedback
Generally very good and thorough answers.
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!
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!
Tuesday, 30 October 2012
Portfolio: Experiment #2
PORTFOLIO #2 Feedback on Worksheets
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.
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.
Tuesday, 16 October 2012
Portfolio: Experiment #1
Notes from today's lesson here
Experiment results
a) The chemicals before mixing - note the colours
b) The chemicals immediately after mixing - are there any differences?
Things to ponder
vi) For the chemical that reacted with bromine water - can you name and draw a structural diagram of the product?
Using a control
In the fume cupboard were three extra test tubes:
*One with just cyclohexane
*One with just cyclohexene and
*One with just Br-water
They just sat there and weren't mixed with anything. Throughout the whole time the Br-water stayed brown and the cyclohexene and cyclohexane stayed clear.
Thus, by comparing our results to the control, we can be sure that the change in colour was due to the reaction between the cyclohexene and the Br-water, ie not just due to some other environmental effect or because the Br-water naturally changes colour.
Reliability
We repeated the experiment with consistent results AND we can use other secondary sources to check the reliability of our results. See the clips below
Q4)
Q8)
Experiment results
a) The chemicals before mixing - note the colours
 |
| Br-water |
 |
| Cyclohexane |
 |
| Cyclohexene |
b) The chemicals immediately after mixing - are there any differences?
 |
| Cyclohexane and Br-water (immediate) |
 |
| Cyclohexene and Br-water (immediate) |
c) The mixtures after 10, 15 and 20 minutes
 |
| Cyclohexane (a) & Cyclohexene (e) 10 min after mixing with Br-water |
 |
| Cyclohexane (a) & Cyclohexene (e) 15 min after mixing with Br-water |
 |
| Cyclohexane (a) & Cyclohexene (e) 20 min after mixing with Br-water |
i) Which chemical reacted more rapidly with the Br-water and what evidence led you to that conclusion
ii) Why were two layers formed when the chemicals were mixed (ie why were they immiscible)?
iii) Which layer was on the bottom and why?
iv) The cyclohexane layer got browner over time suggesting that the bromine was dissolving into it - why is bromine soluble in cyclohexane?
v) For the same reason as (iv) bromine should also be soluble in cyclohexene - why didn't the cyclohexene turn brown?vi) For the chemical that reacted with bromine water - can you name and draw a structural diagram of the product?
Using a control
In the fume cupboard were three extra test tubes:
*One with just cyclohexane
*One with just cyclohexene and
*One with just Br-water
They just sat there and weren't mixed with anything. Throughout the whole time the Br-water stayed brown and the cyclohexene and cyclohexane stayed clear.
Thus, by comparing our results to the control, we can be sure that the change in colour was due to the reaction between the cyclohexene and the Br-water, ie not just due to some other environmental effect or because the Br-water naturally changes colour.
Reliability
We repeated the experiment with consistent results AND we can use other secondary sources to check the reliability of our results. See the clips below
PORTFOLIO #1 Feedback on Worksheets
MOST IMPORTANT TIP – DO NOT EVER leave any of these questions blank. They are a compulsory part of the coursework and are a prerequisite for getting your draft summary marked. IF you cannot do a question you need to:
- Look up the answer in your notes/text
- Ask a classmate
- Ask Dr Burg
Then you need to do the question
Q2)
When a question states structural formula and names you have to give structural formula AND names - amazing! Also – unless you have finished you HSC exam you are obligated to put ALL the hydrogens on ALL your carbon chains. Seeing students take shortcuts in these practise questions makes me want to use octane, nonane and decane for ALL hydrocarbon questions in your exams. You have been warned.
Q3) & Q5)
For combustions make sure you balance the carbon , then hydrogen, then oxygen (CHO)
Q4)
Remember that in substitution reactions (for alkanes) only ONE species is added to the chain in the place of ONE hydrogen (i.e. the hydrogen is substituted for something else)
Q6)
Alkenes undergo addition reactions. TWO species are added to the alkene AT THE SITE OF THE DOUBLE BOND. Thus, if the double bond is between the first and second carbon, the species added will be on the first and second carbon. In addition reactions (unlike substitution reactions) only one product is produced.
E.g. if chlorine gas is added to 1-pentene the product will be 1,2-dichloropentane.
NOTE: there is commas between numbers and hyphen between letters and numbers when naming
E.g. if water is added to hex-2-ene (with a sulfuric acid catalyst) we are adding a H and a OH group to a hexene chain with a double bond between the second and third carbon. Thus the OH will go on either the second or third carbon. So the product could be hexan-2-ol or hexan-3-ol.
NOTE: When adding asymmetrical molecules (water, HCl, HBr etc) to alkenes bigger than ethylene there are always two possible products
Q7)
*It is important to include any necessary catalysts for these reactions.
* Sometimes there will be more than one way to make the chemical.
