Predictions for Chemistry 2014 HSC? (1 Viewer)

[enigma_1]

Brahh don't post the questions like that. I can't access it on my phone.

 

[faisalabdul16]

Brahh don't post the questions like that. I can't access it on my phone.

hahaha -.- get on your computer

 

[enigma_1]

hahaha -.- get on your computer

I'm out, I can't teleport. :/

 

[SuchSmallHands]

yall need to start sticking to concise responses... that number of lines is about a 20 mark question

This one is really hard to do succinctly, since it targets collaboration, monitoring and management and electrochem. I'll try to do it as succinctly as possible for the 5-6 range (and probably fail...)

The lead-acid cell contains lead and H2SO4, with an overall cell reaction as follows:
Pb(s) + PbO(s) + 2H2SO4 ---> 2PbSO4 + 2H2O
- collaboration between environmental and analytical chemists is required to ensure that the disposal of the electrochemical cell does not entail high outputs of lead or H2SO4, a strong acid, into the environment. Lead is bioaccumulative and biomagnifying, thus its entry into waterways and the foodchain would be devastating both to human populations and fauna. Lead causes CNS disruption, kidney failure and anaemia in humans, and has a particularly pronounced impact upon exposed children. The release of H2SO4 into the environment can acidify waterways, hindering the survival of aquatic organisms, and lower soil pH to a level not conducive with plant life. Thus, an analytical chemist is required to perform tests such as AAS to determine lead concentration in soils near disposal sites of lead-acid batteries as well as testing for H2SO4 through the use of a pH meter or indicator solution and collaborate with environmental chemists to benefit from their specialised knowledge of concentrations of these substances that pose a threat to human, plant and animal life.
- a polymer chemist may also collaborate with an analytical chemist, as they can offer insight into how new polymers can be created or existing polymers re-appropriated in order to create more durable casing on lead-acid cells which will minimise the risk of unintentional leakage of the contents of the cell into the environment. This innovation may be necessary if an analytical chemist determines the need for it through recording high concentrations of lead or acid in the environment linked to insufficient casing durability.

Yeah, that last argument isn't great, but for a 6 marker I don't personally feel confident leaving my collaboration between chemists as between only two. This answer also isn't as succinct as I was aiming for. What would you do to make it more concise but still in the 5-6 range, QZP?

 

[SuchSmallHands]

Would you actually write that much though? And how long would it take you to do so? Please don't say 2 minutes :/

My answer for that question took 8 minutes thinking and typing. Handwriting it would probably take a little longer, around 10 minutes all up?

 

[faisalabdul16]

http://www.boardofstudies.nsw.edu.a...s/pdf_doc/chemistry-hsc-sample-answers-12.pdf

Everyone read the sample answer they have provided (Question 33).
It's not even worth 6 marks, max i would give is 4, and that's me being very lenient.

 

[SuchSmallHands]

http://www.boardofstudies.nsw.edu.a...s/pdf_doc/chemistry-hsc-sample-answers-12.pdf

Everyone read the sample answer they have provided (Question 33).
It's not even worth 6 marks, max i would give is 4, and that's me being very lenient.

Marking criteria (6 marks)
• Demonstrates a thorough knowledge and understanding of the identified
roles of chemists and the environmental impact of a correctly named
electrochemical cell
• Assesses the need for collaboration between chemists
• Demonstrates coherence and logical progression of ideas with correct
scientific terminology used

I think I'd give them six based off the criteria. I think they actually satisfied that criteria better than some of us, since we all veered more strongly toward reaction equation/mechanism of environmental impact (you know, like, chemistry?) rather than listing off a bunch of chemical professions which is where the marking criteria gives out a lot of it's marks. They listed four types of chemist, one more than any of us listed. But that's not to say that the marking criteria for this question was perfect, and I don't really think it was.

