# Thread: HSC 2016 Chemistry Marathon

1. ## HSC 2016 Chemistry Marathon

The HSC Chemistry Marathon is an open chain of questions between students. It works by answering a question then posting another question and allowing the cycle to repeat itself.

Rules:
- After answering a question, always provide a new one - this is what keeps the thread alive.
- Allocate a number of marks for any question that you post.
- Do not cheat, if you cannot answer a question, do not search how to answer the question but rather, allow other students to answer the question.
- No copyrighted questions (eg CSSA and Independent) should be posted.

Tips:
- You may post more than one question.
- When possible, after questions have been answered, you can peer mark using the marking scheme.

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Aside: I shall reference anomalousdecay, who is the person I completely pasted pretty much 99% of the wording from.

2. ## Re: HSC Chemistry Marathon 2016

I'll roll off chemistry with an easy question as well.

Explain why the alkene series is generally more chemically reactive than the alkane series. (3)

3. ## Re: HSC Chemistry Marathon 2016

Great initiative leehuan! However the marathon will most likely be inactive till next year, since there are very few 2016ers on bos right now

4. ## Re: HSC Chemistry Marathon 2016

Explain why the alkene series is generally more chemically reactive than the alkane series. (3)[/QUOTE]

Alkene series are generally more reactive than alkanes are due to its functional group, the double bond. The double bond in alkenes are a site of high electron density, therefore, substances are more likely to react with the double bond.

I feel like my answer is too short????

Here's a question:
Compare the type of reactions that occur to produce polyethylene and cellulose. Include relevant formulae equations to assist your answer. (5)

5. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Ekman
Great initiative leehuan! However the marathon will most likely be inactive till next year, since there are very few 2016ers on bos right now
Thanks! True that though, so I'll answer a few questions every now and then to promote activity as well as possible. Maybe one every 2-3 days when there isn't a previous answer. Later on I'll just let the thread flow.

Originally Posted by RachelGreen
Explain why the alkene series is generally more chemically reactive than the alkane series. (3)

Alkene series are generally more reactive than alkanes are due to its functional group, the double bond. The double bond in alkenes are a site of high electron density, therefore, substances are more likely to react with the double bond.

I feel like my answer is too short????

Here's a question:
Compare the type of reactions that occur to produce polyethylene and cellulose. Include relevant formulae equations to assist your answer. (5)
Although I didn't use the word "compare" or "contrast", it helps to state that alkanes only have single bonds within the hydrocarbon chain structure, which are typically chemically unreactive. The third mark was a bit hidden, and required you to justify the process - we can say that the double bond is easily opened up through interaction with other molecules, for example, through addition reactions or polymerisation. Alternatively, use the bromine water practical equations (if you have done so).
Hexane: C6H14(l) + HOBr(l) -(UV)-> C6H12BrOH(l) + H2(g)
1-hexene: C6H12(l) + HOBr(l) -> C6H12BrOH(l)
This is acceptable as the necessity of UV to force a substitution reaction is obviously more energy consuming than the simple addition reaction.

Maybe 1/3, but keep trying!

6. ## Re: HSC Chemistry Marathon 2016

Originally Posted by RachelGreen
Here's a question:
Compare the type of reactions that occur to produce polyethylene and cellulose. Include relevant formulae equations to assist your answer. (5)
(Sometimes I do over-answer, and some information may be unnecessary.)

Polyethylene is regarded as an addition polymer, in which monomer units (in this case, ethylene) continuously join together through the process of addition polymerisation. Addition polymerisation of ethylene can occur through the use of a peroxide initiator molecule, or the Ziegler-Natta process. In the peroxide initiator method, a peroxide molecule is severed [R-O-O-R -> 2 R-O] to become a radical and will attack an ethylene monomer unit. This activates the process [R-O + CH2=CH2 -> R-O-CH2-CH2] and the chain propagates with further monomer units being attacked. The process can be terminated when another radical joins on the chain, thus producing polyethylene. Specifically, the peroxide initiator method (achieved at significantly high temperatures and pressures) produces LDPE with extensive side branching, thus limited dispersion forces, whereas the Ziegler-Natta process (achieved at moderate temperatures and pressures) produces HDPE with limited side branching, thus significant dispersion forces, hence polyethylene can be a hard or flexible polymer.
The overall equation is: n CH2=CH2 -> -(-CH2-CH2-)n-