* You need to make sure your equations are balanced. It is easier to do this if you draw and think about the structures.
E.g. If I want to make 1-bromobutane (CH2BrCH2CH2CH3) from but-1-ene (CH2CHCH2CH3) I need to add an extra hydrogen and a bromine atom. Thus I have to react the but-1-ene with HBr!
Q8)
Where the table says ‘Burns’ it means ‘Combusts’ – so most people still need to write the combustion equations for pentane, hexane and ethanol. Balancing combustion reactions is something we all need to practise (CHO!)
PORTFOLIO #1 – Feedback on Summary Sheets
Note – these summaries were very well done. The addition (lol, that’s very punny Dr Burg!) of skills to this experiment summary made it quite challenging and you all did very well. I had to increase difficulty of the marking scheme to distinguish between you.
NOTE – in this experiment decoloured (which means REMOVE colour) is a better term to use than discoloured (which just means CHANGE colour).
(A) There is something in Engineering (and Science) called the KIS principle (Keep It Simple). Don’t say more than you need to. Answer the question and that is it. You only had to explain why alkenes are more reactive than alkanes. Eg “Unlike single-bonded alkanes, alkenes have a carbon-carbon double bond that allows them to undergo addition reactions. Thus, alkenes are more reactive than alkanes.”
(B) The dependent variable is the factor that is measured in order to determine how it responds to changes made to the independent variable (i.e. the value of the dependent variable DEPENDS on the independent variable) . In this experiment the dependent variable is the reactivity of the molecules (which we measured by the rate at which they decoloured bromine water).
(C) We need to control all variables other than the DV and IV to make the experiment valid – ie so that measured changes to the DV are ONLY due to the changes made to the IV. Some examples from this experiment include temperature, time of measurement, light intensity, and the fact that we used corresponding molecules (same # of carbons with the same shape).
(D) A control is a sample that is left unchanged so that all other results can be compared to it (ie to determine if the IV really had any effect on the DV). In this case we left samples of bromine water, cyclohexene and cyclohexane unmixed over the same time period as the experiment. The fact that they did not change colour indicates that the observed colour changes in the experiment were due to the reaction between cyclohexene and bromine water.
(E) Make sure you name the reactants and products. Remember that when naming hydrocarbons there are commas between numbers and hyphens between letters and numbers. Eg CCl3-CClF2 is called 1,1,1,2-tetrachloro-2,2-difluoroethane. Also - bromine (the element) is a liquid but bromine water is a aqueous solution of bromine – ie Br2 (aq)
(F) Even though cyclohexane was the only chemical identified as being dangerous for the environment, we should never tip harmful chemicals down the sink if we can avoid it. All three were stored for proper disposal.
(G) Reliability is a measure of whether the results are reproducible. This requires the experiment to be repeated with consistent results. Validity is a measure of whether the investigation was able to measure what was intended. The most important way to ensure a valid experiment is to ensure that all variables are controlled so that the only thing affecting the DV is the chnages made to the IV (but it also requires accurate measurement and reliability).
(H) Don’t forget that we did the experiment in a fume cupboard!
(I) A cause is a REASON for something happening. An effect is what happens as a RESULT of the cause. A great ay to explain a cause-and-effect situation is this awesome word: ‘because’ (it introduces a cause). Eg “Because alkenes have a reactive double bond (the cause), they undergo an addition reaction with bromine water (the effect)”.
(J) To explain reliability we need to consider whether we repeated the experiment with consistent results. Even though no individual in the class repeated the experiment, each group did the same experiment and all groups obtained the same result.
(K) You have been asked to write four different conclusion sentences (from slightly different perspectives). You need to treat each as if it was a stand-alone conclusion. So each needs to say (somehow) that the cyclohexene reacted faster than cyclohexane with bromine water and thus that alkenes are more reactive than alkanes.
(L) Quantitative results involve measured numerical values (quantities, eg mass, time, volume). Qualitative results involve descriptive measurements of observations (eg colour, shape, size, identity).
(M) You need to include labels on your diagrams. If your diagrams are not coloured you need to label the colours. You should label that the bottom layer was always the aqueous (bromine water) layer. It is important to note that the hydrocarbon layer NEVER mixed with the aqueous layer (remember ‘like dissolves like’). Even when the bromine water was completely decoloured there were still two layers: a clear cyclohexene mixture on top and a clear aqueous mixture below.
(N) It is a good idea to make your safety section concise by combining all the similar risks together but be careful not to make erroneous statements. Eg you cannot say that the bromine water risks and precautions are identical to cyclohexene, as cyclohexene is flammable and bromine water is not.
(O) All hydrocarbon reaction equations (apart from combustion, photosynthesis and fermentation should be structural equations - with states.