 

[faisalabdul16]

Marking criteria (6 marks)
• Demonstrates a thorough knowledge and understanding of the identified
roles of chemists and the environmental impact of a correctly named
electrochemical cell
• Assesses the need for collaboration between chemists
• Demonstrates coherence and logical progression of ideas with correct
scientific terminology used

I think I'd give them six based off the criteria. I think they actually satisfied that criteria better than some of us, since we all veered more strongly toward reaction equation/mechanism of environmental impact (you know, like, chemistry?) rather than listing off a bunch of chemical professions which is where the marking criteria gives out a lot of it's marks. They listed four types of chemist, one more than any of us listed. But that's not to say that the marking criteria for this question was perfect, and I don't really think it was.

Ohyeah obviously they'd get a 6 because the sample answer addresses the marking criteria, whereas us, in an exam have no clue as to how to structure or where to get the 6 marks from. Come on, as if you're not going to brain dump everything you know in detail in order to ensure your 6 marks.

We have no idea of what the marking criteria is, hence why we all go in so much detail and we have to.

 

[SuchSmallHands]

Ohyeah obviously they'd get a 6 because the sample answer addresses the marking criteria, whereas us, in an exam have no clue as to how to structure or where to get the 6 marks from. Come on, as if you're not going to brain dump everything you know in detail in order to ensure your 6 marks.

We have no idea of what the marking criteria is, hence why we all go in so much detail and we have to.

Not necessarily true, you have to be really careful not to take them as definite full mark answers according to my teacher. I guess that's why they have this caveat on the first page every year: 'the ‘sample answers’ or similar advice contained in this document are not intended
to be exemplary or even complete answers or responses.'

 

[SuchSmallHands]

Some of the chem of art sample answers in particular suck and wouldn't get full marks in a million years.

 

[faisalabdul16]

New question?

 

[SuchSmallHands]

Explain how a buffer works with reference to a specific example in a natural system. (4 marks)

 

[faisalabdul16]

Explain how a buffer works with reference to a specific example in a natural system. (4 marks)

A buffer is a solution which is made of equimolar amounts of an acid with its conjugate base (weak acid) and its role is to withstand pH change when small amounts of an acid or base is added. An example of a buffer in a natural system is the carbonic acid buffer, which helps to try and maintain the pH of blood at 7.4, to allow for enzymes in our body to operate at optimum capacity and allow for metabolic functioning.
H2CO3(aq) + H2O(l) (equilibrium) HCO3(-) + H3O(+)

As carbon dioxide concentration increases, it dissolves with water forming carbonic acid, and the buffer is there to withstand the pH change and not allow for a decrease in pH. Without a buffer like such, there would be fluctuations in blood pH, resulting in enzymes either ceasing to exist or denaturation, leading to decreased metabolic functioning.

 

[QZP]

A buffer is a solution which consists of equimolar amounts of an acid with its conjugate base (weak acid preferentially) and its role is to withstand pH change when small amounts of an acid or base is added. An example of a buffer in a natural system is the carbonic acid buffer, which helps to try and maintain the pH of blood at 7.4, to allow for enzymes in our body to operate at optimum capacity and allow for metabolic functioning.
H2CO3(aq) + H2O(l) (equilibrium) HCO3(-) + H3O(+)

As carbon dioxide concentration increases, it dissolves with water forming carbonic acid, and the buffer is there to withstand the pH change and not allow for a decrease in pH. Without a buffer like such, there would be fluctuations in blood pH, resulting in enzymes either ceasing to exist or denaturation, leading to decreased metabolic functioning.

- Equimolar? That's not necessarily true
- Buffers only apply to weak acids/bases because it involves an equilibrium system
- You didn't explain how a buffer works (didn't answer the Q)

 

[faisalabdul16]

- Equimolar? That's not necessarily true
- Buffers only apply to weak acids/bases because it involves an equilibrium system
- You didn't explain how a buffer works (didn't answer the Q)

Bold part - i worded it wrong. It is made of equimolar amounts of acid and base.

hahah write your answer?

