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Author Topic: HSC Chemistry Question Thread  (Read 1040661 times)  Share 

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jakesilove

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Re: Chemistry Question Thread
« Reply #780 on: September 24, 2016, 02:36:32 pm »
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What would be the answer to the following question:

Which of the following does NOT usually occur during the cracking of high boiling fractions of crude oil?
A) a decrease in the pressure of the reaction vessel
B) the formation of products with a higher total chemical potential energy than the reactants
C) the production of smaller saturated hydrocarbons
D) the formation of unsaturated hydrocarbons

Hey! First, for anyone reading this, this question is outside the HSC curriculum. As such, I'm not really confident with my response. However, since we're boiling stuff, I would imagine that liquids are turning into gases. Therefore, the pressure of the vessel should really INCREASE almost every time, not decrease. Thus, I think the answer should be A.

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lha

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Re: Chemistry Question Thread
« Reply #781 on: September 25, 2016, 11:11:53 am »
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What mark do you need in chemistry to get a decent atar of 90+?


Does anyone have an easy method of memorising solvay process?

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« Last Edit: September 25, 2016, 11:14:57 am by RuiAce »

RuiAce

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Re: Chemistry Question Thread
« Reply #782 on: September 25, 2016, 11:13:57 am »
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What mark do you need in chemistry to get a decent atar of 90+?
Your ATAR is dependent on all of your subjects. If you got 100 in chemistry but only 50 in everything else then it was all for nothing (except maybe a glorious state rank).

What mark do you need in chemistry to get a 90+ ATAR? Think about all of your subjects all at once, not just chemistry.
Does anyone have an easy method of memorising solvay process?
This is probably one of the most rigorous things to memorise. The reason is that there's heaps of processes that go on in the Solvay process.

I would recommend the following three:
- A flow chart
- A list of chemical equations
- The steps, in your notes

You should read over your notes ample times so that the steps are actually presentable, however it may be too hard to process. Hence, utilise the flow chart and compare your notes to the chart. The flow chart gives a much more visual instead of mental approach to the things at hand.

As for the chemical equations, you may choose to put the equations ON the flow chart as well, but you may choose to just write it out under a subheading
E.g. Brine purification: ...
Decomposition of sodium carbonate into sodium hydrogen carbonate: ...
(I'm fairly sure there are 6 subheadings if you take this approach)
« Last Edit: September 25, 2016, 11:21:06 am by RuiAce »

zoeh

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Re: Chemistry Question Thread
« Reply #783 on: September 25, 2016, 01:35:23 pm »
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In the shipwrecks option, there was a past paper question (from 2005 hsc) showing an electrolytic cell with 2 inert electrodes, with Cl- coating the anode and H20 around the cathode, in a solution of aqueous NaCl. The question asked to indicate one possible anode and cathode reaction that may occur (it was only a 1 mark question). How do you do this? Like how do you know which ones can act as an anode or cathode?

marynguyen18

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Re: Chemistry Question Thread
« Reply #784 on: September 25, 2016, 03:57:51 pm »
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does dry cells and fuel cells have a chemical equation that you have to remember?

RuiAce

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Re: Chemistry Question Thread
« Reply #785 on: September 25, 2016, 04:07:46 pm »
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does dry cells and fuel cells have a chemical equation that you have to remember?
Yes

For example, for the dry cell we have

Anode: Zn(s) → Zn2+ + 2 e-
Cathode: 2 MnO2(s) + 2 NH4+ + 2 e- → Mn2O3(s) + H2O(l) + 2 NH3(aq)

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Re: Chemistry Question Thread
« Reply #786 on: September 26, 2016, 10:36:25 pm »
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for the Winkler titration...how important is it to know it o.O
I mean, using a dissolved oxygen sensor is also a quantitative method of measuring DO, would give a more accurate reading and is way easier to explain. I guess is it only important to be aware it's a feasible method for DO testing, maybe in case it comes up in MCs or something? Have there been any past HSC/trial exam questions you're aware of that have required a thorough explanation of the Winkler titration.

Same with the EDTA titration. Like, do we need to know Ca2+ + EDTA4- --> Ca(EDTA)2- and the water sample should turn from red to blue with Eriochrome Black T indicator blahblahlah.....

lmao.

