# Thread: HSC 2012-2015 Chemistry Marathon (archive)

1. ## Re: HSC 2012 Chemistry Marathon

This is from Independent 09 Trial Exam

Multiple Choice Q3:

3. The molar heat of combustion of ethanol is 1367kJ/mol
What mass of ethanol is required to heat 1.0 moles of water by 10 degrees.

a - 136.7g
b - 46.0g
c - 25.3g
d - 0.025g

2. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by nightweaver066
Question:

A. Amphiprotic species are those that can act as proton donors as well as proton acceptors. For example when HCl is added, HCO3- + H+ -> H2CO3. In this case the bicarbonate ion has accepted a proton thus act as a proton acceptor. However when titrated against NaOH, HCO3- + OH- -> H2O(l) + CO32-, where the bicaronate ion act as a proton donor, donating a hydrogen ion to the hydroxide. HCO3-, able to donate as well as accept hydrogen ions is thus amphiprotic.
B. 8

QUESTIOn
Explain the process of filtration by microscopic membrane filters with reference to their design and composition.

3. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by EazyEEE
A. Amphiprotic species are those that can act as proton donors as well as proton acceptors. For example when HCl is added, HCO3- + H+ -> H2CO3. In this case the bicarbonate ion has accepted a proton thus act as a proton acceptor. However when titrated against NaOH, HCO3- + OH- -> H2O(l) + CO32-, where the bicaronate ion act as a proton donor, donating a hydrogen ion to the hydroxide. HCO3-, able to donate as well as accept hydrogen ions is thus amphiprotic.
B. 8

QUESTIOn
Explain the process of filtration by microscopic membrane filters with reference to their design and composition.
Microscopic membrane filters have microscopic pores and the use of appropriate sized filters can avoid the need to chemically treat the water. The filters can be classified as microfiltration, ultrafiltration, nanofiltration or reverse osmosis membranes depending on the size of the pore.
The membrane is made from synthetic polymers dissolved in a mixture of solvents.

Semi-permeable membranes used in reverse osmosis are either made of cellulose acetate or a layer of polyamide attached to another polymer. Under pressure these polymers allow the passage of water molecules but not that of most atoms, ions or other molecules.

Water is made to flow across the membrane not through it. This reduces the blockage of the pores and contaminants are carried away as waste. The membrane is housed in a pressure vessel and is either made as a wound spiral or hollow fibres.

Microfiltration removes protozoans, bacteria, colloids, some colouration and some viruses. The size of the pore determines which sized particle or organism may pass through the membrane. The finer the pore size the smaller the particles trapped and the more expensive the membrane.

Question- explain, with an example, how oxidation states allow chemists to decide whether or not a reaction involves electron transfer. (assume 3 marks for this quest)

4. ## Re: HSC 2012 Chemistry Marathon

bump
explain, with an example, how oxidation states allow chemists to decide whether or not a reaction involves electron transfer. (assume 3 marks for this quest)

5. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by VJ30
bump
explain, with an example, how oxidation states allow chemists to decide whether or not a reaction involves electron transfer. (assume 3 marks for this quest)
Your response to that membrane filter was great.

Of course, in an exam, diagram!

If oxidation state increases, oxidation has occured and an electron has been released.

If oxidation state decreases, reduction has occured and an electron has been captured.

In redox reactions, if oxidation occurs, a reduction reaction must occur.

If the oxidation state of one reactant increases, and decreases for another reactant in the same reaction, this means an electron has been released by one of the reactants the same electron was captured by the other revealing that the reaction involves an electron transfer.

Therefore, by monitoring the oxidation states of reactants in a reaction, if one increases and another decreases of the same magnitude, a reaction involving an electron transfer has occured.

Now,

Assess the effectiveness of monitoring and managing CFC & halon production & usage. (5)

6. ## Re: HSC 2012 Chemistry Marathon

In this option you studies one natural product that was not a fossil fuel. Describe the issues associated with shrinking world supplies of this natural product, and evaluate progress being made to solve the problems identified.

Marks : 10

Criteria
Identifies an appropriate natural product
Provides a judgement
Provides characteristics and features of at least TWO issues associated with shrinking world supplies of the natural product
Provides characteristics and features of progress being made to find replacement materials
Provides a response that demonstrate coherence and logical progression and includes correct use of scientific principles and idea

7. ## Re: HSC 2012 Chemistry Marathon

Industrial Chemistry Question ^

8. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by nightweaver066

- Ethene is usually produced from the cracking of large hydrocarbons.
- The cracking of a large alkane always produces a small alkene, usually ethene and occasionally propene, and a large alkane.
- The cracking process involves heating the hydrocarbons to 500C with a zeolite catalyst in the absence of oxygen to prevent combustion.
--> Example: C8H18 --> C2H4 + C6H14
- The zeolite catalyst adsorb alkanes, weakening their bonds is required because the activation energy of cracking without a catalyst is very high, requiring immensely high temperatures making it uneconomical.
- Ethene can then be separated from the hydrocarbons by cooling the gas where 5-carbon alkyl chains will condense into liquids.
- Ethene can be separated from small the other smaller hydrocarbons such as propene by fractional distillation, if necessary.

9. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by someth1ng
- Ethene is usually produced from the cracking of large hydrocarbons.
- The cracking of a large alkane always produces a small alkene, usually ethene and occasionally propene, and a large alkane.
- The cracking process involves heating the hydrocarbons to 500C with a zeolite catalyst in the absence of oxygen to prevent combustion.
--> Example: C8H18 --> C2H4 + C6H14
- The zeolite catalyst adsorb alkanes, weakening their bonds is required because the activation energy of cracking without a catalyst is very high, requiring immensely high temperatures making it uneconomical.
- Ethene can then be separated from the hydrocarbons by cooling the gas where 5-carbon alkyl chains will condense into liquids.
- Ethene can be separated from small the other smaller hydrocarbons such as propene by fractional distillation, if necessary.
Great response

10. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by nightweaver066
Great response

- There are many ways a radioisotope can be produced.
- Two such methods include neutron bombardment and nuclear fission.
--> Both of these methods are frequent used as induced nuclear transformations.
- Neutron bombardment usually involves bombarding a certain nuclei with a neutron but does not necessarily involve the product to undergo fission to produce smaller products, as seen in the case of Technetium-99m.
[Equation] Mo-98+neutron-->Mo-99
[Equation] Mo-99-->Tc-99m+electron+gamma
- Nuclear fission often involves the bombardment of a certain nuclei or particle (including neutrons) causing it to spontaneously undergo fission to produce smaller products, as seen in the case of Strontium-90.
[Equation] U-235+neutron-->Xe-138+Sr-90+8neutrons+gamma
- Evidently, although both methods are different, they are both used to produce radioisotopes.

Fairly plain response but I think it'd get 3/4, not exactly a 4/4 response, in my opinion.

Assess the impact of atomic absorption spectroscopy (AAS) on our understanding of trace elements. (3 marks)

11. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by someth1ng
- There are many ways a radioisotope can be produced.
- Two such methods include neutron bombardment and nuclear fission.
--> Both of these methods are frequent used as induced nuclear transformations.
- Neutron bombardment usually involves bombarding a certain nuclei with a neutron but does not necessarily involve the product to undergo fission to produce smaller products, as seen in the case of Technetium-99m.
[Equation] Mo-98+neutron-->Mo-99
[Equation] Mo-99-->Tc-99m+electron+gamma
- Nuclear fission often involves the bombardment of a certain nuclei or particle (including neutrons) causing it to spontaneously undergo fission to produce smaller products, as seen in the case of Strontium-90.
[Equation] U-235+neutron-->Xe-138+Sr-90+8neutrons+gamma
- Evidently, although both methods are different, they are both used to produce radioisotopes.

Fairly plain response but I think it'd get 3/4, not exactly a 4/4 response, in my opinion.
I think you should include the technology/machinery involved or comparing something else to get 4/4

Assess the impact of atomic absorption spectroscopy (AAS) on our understanding of trace elements. (3 marks)
- AAS is a quantitative analysis technique for determining small concentrations of metals in samples.
- Using AAS, scientists have been able to monitor and understand the effect of certain concentrations of trace elements such as cobalt and copper in agricultural land for optimum crop growth
- However, AAS has detection limits in determining concentrations (e.g. detection limit of 1ppb for lead in water samples) and so cannot monitor the effects of such small concentrations
- Overall, AAS has made a profound impact on our understanding of trace elements as farmers are able to optimise plant growth as [Pb] < 1 ppb does not greatly effect plant growth (made this part up, is it fine? lol)

Question:

12. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by nightweaver066
I think you should include the technology/machinery involved or comparing something else to get 4/4

- AAS is a quantitative analysis technique for determining small concentrations of metals in samples.
- Using AAS, scientists have been able to monitor and understand the effect of certain concentrations of trace elements such as cobalt and copper in agricultural land for optimum crop growth
- However, AAS has detection limits in determining concentrations (e.g. detection limit of 1ppb for lead in water samples) and so cannot monitor the effects of such small concentrations
- Overall, AAS has made a profound impact on our understanding of trace elements as farmers are able to optimise plant growth as [Pb] < 1 ppb does not greatly effect plant growth (made this part up, is it fine? lol)
Yeah, I know...I felt that the marking scheme would expect at least two valid comparisons (2 marks, 1 mark for each comparison), identifying two methods (1 mark), identifying two examples (1 mark).

1. Another comparison could be:
Neutron bombardment usually does not require acceleration as neutrons hold no electrical charge, as it is not repelled by the positive nuclei by electrostatic repulsion, it won't need to be forces into the nucleus.
Nuclear fission can often require bombardment of a positively charged nuclei, this means that it must be accelerated to very high speeds to overcome electrostatic repulsion and allowing a new nucleus to be formed which could spontaneously decay into smaller molecules.

2. How? Give an example such as in South Australia, what appeared to be land very suitable for agriculture did not thrive and it was found that cobalt deficiency did not allow grass to grow for grazing.

