HSC 2012-2015 Chemistry Marathon (archive) (11 Viewers)

[InteGrand]

re: HSC Chemistry Marathon Archive

You do need to use natural log to calculate pH though...and calculation of pH questions are in exam paper.

You don't need natural log, just base 10.

 

[someth1ng]

re: HSC Chemistry Marathon Archive

You do need to use natural log to calculate pH though...and calculation of pH questions are in exam paper.

Ummm...are you being sarcastic?

 

[strawberrye]

re: HSC Chemistry Marathon Archive

You don't need natural log, just base 10.

Yep, I apologise for this mistake, I am not particularly good with maths terminology. Essentially, I misinterpret the question as not requiring to know the logarithm formula specified in the syllabus to calculate the pH.

Ummm...are you being sarcastic?

No, I am not being sarcastic, was rusty on my maths terminology. Genuine mistake, forgotten natural log stands for the base e. I avoid sarcasm at all costs, whether it be online or in real life.

 

[Librah]

re: HSC Chemistry Marathon Archive

Nitrate ions its one extra step

I'm a bit skeptical about this, wouldn't the nitrate ion concentration remain virtually the same? Assuming the solution remains the same volume. Maybe my electro-chemical theory is a bit rusty or i've misunderstood something.

 

[Kaido]

re: HSC Chemistry Marathon Archive

Yep, I apologise for this mistake, I am not particularly good with maths terminology. Essentially, I misinterpret the question as not requiring to know the logarithm formula specified in the syllabus to calculate the pH.



No, I am not being sarcastic, was rusty on my maths terminology. Genuine mistake, forgotten natural log stands for the base e. I avoid sarcasm at all costs, whether it be online or in real life.

The feels for berry.

 

[Kaido]

re: HSC Chemistry Marathon Archive

1. The HSC will never specify Tc-99m as the isotope you're required to write about. You have a choice of industrial/medical isotopes to study.

2. Tc-99m is an isotope used overwhelmingly in medicine, it's not really used in industry. So it's placement in this question doesn't make much sense.

A question like: outline the use of one radioactive isotope of your own choosing in industry (3 marks) conforms much better to the HSC syllabus. I really suggest you start familiarising yourself with the structure of HSC questions so you'll know what to expect.

That's a very good point. I've been doing heaps of trials now, and I feel the questions are quite specific and the marking criteria is even more specific.
When I looked at past hsc papers, most of the questions targeted a variety of dotpoints at once instead of the specificity like you mentioned

Any other differences between hsc and trials?

 

[teridax]

re: HSC Chemistry Marathon Archive

off topic, but i here lots of ppl perpetuate that hsc chem is rife with rote learning. is this legit?

 

[SuchSmallHands]

re: HSC Chemistry Marathon Archive

That's a very good point. I've been doing heaps of trials now, and I feel the questions are quite specific and the marking criteria is even more specific.
When I looked at past hsc papers, most of the questions targeted a variety of dotpoints at once instead of the specificity like you mentioned

Any other differences between hsc and trials?

Depends on who writes the trials. Some are quite good and reflect the HSC quite decently. Some teachers write or alter a paper with their class in mind though, so they tend to have more specific questions. Most trial papers are decent resources to use.

off topic, but i here lots of ppl perpetuate that hsc chem is rife with rote learning. is this legit?

I didn't do much rote for chem. People will say you need to memorise loads of equations, etc. but you only really need to do that if you haven't learned any chemistry in two years. I never learned the acid rain equations for example, but if you know anything about chemistry it's not a hard equation to work out in the exam. I know people who sat down and rote learned things like that I still have no idea why.
Basically, if you're not interested in learning chemistry and you just want a certain mark you'll have to rote lean a lot. However if you're focusing on understanding how a process works rather than just remembering cause ---> effect chemistry has no more rote learning than any other subject.

 

[Fizzy_Cyst]

re: HSC Chemistry Marathon Archive

I'm a bit skeptical about this, wouldn't the nitrate ion concentration remain virtually the same? Assuming the solution remains the same volume. Maybe my electro-chemical theory is a bit rusty or i've misunderstood something.

if the Mg ions are escaping into solution from the electrode, this would cause the solution to become positively charged, this would impact the voltage in the external circuit. Nitrate ions migrate through the salt bridge to ensure that the solution remains neutral.

 

[Librah]

re: HSC Chemistry Marathon Archive

if the Mg ions are escaping into solution from the electrode, this would cause the solution to become positively charged, this would impact the voltage in the external circuit. Nitrate ions migrate through the salt bridge to ensure that the solution remains neutral.