Thanks :)
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RuiAce

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Re: Chemistry Question Thread
« Reply #787 on: September 26, 2016, 10:38:04 pm »
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for the Winkler titration...how important is it to know it o.O
I mean, using a dissolved oxygen sensor is also a quantitative method of measuring DO, would give a more accurate reading and is way easier to explain. I guess is it only important to be aware it's a feasible method for DO testing, maybe in case it comes up in MCs or something? Have there been any past HSC/trial exam questions you're aware of that have required a thorough explanation of the Winkler titration.

Same with the EDTA titration. Like, do we need to know Ca2+ + EDTA4- --> Ca(EDTA)2- and the water sample should turn from red to blue with Eriochrome Black T indicator blahblahlah.....

lmao.

Thanks :)
I did memorise that EDTA equation just in case, but in all honesty I just memorised what they were used for. I knew nothing about the methods themselves to EDTA or Winkler at all.

Jake might dispute me on this one though.

ml125

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Re: Chemistry Question Thread
« Reply #788 on: September 27, 2016, 01:41:38 am »
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In the shipwrecks option, there was a past paper question (from 2005 hsc) showing an electrolytic cell with 2 inert electrodes, with Cl- coating the anode and H20 around the cathode, in a solution of aqueous NaCl. The question asked to indicate one possible anode and cathode reaction that may occur (it was only a 1 mark question). How do you do this? Like how do you know which ones can act as an anode or cathode?
There are two possible cathode reactions which can occur:


The standard potential of the reduction of water is lower than that of Na+, therefore the second reaction is more likely to occur.

There are two possible anode reactions which can occur:


Notice that the two standard potentials are relatively close to each other. The reaction to take place will depend on the concentration of Cl- in solution. It would be fine to state either of the reactions, given that you also state conditions eg. if [Cl-]<1M, water is oxidised. However without stating conditions, I'd say that it would be safe to state that the chlorine reaction would occur.

Hi i was wonder if i could my answers to these questions checked please.
There are answers so im not sure whether i got them right or not
In your last line of working, you divide by 0.001 instead of 0.01! The answer should be 0.423M instead of 4.23M. Other than that, the rest should be fine :)
« Last Edit: September 27, 2016, 02:10:47 am by ml125 »
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jakesilove

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Re: Chemistry Question Thread
« Reply #789 on: September 27, 2016, 08:45:06 am »
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I did memorise that EDTA equation just in case, but in all honesty I just memorised what they were used for. I knew nothing about the methods themselves to EDTA or Winkler at all.

Jake might dispute me on this one though.

I just knew the methods, and didn't remember the formulas. So, basically, you just need enough specific information SOMEWHERE (what it is is up to you!)
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anotherworld2b

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Re: Chemistry Question Thread
« Reply #790 on: September 27, 2016, 09:48:29 pm »
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Could i please get help with these questions? I am not sure what to do  :-\

RuiAce

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Re: Chemistry Question Thread
« Reply #791 on: September 27, 2016, 09:55:33 pm »
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Could i please get help with these questions? I am not sure what to do  :-\
Hints

Q9: Limewater is calcium hydroxide (CaOH). You should have heard about it, because limewater is the most commonly used chemical used to test for presence of CO2. But the equation may not be too obvious. Giveaway: One of the products is CaCO3.

Limewater turns white ("milky") in the presence of carbon dioxide. This is essentially the insoluble calcium carbonate forming

Q7: This is just a metal displacement reaction.

anotherworld2b

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Re: Chemistry Question Thread
« Reply #792 on: September 28, 2016, 10:35:47 am »
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Thank you very much for your help ;)

I was wondering would my answers yo these two questions be correct?

Hints

Q9: Limewater is calcium hydroxide (CaOH). You should have heard about it, because limewater is the most commonly used chemical used to test for presence of CO2. But the equation may not be too obvious. Giveaway: One of the products is CaCO3.

Limewater turns white ("milky") in the presence of carbon dioxide. This is essentially the insoluble calcium carbonate forming

Q7: This is just a metal displacement reaction.

RuiAce

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Re: Chemistry Question Thread
« Reply #793 on: September 28, 2016, 11:10:29 am »
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Thank you very much for your help ;)

I was wondering would my answers yo these two questions be correct?
Q6 is definitely fine

Your working for Q7 is a bit over the place but I saw nLiOH = 0.253 * 0.01673 which is good. And it looks like you divided back out by 0.01 to get the concentration of HBr so it should be fine

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Re: Chemistry Question Thread
« Reply #794 on: September 28, 2016, 02:57:07 pm »
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could someone please explain the difference between endpoint and equivalence point in a titration?