By contrast, cellulose is classified as a condensation polymer, through which monomer units (in this case, beta-glucose) ultimately join to form cellulose through condensation polymerisation. This process involves the expulsion of a small molecule (in this case, water) during the polymerisation process. Of note is the alternating hydroxyl (-OH) groups in the glucose monomers, and the process begins through two glucose monomers joining on carbons 1 and 4 to form a beta maltose dimer [2 C6H12O6 -> C12H22O11 + H2O]. These maltose dimers will then join together on carbons 1 and 4 again, continuously such that the chain will propagate. The result is the linear, rigid polymer cellulose.
The overall equation is: n C6H12O6 -> -(-C6H10O5-)n- + (n-1) H2O
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NEXT QUESTION:
Explain why ethanol has extensive uses as a solvent in various chemical industries. (4)

7. ## Re: HSC Chemistry Marathon 2016

Originally Posted by leehuan
Explain why ethanol has extensive uses as a solvent in various chemical industries. (4)
Ethanol as a molecule has both polar and non-polar ends which allows it to dissolve polar and non-polar substances. The OH functional group on ethanol is very polar as oxygen is very electronegative. Furthermore, the hydrogen atom can from strong hydrogen bonds with other molecules. This makes ethanol a useful solvent for polar substances. The other end of the ethanol molecule consists of non-polar bonds between carbon and hydrogen. This makes ethanol a useful solvent for non-polar substances.

Finally, ethanol can be readily supplied and produced in bulk from hydration of ethylene or fermentation of glucose which increases the availability of ethanol in chemical industries as a solvent.

Next question: Outline the three major steps in the polymerisation of ethylene.

8. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Ambility
Next question: Outline the three major steps in the polymerisation of ethylene.
Assuming this is for 3 marks:

The monomer of ethylene undergoes three main steps during the process of polymerisation in order to produce the polymer polyethylene.
Initiation
An initiator molecule, such as benzyl peroxide, is added to the monomer of ethylene. The peroxide's one free radical reacts with the ethylene, breaking the double bond and allowing it to attach itself to the bonding site, creating an ethylene-initiator radical.
Propagation
The ethylene-initiator radical then reacts with another ethylene monomer, opening another bonding site and allowing for the monomer to attach. This process repeats itself, allowing for the chain of the polymer to get longer.
Termination
This is where the reacting process stops, when two similar chains collide and the two radical's react with each other, completing the reaction and terminating the sequence. Termination is a random process and can occur at any time for different reactions

Also, this process of polymerisation of ethylene produces the polyethylene, however the incorporation of a catalyst (such as titanium trialkylaluminium) in the process produces straight, rigid chains called high-density polyethylene, as opposed to the regular branched structure known as low-density polyethylene.

Next question: Discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry [6 marks]

9. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Ambility
Ethanol as a molecule has both polar and non-polar ends which allows it to dissolve polar and non-polar substances. The OH functional group on ethanol is very polar as oxygen is very electronegative. Furthermore, the hydrogen atom can from strong hydrogen bonds with other molecules. This makes ethanol a useful solvent for polar substances. The other end of the ethanol molecule consists of non-polar bonds between carbon and hydrogen. This makes ethanol a useful solvent for non-polar substances.

Finally, ethanol can be readily supplied and produced in bulk from hydration of ethylene or fermentation of glucose which increases the availability of ethanol in chemical industries as a solvent.

Next question: Outline the three major steps in the polymerisation of ethylene.
3 or maybe 4/4, mainly cause you only referred to one intermolecular force (hydrogen bonding). You should relate the non-polar bonds to interaction with molecules such as say cyclohexene through dispersion forces, and whilst the statement that oxygen is electronegative may be able to suffice (in fact, quite likely so), I personally would relate to dipole-dipole interactions.
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Comment on your question, you may want to state LDPE, because HDPE is not produced using those three steps which you were likely seeking information for.