(P) You need to make your statements generalised - ie we studied this one simple system of cyclohexene and cyclohexane (ignore the hexane) so we know that the only thing affecting the rate of reaction is the double bond. Thus, we can conclude more than just cyclohexene reacts faster than cyclohexane - we can make the generalisation that alkenes are more reactive than...
(Q) Specify what your controlled variables were
PORTFOLIO #1 – Feedback on Summary Sheets
Note – these summaries were very well done. The addition (lol, that’s very punny Dr Burg!) of skills to this experiment summary made it quite challenging and you all did very well. I had to increase difficulty of the marking scheme to distinguish between you.
NOTE – in this experiment decoloured (which means REMOVE colour) is a better term to use than discoloured (which just means CHANGE colour).
(A) There is something in Engineering (and Science) called the KIS principle (Keep It Simple). Don’t say more than you need to. Answer the question and that is it. You only had to explain why alkenes are more reactive than alkanes. Eg “Unlike single-bonded alkanes, alkenes have a carbon-carbon double bond that allows them to undergo addition reactions. Thus, alkenes are more reactive than alkanes.”
(B) The dependent variable is the factor that is measured in order to determine how it responds to changes made to the independent variable (i.e. the value of the dependent variable DEPENDS on the independent variable) . In this experiment the dependent variable is the reactivity of the molecules (which we measured by the rate at which they decoloured bromine water).
(C) We need to control all variables other than the DV and IV to make the experiment valid – ie so that measured changes to the DV are ONLY due to the changes made to the IV. Some examples from this experiment include temperature, time of measurement, light intensity, and the fact that we used corresponding molecules (same # of carbons with the same shape).
(D) A control is a sample that is left unchanged so that all other results can be compared to it (ie to determine if the IV really had any effect on the DV). In this case we left samples of bromine water, cyclohexene and cyclohexane unmixed over the same time period as the experiment. The fact that they did not change colour indicates that the observed colour changes in the experiment were due to the reaction between cyclohexene and bromine water.
(E) Make sure you name the reactants and products. Remember that when naming hydrocarbons there are commas between numbers and hyphens between letters and numbers. Eg CCl3-CClF2 is called 1,1,1,2-tetrachloro-2,2-difluoroethane. Also - bromine (the element) is a liquid but bromine water is a aqueous solution of bromine – ie Br2 (aq)
(F) Even though cyclohexane was the only chemical identified as being dangerous for the environment, we should never tip harmful chemicals down the sink if we can avoid it. All three were stored for proper disposal.
(G) Reliability is a measure of whether the results are reproducible. This requires the experiment to be repeated with consistent results. Validity is a measure of whether the investigation was able to measure what was intended. The most important way to ensure a valid experiment is to ensure that all variables are controlled so that the only thing affecting the DV is the chnages made to the IV (but it also requires accurate measurement and reliability).
(H) Don’t forget that we did the experiment in a fume cupboard!
(I) A cause is a REASON for something happening. An effect is what happens as a RESULT of the cause. A great ay to explain a cause-and-effect situation is this awesome word: ‘because’ (it introduces a cause). Eg “Because alkenes have a reactive double bond (the cause), they undergo an addition reaction with bromine water (the effect)”.
(J) To explain reliability we need to consider whether we repeated the experiment with consistent results. Even though no individual in the class repeated the experiment, each group did the same experiment and all groups obtained the same result.
(K) You have been asked to write four different conclusion sentences (from slightly different perspectives). You need to treat each as if it was a stand-alone conclusion. So each needs to say (somehow) that the cyclohexene reacted faster than cyclohexane with bromine water and thus that alkenes are more reactive than alkanes.
(L) Quantitative results involve measured numerical values (quantities, eg mass, time, volume). Qualitative results involve descriptive measurements of observations (eg colour, shape, size, identity).
(M) You need to include labels on your diagrams. If your diagrams are not coloured you need to label the colours. You should label that the bottom layer was always the aqueous (bromine water) layer. It is important to note that the hydrocarbon layer NEVER mixed with the aqueous layer (remember ‘like dissolves like’). Even when the bromine water was completely decoloured there were still two layers: a clear cyclohexene mixture on top and a clear aqueous mixture below.
(N) It is a good idea to make your safety section concise by combining all the similar risks together but be careful not to make erroneous statements. Eg you cannot say that the bromine water risks and precautions are identical to cyclohexene, as cyclohexene is flammable and bromine water is not.
(O) All hydrocarbon reaction equations (apart from combustion, photosynthesis and fermentation should be structural equations - with states.
(P) You need to make your statements generalised - ie we studied this one simple system of cyclohexene and cyclohexane (ignore the hexane) so we know that the only thing affecting the rate of reaction is the double bond. Thus, we can conclude more than just cyclohexene reacts faster than cyclohexane - we can make the generalisation that alkenes are more reactive than...
(Q) Specify what your controlled variables were
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