 

[enigma_1]

- Equimolar? That's not necessarily true
- Buffers only apply to weak acids/bases because it involves an equilibrium system
- You didn't explain how a buffer works (didn't answer the Q)

True that. A buffer is a solution consisting of a weak Bronsted-Lowry acid or base and its conjugate. The buffer is able to maintain pH despite the addition of small amounts of strong acid or base due to the system being in equilibrium.
Didn't mention Le Chatelier once - disappointing.

You didn't specify how the CO2 is increased in concentration.

insert your equation here

You could say, via natural cellular respiration, humans inhale Oxygen and expel Carbon dioxide. By doing so, the carbon dioxide concentration in the blood decreases in the equilibrium ie the H2CO3 concentration decreases and since there is less CO2 dissolved, the pH increases because CO2 is the cause of acidity (in the form of H2CO3) carbonic acid. And according to Le Chatelier's principle, when a system is in equilibrium and a stress is induced, the system will act in order to minimise the change and return the system to equilibrium. Thus, the system will shift to the left (reverse) direction to compensate for the reduced concentration of carbonic acid (increased pH) by lowering the pH again and this system acts as a buffer to allow the pH to be maintained within the range of 7.35 to 7.45 which allows the regular blood pH to be maintained, preventing decreased metabolism.

 

[faisalabdul16]

True that. A buffer is a solution consisting of a weak Bronsted-Lowry acid or base and its conjugate. The buffer is able to maintain pH despite the addition of small amounts of strong acid or base due to the system being in equilibrium.
Didn't mention Le Chatelier once - disappointing.

You didn't specify how the CO2 is increased in concentration.

insert your equation here

You could say, via natural cellular respiration, humans inhale Oxygen and expel Carbon dioxide. By doing so, the carbon dioxide concentration in the blood decreases in the equilibrium ie the H2CO3 concentration decreases and since there is less CO2 dissolved, the pH increases because CO2 is the cause of acidity. And according to Le Chatelier's principle, when a system is in equilibrium and a stress is induced, the system will act in order to minimise the change and return the system to equilibrium. Thus, the system will shift to the left (reverse) direction to compensate for the reduced concentration of carbonic acid and this system acts as a buffer to allow the pH to be maintained within the range of 7.35 to 7.45 which allows the regular blood pH to be maintained, preventing decreased metabolism.

My equation shows it's in equilibrium

haha nah my answers is shit, i couldnt be bothered

 

[SuchSmallHands]

- Equimolar? That's not necessarily true
- Buffers only apply to weak acids/bases because it involves an equilibrium system
- You didn't explain how a buffer works (didn't answer the Q)

Ok QZP, how does this go
- A buffer is a substance which works to minimise fluctuations in pH if small quantities of acids or bases are adding (not strictly necessary to answer the question, I want to put this in regardless)
- Generally comprised of a weak acid and its conjugate base (but can be vice-versa)
- The bicarbonate ion, found in human blood, works to maintain a pH of 7.4 to within a 0.5pH range if acidic or basic substances enter the blood stream.
- H2CO3 <----> HCO3- + H+
- The buffer system works as, in accordance with LCP, addition of an acidic substance shifts the equilibrium to the left in order to reduce the impact of the acidification and thus prevent acidemia. By contrast, addition of a basic substance pushes the equilibrium to the right and favours the production of H+ ions, thus restoring pH and preventing alkylosis.

 

[enigma_1]

My equation shows it's in equilibrium

haha nah my answers is shit, i couldnt be bothered

k den

 

[SuchSmallHands]

Found the sample answer, just as a reference

Buffers are solutions that resist changes in pH when small quantities of acid or base are added.
A buffer is made by mixing together a weak acid and its conjugate base, or a base with its – conjugate acid in approximately equal proportions. A natural example is seen in the HCO3
buffering system that maintains blood pH.
H2CO3(aq) � HCO3–(aq) + H+(aq)
When there are changes in acid/base level of the blood, the system is pushed either to the left on
addition of an acidic substance or the right on the addition of a basic substance and the pH is
maintained.