10. ## Re: HSC Chemistry Marathon 2016

Originally Posted by leehuan
_______________________
Comment on your question, you may want to state LDPE, because HDPE is not produced using those three steps which you were likely seeking information for.
I thought i remembered reading from my textbook that they chuck in a catalyst and it prevents backbiting thus producing HDPE but it still follows the same procedure (with differences in temperatures and pressures). Correct me if I'm wrong, it was a while ago

11. ## Re: HSC Chemistry Marathon 2016

Originally Posted by leehuan
3 or maybe 4/4, mainly cause you only referred to one intermolecular force (hydrogen bonding). You should relate the non-polar bonds to interaction with molecules such as say cyclohexene through dispersion forces, and whilst the statement that oxygen is electronegative may be able to suffice (in fact, quite likely so), I personally would relate to dipole-dipole interactions.
_______________________
Comment on your question, you may want to state LDPE, because HDPE is not produced using those three steps which you were likely seeking information for.
Fair enough, I was thinking about that in my head, just didn't explicitly mention it.

Regarding LDPE/HDPE, this is what my textbook (conquering chemistry) says:

In both these processes the initiator of catalyst activates an ethylene molecule by attaching to it. An ordinary ethylene molecule then attaches to this, forming a new activated species. Polymerisation continues by ethylene molecules one after the other adding to the growing chain. Finally the process stops by two such activated chains colliding with each other; they either join together to form a stable polymer molecule or they exchange a hydrogen atom to form two stable molecules.

12. ## Re: HSC Chemistry Marathon 2016

Originally Posted by DatAtarLyfe
Assuming this is for 3 marks:

The monomer of ethylene undergoes three main steps during the process of polymerisation in order to produce the polymer polyethylene.
Initiation
An initiator molecule, such as benzyl peroxide, is added to the monomer of ethylene. The peroxide's one free radical reacts with the ethylene, breaking the double bond and allowing it to attach itself to the bonding site, creating an ethylene-initiator radical.
Propagation
The ethylene-initiator radical then reacts with another ethylene monomer, opening another bonding site and allowing for the monomer to attach. This process repeats itself, allowing for the chain of the polymer to get longer.
Termination
This is where the reacting process stops, when two similar chains collide and the two radical's react with each other, completing the reaction and terminating the sequence. Termination is a random process and can occur at any time for different reactions

Also, this process of polymerisation of ethylene produces the polyethylene, however the incorporation of a catalyst (such as titanium trialkylaluminium) in the process produces straight, rigid chains called high-density polyethylene, as opposed to the regular branched structure known as low-density polyethylene.

Next question: Discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry [6 marks]
I'm sure that would be sufficient for 3/3. The marking guidelines from the test this question came form says 1/2 mark for naming each process, 1/2 mark for explaining each process.

13. ## Re: HSC Chemistry Marathon 2016

The peroxide initiator should not be regarded as a "catalyst" in the production of LDPE. The TiCl3-Triaklylaluminium surface, used in the Ziegler Natta process for HDPE production, however, is. Honestly, Conquering Chemistry explains things so much more clearly than Jacaranda. The correction I need to make is that the initiation-propagation-termination process still holds true. The only difference is whether the term initiator or catalyst should be coined.

Which is actually really disappointing, because there was a question on it in the HSC exam on this and I garbled about the three processes, but I referred it to LDPE only. I only made a brief reference to HDPE.

14. ## Re: HSC Chemistry Marathon 2016

Here's a question
Relate the structure, properties and uses of polyvinyl chloride and polystyrene. (4 marks)

15. ## Re: HSC Chemistry Marathon 2016

Polyvinyl chloride is used in raincoats and shower curtains because PVC is water-resistant due to its structure containing many non-polar C-C & C-H bond. Its water-resistant nature combined with its rigid and toughness (as a result of chain stiffening) is used in water pipes. In addition, these water pipes must be used underground because PVC has a C-Cl bond that is not stable in sunlight.

Polystyrene is a clear amorphous polymer that exhibits the property chain stiffening that makes the substance rigid/stiff, and has god electrical insulation properties. As a result, it is used as an insulation for electrical wires. The high stiffness makes the polymer suitable for rigid items such as car battery cases, handles for screwdrivers etc.

