HSC 2012-2015 Chemistry Marathon (archive) (2 Viewers)

Lieutenant_21 · Mar 22, 2013

re: HSC Chemistry Marathon Archive

nightweaver066 said:
Chemistry MC answers:
1. B
2. D
3. A
4. C
5. B
6. C
7. D
8. D
9. B
10. B
11. B
12. D(?)
13. D
14. A
15. C
16. A
17. C
18. C
19. A
20. A

Unsure about 12, the rest are 100% correct.

Thank you so much!
I think 12 is D because they have to be in alphabetical order. Some one correct me if I am wrong?

HeroicPandas · Apr 8, 2013

re: HSC Chemistry Marathon Archive

Question 1:
Are all amphiprotic substances amphoteric? Explain

starshine02 · Apr 8, 2013

re: HSC Chemistry Marathon Archive

HeroicPandas said:
Question 1:
Are all amphiprotic substances amphoteric? Explain

1. Yes, all amphiprotic substances are amphoteric. Amphiprotic substance refers to those which are able to either donate or accept a proton. An amphoteric substance is one that is able to behave as both an acid or a base depending on its environment. Since an acid can be defined as a proton donor through the Bronsted-Lowry theory, an amphiprotic substance must also be amphoteric.

((This always confuses me, is this right? Would I have to explain how not all amphoteric substances aren't amphiprotic?))

HeroicPandas · Apr 9, 2013

re: HSC Chemistry Marathon Archive

starshine02 said:
1. Yes, all amphiprotic substances are amphoteric. Amphiprotic substance refers to those which are able to either donate or accept a proton. An amphoteric substance is one that is able to behave as both an acid or a base depending on its environment. Since an acid can be defined as a proton donor through the Bronsted-Lowry theory, an amphiprotic substance must also be amphoteric.

((This always confuses me, is this right? Would I have to explain how not all amphoteric substances aren't amphiprotic?))

Nice! Yeh u are correct, u need dont need to explain anything else (there is another way to look at this question though)

HeroicPandas · Apr 29, 2013

re: HSC Chemistry Marathon Archive

BUMP!

An insanely difficult question: What machine is used to accurately find the concetration (ppm,ppb) of an ion in a substance that is more accurate than using traditional gravimetric/volumetric analysis? 1 mark

Sy123 · Apr 29, 2013

re: HSC Chemistry Marathon Archive

HeroicPandas said:
BUMP!

An insanely difficult question: What machine is used to accurately find the concetration (ppm,ppb) of an ion in a substance that is more accurate than using traditional gravimetric/volumetric analysis? 1 mark

The Photomultiplier from Atomic Absorbtion Spectroscopy?

HeroicPandas · Apr 30, 2013

re: HSC Chemistry Marathon Archive

Sy123 said:
The Photomultiplier from Atomic Absorbtion Spectroscopy?

Yes!

Sy123 · Jun 10, 2013

re: HSC Chemistry Marathon Archive

$The Haber process is given by the chemical equation$

$3H_{2(g)} + N_{2(g)} \leftrightharpoons 2NH_{3(g)}$

$i) Write down the equilibrium constant for this expression$

$ii) Hence mathematically explain with reference to the reaction quotient$
$that why an increase in pressure$
$necessitates a shift of the reaction to the right$

Streetboy · Jun 10, 2013

re: HSC Chemistry Marathon Archive

Sy123 said:
$The Haber process is given by the chemical equation$

$3H_{2(g)} + N_{2(g)} \leftrightharpoons 2NH_{3(g)}$

$i) Write down the equilibrium constant for this expression$

$ii) Hence mathematically explain with reference to the reaction quotient$
$that why an increase in pressure$
$necessitates a shift of the reaction to the right$

i)

ii) The synthesis of ammonia is a reversible reaction. Under normal conditions, the equilibrium lies far to the left, and little ammonia is produced. However, to ensure that sufficient ammonia is produced, condition need to be established that shift the equilibrium position to the right.

N2(g) + 3 H2(g) 2 NH3(g) There molar ratio is: 4:2, in accordance with Le Chatelier’s Principle, increasing the pressure (number of molecules per volume) shifts the equilibrium to the right producing more ammonia, so that the total number of moles/molecules is decreased.

Sy123 · Jun 10, 2013

re: HSC Chemistry Marathon Archive

Streetboy said:
i)

ii) The synthesis of ammonia is a reversible reaction. Under normal conditions, the equilibrium lies far to the left, and little ammonia is produced. However, to ensure that sufficient ammonia is produced, condition need to be established that shift the equilibrium position to the right.

N2(g) + 3 H2(g) 2 NH3(g) There molar ratio is: 4:2, in accordance with Le Chatelier’s Principle, increasing the pressure (number of molecules per volume) shifts the equilibrium to the right producing more ammonia, so that the total number of moles/molecules is decreased.

You have restated the question for part (ii). I'm asking you to prove (mathematically) that an increase in pressure shifts the reaction to the right (which is why I asked for the equilibrium constant)

Reaction Quotient, (Q), is core module right? Or is it Industrial chem.

omgiloverice · Jun 10, 2013

re: HSC Chemistry Marathon Archive

Sy123 said:
You have restated the question for part (ii). I'm asking you to prove (mathematically) that an increase in pressure shifts the reaction to the right (which is why I asked for the equilibrium constant)

Reaction Quotient, (Q), is core module right? Or is it Industrial chem.

I think it is only for Industrial chem.