3. Never make information up, you get 0 marks for it. If you said "extremely miniscule amounts of many heavy metals (eg lead) such that it is undetectable by AAS usually does not have a significant adverse effect on plant growth", you could have gotten a good mark for it and being general but "academic" makes your work more favourable.

13. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by someth1ng
Yeah, I know...I felt that the marking scheme would expect at least two valid comparisons (2 marks, 1 mark for each comparison), identifying two methods (1 mark), identifying two examples (1 mark).

1. Another comparison could be:
Neutron bombardment usually does not require acceleration as neutrons hold no electrical charge, as it is not repelled by the positive nuclei by electrostatic repulsion, it won't need to be forces into the nucleus.
Nuclear fission can often require bombardment of a positively charged nuclei, this means that it must be accelerated to very high speeds to overcome electrostatic repulsion and allowing a new nucleus to be formed which could spontaneously decay into smaller molecules.

2. How? Give an example such as in South Australia, what appeared to be land very suitable for agriculture did not thrive and it was found that cobalt deficiency did not allow grass to grow for grazing.

3. Never make information up, you get 0 marks for it. If you said "extremely miniscule amounts of many heavy metals (eg lead) such that it is undetectable by AAS usually does not have a significant adverse effect on plant growth", you could have gotten a good mark for it and being general but "academic" makes your work more favourable.
Didn't think i needed to be very specific with the example.

Thanks for the feedback

14. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by nightweaver066
Didn't think i needed to be very specific with the example.

Thanks for the feedback
I just thought it wasn't detailed enough, how have they understood it? Those sort of questions, in a way, undermine your answer to some extent since it doesn't fully address all aspects of the question. It's probably better to do too much than too little.

That's my thoughts, at least. But otherwise, you're doing well.

15. ## HSC 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.

16. ## re: HSC Chemistry Marathon Archive

$HCl \rightarrow H^+ + Cl^-$

$n(HCl) = CV = 0.1 \cdot 0.005 = 0.0005 \ mol$

There is a one to one ratio between HCl and H+

Hence

$n(H^+)= 0.0005$

$\therefore [H^+]= \frac{0.0005}{0.105} =0.0047...$

We get 0.105 from the Volume of the soln which is 105 ml
We now have conecntration of Hydrogen ions, so we take -log of it

$pH = -\log 0.0047 = 2.32$

I hope I'm right lol

=======================

How is it possible to have NEUTRAL water at ph6?

(just brief and straight to the point)

17. ## re: HSC Chemistry Marathon Archive

Correct.

Isn't neutral water defined with pH 7?

18. ## re: HSC Chemistry Marathon Archive

Originally Posted by bleakarcher
Correct.

Isn't neutral water defined with pH 7?
I would tell you why this is not the case, but it would give the answer away haha.

EDIT: I didn't want to kill the thread this early, here is another question in the mean time while people think about it

Upon analysis of mass of a hydrocarbon was found to contain 82.6% Carbon and 17.4% Hydrogen. Calculate its empirical formula

19. ## re: HSC Chemistry Marathon Archive

Originally Posted by Sy123
Upon analysis a hydrocarbon was found to contain 82.6% Carbon and 17.4% Hydrogen. Calculate its empirical formula
What sort of analysis?

Without knowing the type of analysis you don't know whether it is mol% or mass%

20. ## re: HSC Chemistry Marathon Archive

Originally Posted by Riproot
What sort of analysis?

Without knowing the type of analysis you don't know whether it is mol% or mass%
Fixed. But as far as I know it is impossible for there to be those numerical percentages and it meaning mol%, the Carbon - Hydrogen ratio is too high for it to be mol.

21. ## re: HSC Chemistry Marathon Archive

Umm just curious coz I'm kinda wtf-ing, but are both of your questions HSC level?

22. ## re: HSC Chemistry Marathon Archive

Originally Posted by Sy123
Fixed. But as far as I know it is impossible for there to be those numerical percentages and it meaning mol%, the Carbon - Hydrogen ratio is too high for it to be mol.
But still! It's the principle of the thing.

(It's gravimetric analysis or something, right?)

23. ## re: HSC Chemistry Marathon Archive

Originally Posted by theind1996
Umm just curious coz I'm kinda wtf-ing, but are both of your questions HSC level?
The pH 6 one would be one of the tricky ones but the percentage one is fine. I think we did more of that stuff in year 11 though iirc.

24. ## re: HSC Chemistry Marathon Archive

Originally Posted by theind1996
Umm just curious coz I'm kinda wtf-ing, but are both of your questions HSC level?
They are both within syllabus standards not sure what you mean by HSC level.

25. ## re: HSC Chemistry Marathon Archive

Originally Posted by Riproot
The pH 6 one would be one of the tricky ones but the percentage one is fine. I think we did more of that stuff in year 11 though iirc.
Oh yeah shit.

The empirical formula is from moles shit in Yr 11.

Post some more Production questions guys? I think most people haven't done Acidic in great depth.

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