Next question:
Tincture of iodine is an antiseptic often found in medical kits. It is a solution of iodine in ethanol. Ethanol is used as the solvent as iodine is relatively insoluble in water. When an aqueous solution of iodine is required, iodine ions are added to iodine to form the triiodide ion which is more soluble in water.

(a) Write an equation for the reaction above for the solution of iodine
(b) Draw a labelled diagram to explain the solubility of iodine in ethanol.
(c) Explain why the triiodide ion is more soluble in water than is iodine.

16. ## Re: HSC Chemistry Marathon 2016

Originally Posted by RachelGreen
Polyvinyl chloride is used in raincoats and shower curtains because PVC is water-resistant due to its structure containing many non-polar C-C & C-H bond. Its water-resistant nature combined with its rigid and toughness (as a result of chain stiffening) is used in water pipes. In addition, these water pipes must be used underground because PVC has a C-Cl bond that is not stable in sunlight.

Polystyrene is a clear amorphous polymer that exhibits the property chain stiffening that makes the substance rigid/stiff, and has god electrical insulation properties. As a result, it is used as an insulation for electrical wires. The high stiffness makes the polymer suitable for rigid items such as car battery cases, handles for screwdrivers etc.
Yep, 4/4. Also nice question. If you want though, the chain stiffening can be related to the hexagonal structure of the benzene ring in PS.

17. ## Re: HSC Chemistry Marathon 2016

Originally Posted by RachelGreen
Next question:
Tincture of iodine is an antiseptic often found in medical kits. It is a solution of iodine in ethanol. Ethanol is used as the solvent as iodine is relatively insoluble in water. When an aqueous solution of iodine is required, iodine ions are added to iodine to form the triiodide ion which is more soluble in water.

(a) Write an equation for the reaction above for the solution of iodine
(b) Draw a labelled diagram to explain the solubility of iodine in ethanol.
(c) Explain why the triiodide ion is more soluble in water than is iodine.
Is this question from the production of materials module?

18. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Ambility
Is this question from the production of materials module?
Solubility in ethanol is by default Production of Materials.

Part c) is interesting, however; it draws on knowledge from the preliminary course.

19. ## Re: HSC Chemistry Marathon 2016

Originally Posted by DatAtarLyfe
Next question: Discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry [6 marks]
.

20. ## Re: HSC Chemistry Marathon 2016

Originally Posted by DatAtarLyfe
Next question: Discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry [6 marks]
I personally do not agree that this is a 6 mark question. You are asking about only the situation of the world's fuel supplies, and not actually explicitly relating it to ethanol and it's potentials. The need for a new fuel source altogether is a small dot point.

21. ## Re: HSC Chemistry Marathon 2016

Originally Posted by RachelGreen
Next question:
Tincture of iodine is an antiseptic often found in medical kits. It is a solution of iodine in ethanol. Ethanol is used as the solvent as iodine is relatively insoluble in water. When an aqueous solution of iodine is required, iodine ions are added to iodine to form the triiodide ion which is more soluble in water.

(a) Write an equation for the reaction above for the solution of iodine
(b) Draw a labelled diagram to explain the solubility of iodine in ethanol.
(c) Explain why the triiodide ion is more soluble in water than is iodine.
Note, you wouldn't expect this type of question in the HSC though.

(a) I2(aq) + I-(aq) <--> I3-(aq)
(b) I-I
Molecular iodine is a linear molecule and non-polar, similar to the hydrocarbon chain in ethanol. The similar molecular structure of these molecules allows for dispersion forces to form between the molecules, which accounts for the solubility of iodine in ethanol.
(c) Whereas iodine is non-polar, the triiodide ion is a molecule with a -1 charge. Noting how water is a polar molecule; the hydrogen atoms being slightly positive and the oxygen atom slightly negative, ion-dipole interactions will form between the molecules, allowing for a sound solubility of the ion. By contrast, iodine is also a linear molecule, opposed to the bent structure of water, hence no significant intermolecular forces can form between molecules to promote solubility.

22. ## Re: HSC Chemistry Marathon 2016

Oh wow that's perfect!

23. ## Re: HSC Chemistry Marathon 2016

Originally Posted by DatAtarLyfe
Assuming this is for 3 marks:
Next question: Discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry. [6 marks]
Let's reword this.