I'm assuming you mean nitrate ions somehow migrates from the Copper Nitrate solution to the Magnesium nitrate one via the salt bridge since if nitrate moved out of the magnesium solution, that'd just make the solution relatively more positive. But i was under the impression ions from the solutions don't travel INTO the salt bridges, but that the anions already in the salt bridge will diffuse into the anode (where positive charge accumulates in the solution) and positive ions will diffuse into cathode to restore lost positive charge in the solution to maintain electrical neutrality.

 

[superstar12]

re: HSC Chemistry Marathon Archive

Couple of questions on galvanic cells:
- Why do electrons flow via the external wire, not via the salt bridge?
- Why do the anode and cathode have to be physically separated?

 

[Drsoccerball]

re: HSC Chemistry Marathon Archive

Couple of questions on galvanic cells:
- Why do electrons flow via the external wire, not via the salt bridge?
- Why do the anode and cathode have to be physically separated?

Correct me if im wrong but my perception on this is:
-Because the purpose of the salt bridge is to allow the transfer of ions not electrons and it is known that the electrons travel from anode to cathode
-Increased the life span of the battery

 

[superstar12]

re: HSC Chemistry Marathon Archive

Correct me if im wrong but my perception on this is:
-Because the purpose of the salt bridge is to allow the transfer of ions not electrons and it is known that the electrons travel from anode to cathode
-Increased the life span of the battery

But why can't electrons flow through the salt bridge?

How does the physical separation of electrodes lead to increased lifespan of battery?


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[someth1ng]

re: HSC Chemistry Marathon Archive

But why can't electrons flow through the salt bridge?

How does the physical separation of electrodes lead to increased lifespan of battery?


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Electrons can't be move through the water (at least, no alone), only ions are passed through water, which is why salt solutions can conduct.

 

[Kaido]

re: HSC Chemistry Marathon Archive

- Oh i thought it was the relative ease of conductivity through an actual wire, hence why electrons flow through yeah...
- separated because each half-cell has a different reaction occurring (ox/red)

 

[SuchSmallHands]

re: HSC Chemistry Marathon Archive

- separated because each half-cell has a different reaction occurring (ox/red)

The anode and cathode have to be separated to allow you to have a battery. You need a potential difference, throwing together a bunch of metals and ionic solutions won't give you that.

 

[Gabriel Moussa]

re: HSC Chemistry Marathon Archive

The electrolyte in car batteries is sulfuric acid. A curious student decided to determine the concentration of this acid in a well-charged car battery by taking exactly 2 mL by pipette and titrating it with 1.16 mol/L sodium hydroxide solution. 17.1 mL was needed to reach the equivalence point. Calculate the molarity of the sulfuric acid in the battery.

(2 marks)

 

[zhertec]

re: HSC Chemistry Marathon Archive

C (acid) x V (acid) = C (base) x V (base)

therefore: C (acid) x (2/1000) = 1.16 x (17.1/1000)

Since H2SO4 is diprotic : 2(C (acid) x (2/1000)) = 1.16 x (17.1/1000)

C (acid) = 4.96 mol/L 3 sig fig.

 

[Fizzy_Cyst]

re: HSC Chemistry Marathon Archive

I'm assuming you mean nitrate ions somehow migrates from the Copper Nitrate solution to the Magnesium nitrate one via the salt bridge since if nitrate moved out of the magnesium solution, that'd just make the solution relatively more positive. But i was under the impression ions from the solutions don't travel INTO the salt bridges, but that the anions already in the salt bridge will diffuse into the anode (where positive charge accumulates in the solution) and positive ions will diffuse into cathode to restore lost positive charge in the solution to maintain electrical neutrality.

I am not referring to nitrate ions from Magnesium Nitrate migrating through the salt bridge, rather the nitrate ions already in the salt bridge (from KNO3) migrating.

I told my students to always assume KNO3 is used in the salt bridge, unless otherwise specified.

 

[Librah]

re: HSC Chemistry Marathon Archive

I am not referring to nitrate ions from Magnesium Nitrate migrating through the salt bridge, rather the nitrate ions already in the salt bridge (from KNO3) migrating.

I told my students to always assume KNO3 is used in the salt bridge, unless otherwise specified.

Don't think you can assume something like that in a question like this, past HSC calculation questions for these cells usually provides labelled diagrams of the cells in use. The salt bridge could contain any inert salt like NaCl/KCl or some other salt depending on the contents of the half cells, even though a NO3- based one would probably be best suited for this scenario.

Someone should attempt the question though.