Explain the following statement: "The release of CFCs into the environment devastate the ozone layer, due to its endless chlorine catalytic cycle." Include all relevant equations in your answer. (5 marks)

Sy123 · Jun 10, 2013

re: HSC Chemistry Marathon Archive

$$A lawn fertiliser's sulfate content is$ \ \ P \% (w/w)$

$A sample of$ \ \ m \ \ $grams of fertiliser is analysed, by reacting with excess barium nitrate$

$What is the mass of the Barium Sulfate precipitate expected to form, in terms of the above pronumerals?$

skillstriker · Jun 11, 2013

re: HSC Chemistry Marathon Archive

omgiloverice said:
I think it is only for Industrial chem.

Explain the following statement: "The release of CFCs into the environment devastate the ozone layer, due to its endless chlorine catalytic cycle." Include all relevant equations in your answer. (5 marks)

- chloroflurocarbons = compounds containing chlorine, fluorine and carbon
- thousands of tonnes released into atmosphere due to use in refrigerants, propellants etc.
- cfc gradually photodissociates to produce chlorine free radical (eqn)
- chlorine free radical reacts with ozone to produce chlorine monoxide free radical (eqn)
- chlorine monoxide radical reacts with atomic oxygen to regenerate chlorine free radical (eqn)
- thus the chlorine acts as a catalyst which can further destroy ozone molecules (net reaction = O + O3 --> 2O2) until it reacts with methane

omgiloverice · Jun 11, 2013

re: HSC Chemistry Marathon Archive

skillstriker said:
- chloroflurocarbons = compounds containing chlorine, fluorine and carbon
- thousands of tonnes released into atmosphere due to use in refrigerants, propellants etc.
- cfc gradually photodissociates to produce chlorine free radical (eqn)
- chlorine free radical reacts with ozone to produce chlorine monoxide free radical (eqn)
- chlorine monoxide radical reacts with atomic oxygen to regenerate chlorine free radical (eqn)
- thus the chlorine acts as a catalyst which can further destroy ozone molecules (net reaction = O + O3 --> 2O2) until it reacts with methane

I'm not too sure how you manage to get away answering like that. :L But anyhow I asked you explain the statement not, list the process of how cfc disintegrate the ozone. So I'll give you a 3 out of 5, one for not actually explaining the statement, and another for not diving into enough detail about the chlorine cycle (mention co-ordinate bonds, and the electronegativity of chlorine)

someth1ng · Jun 11, 2013

re: HSC Chemistry Marathon Archive

skillstriker said:
- chloroflurocarbons = compounds containing chlorine, fluorine and carbon
- thousands of tonnes released into atmosphere due to use in refrigerants, propellants etc.
- cfc gradually photodissociates to produce chlorine free radical (eqn)
- chlorine free radical reacts with ozone to produce chlorine monoxide free radical (eqn)
- chlorine monoxide radical reacts with atomic oxygen to regenerate chlorine free radical (eqn)
- thus the chlorine acts as a catalyst which can further destroy ozone molecules (net reaction = O + O3 --> 2O2) until it reacts with methane

The bolded parts are wrong:
1. What about hydrogen?
2. It doesn't NEED to be methane.

omgiloverice said:
I'm not too sure how you manage to get away answering like that. :L But anyhow I asked you explain the statement not, list the process of how cfc disintegrate the ozone. So I'll give you a 3 out of 5, one for not actually explaining the statement, and another for not diving into enough detail about the chlorine cycle (mention co-ordinate bonds, and the electronegativity of chlorine)

He technically answered it correctly but not very well.
He wasn't listing, he was putting it in steps of how CFCs catalyse the decomposition of ozone. Therefore, he DID explain how CFCs released into the atmosphere have a devastating effect on ozone.

In my opinion, this wasn't a very well written question.

skillstriker · Jun 11, 2013

re: HSC Chemistry Marathon Archive

CFCs don't contain hydrogen.

omgiloverice · Jun 11, 2013

re: HSC Chemistry Marathon Archive

Fair enough~ I agree so here is a better question.

$A student carried out an investigation to analyse the sulfate content of lawn fertilizer. The student weighed out 1.0g of fertilizer into a beaker, added water and then stirred. 50 mL of 0.30 mol\cdot L^{-1}$ $barium chloride solution was then added. A white precipitate formed, filtered and then weighed to be 1.2g.$

$a) Assess the validity of his experimental method for determining the sulfate content of lawn fertilizer$

$b) Determine the percentage of sulfate ion, by mass, in the fertiliser sample$

someth1ng · Jun 11, 2013

re: HSC Chemistry Marathon Archive

Oh yeah, they don't contain hydrogen - that's HCFCs, lol.

In your answer, you should restate or make reference to the quote "The release of CFCs into the environment devastate the ozone layer, due to its endless chlorine catalytic cycle". You went straight to explaining and that would probably render you limited to 1 mark because there's usually a criteria something like "Identifies that CFCs are destructive to the ozone layer".

I can see that you did it "in a way" but it is not explicit because it's more along the lines of "chlorine radicals are destructive" but not "CFCs are destructive".

Last thing I want to nit-pick is that when you are showing that something is a step-by-step process, number it (like below), not dot point.

1. Step 1
2. Step 2
3. Step 3

skillstriker · Jun 11, 2013

re: HSC Chemistry Marathon Archive

also, the cycle technically isn't 'endless' because it does eventually stop when chlorine reacts with another chemical e.g. methane

someth1ng · Jun 11, 2013

re: HSC Chemistry Marathon Archive

skillstriker said:
also, the cycle technically isn't 'endless' because it does eventually stop when chlorine reacts with another chemical e.g. methane

Yeah, that too.

HSC 2012-2015 Chemistry Marathon (archive) (2 Viewers)

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