Using ethanol as an example, discuss the need for an alternative source of energy and fuel in the future, with key references to the current situation in the petrochemical industry. Explain the benefits that are carried along with the use of ethanol as said substitute. (6)

24. ## Re: HSC Chemistry Marathon 2016

Petroleum is a non-renewable fossil fuel, and supplies are being rapidly depleted due to heavy demands for petrochemicals for a variety of uses such as fuels, plastics, lubricants, solvents etc. The demand is increasing as world's populations increases. Consequently, crude oil is being rapidly depleted due to HIGH demand and limited crude oil reserves. Furthermore, the need for alternative sources of petrochemical products derivatives come down to two significant points: environmental impact and scarcity.

- Majority of crude oil is used up as a fuel, the consumption of fuel products is detrimental upon the environment which releases pollutants in the atmosphere (octane - major component of petrol, burns incompletely). In comparison to other potential fuels such as ethanol, the current petrol products burns relatively uncleanly, leading to environmental problems.

- Most polymers are currently derived from petroleum are currently non-biodegradable which places a considerable strain on our landfills. Alternative sources esp. biopolymers, are biodegrable and would alleviate such problems

- Another reason why alternative sources are needed because petrochemical products are derived from non-renewable sources of crude oil, with chemists placing the lifespan on current petroleum supplies well under 50 years, alternative sources are required simply due unsustainable trends

Alternatives sources such as biomass may be used, however they are more expensive than crude oil. In addition, using ethanol as an example for alternative sources; new infrastructure needs to be constructed (such as fermentation plants for ethanol), a process which takes time and money and just isn't economically viable currently.

84% of crude oil is used to produce energy. This includes petrol and diesel for cars. Potential alternative sources too alleviate these problems would be ethanol where it will meet with not only future energy needs, but with material needs as well. Ethanol can be produced by fermenting sugars from sugar cane crops:
C6H12O6 (aq) ----(yeast enzymes) ---> 2C2H5OH (aq) + 2CO2(g)

Benefits:
- Ethanol is able to be used as a petrol supplement, because it undergoes combustion: C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (l) + heat
It can be used as a substitute because it is a renewable resource as it is manufactured from carbohydrates such as glucose and starch that are produced by photosynthesis by plants. The products of its combustion (CO2 and H2O) are the reacts needed by plants for photosynthesis: 6CO2 (g) +
6H2O (l) ----light----> C6H12O6 (aq) + 6O2 (g)

- Burns completely in oxygen (cleaner burning fuel) in contrast to other fuels that undergoes incomplete combustion thus producing toxic CO and carcinogenic soot

Ethanol combined with its high energy-per-mole output, cleaner burning nature and ease of transport, ethanol has great potential as an alternative fuel.

NEXT QUESTION:
Outline two methods of producing ethanol. In your answer, construct chemical equations to demonstrate ONE renewable and ONE non-renewable method of ethanol production. (4 marks)

25. ## Re: HSC Chemistry Marathon 2016

Originally Posted by RachelGreen
Outline two methods of producing ethanol. In your answer, construct chemical equations to demonstrate ONE renewable and ONE non-renewable method of ethanol production. (4 marks)
Ethanol can be produced non-renewably from crude oil. Petroleum is separated into its constituent hydrocarbon molecules through a process of fractional distillation using boiling point as a difference in properties. One of the distillate fractions will contain ethylene but if more ethylene is required then higher molecular weight fractions can be catalytically or thermally cracked to ethylene which is the process of breaking the larger molecules down into smaller ones. Ethylene is then converted to ethanol through a hydration reaction which is an addition of a water molecule over ethylene's double bond. This requires 300 degrees Celsius and a sulfuric acid catalyst.

$C_2H_4+H_2O \to C_2H_5OH$

Ethanol can be produced renewably from fermentation. Biomass from plants containing cellulose is broken down both into glucose monomers and then into ethanol through a process of fermentation. This process produces ethanol along with carbon dioxide, requires an enzyme (yeast) and about 37 degrees Celsius. This process will only produce ethanol up to a concentration of 15%, further concentrations of ethanol can be derived through distillation.

$C_6H_{12}O_6 \to 2C_2H_5OH + 2CO_2$

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