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Author Topic: VCE Chemistry Question Thread  (Read 2834272 times)  Share 

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Freddie Hg

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Re: VCE Chemistry Question Thread
« Reply #7575 on: November 10, 2018, 09:46:49 pm »
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I'm pretty sure that molar heat of combustion and enthalpy are not the same thing. The value of the enthalpy change can be positive or negative depending on whether it's an endo or exothermic reaction, however the molar heat of combustion will always be positive.

Enthalpy = delta H
Molar heat of combustion = delta Hc

-----ADD-----
Just read through VCAA's 2017 NHT examiners' report - they've got negative delta Hc values in there, even though I was taught that they're always positive. Now I'm confused lol.
If you refer to the data book you'll see that all molar heat of combustion values are positive. Possible mistake on VCAA's part?
(Image removed from quote.)
most likely a VCAA mistake.
ive noticed they tend to have mistakes in the nht exam reports.
maybe they dont care about the the northern hemisphere timetable?

Most times (I think) the solid reagents also become liquid, because molten compounds are usually maintained at extremely high temperature
i thought about that too. but i dont remember ever doing a question like that.
can you refer to any question in mind? if not we need someone else who is certain.
guys we need help

mod edit: merged posts. Please refrain from posting multiple posts in a row
« Last Edit: November 10, 2018, 10:35:46 pm by insanipi »

hums_student

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Re: VCE Chemistry Question Thread
« Reply #7576 on: November 10, 2018, 09:52:54 pm »
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For electrolysis in molten ionic compounds, what are the states of all the reactants and products? Are they all liquid or does only aq stuff become liquid?
I think it's a bit of a grey area in VCE chem - both works.
I was told that for electrolysis questions, it's best to just play safe and leave off states altogether (in electrolysis questions VCAA doesn't penalise you for not having states).

most likely a VCAA mistake.
ive noticed they tend to have mistakes in the nht exam reports.
maybe they dont care about the the northern hemisphere timetable?
Very likely. I noticed some mistakes in the 2018 NHT as well.
Also a lot of their equations in the NHT reports don't have states when states are required. (Poor northern hemisphere people)
By the way I'm going way off topic now but when I read your personal text I sang it in my head. Love it.
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zenith101

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Re: VCE Chemistry Question Thread
« Reply #7577 on: November 10, 2018, 10:00:24 pm »
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any tips on how to word answers to questions asking about how the HNMR supports the possible structure or whatever? I feel like there's so much you can refer to (splitting pattern, no. of environments, chemical shift, relative peak height) , but don't know how much and how to structure it. For reference, VCAA sample exam q4b.

Freddie Hg

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Re: VCE Chemistry Question Thread
« Reply #7578 on: November 10, 2018, 10:01:50 pm »
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I was told that for electrolysis questions, it's best to just play safe and leave off states altogether (in electrolysis questions VCAA doesn't penalise you for not having states).
Dude are you sure? states were always emphasised as being seriously important in ensuring full marks for an equation. again i think we need someone with a more confident answer, it seems our teachers are teaching conflicting information. (this is why i think vcaa should write the textbooks, turn those study design points into full pages of info and sell it!)


and about the personal message, i thought it would make it easier for people to get my name. but not to easy  :)

any tips on how to word answers to questions asking about how the HNMR supports the possible structure or whatever? I feel like there's so much you can refer to (splitting pattern, no. of environments, chemical shift, relative peak height) , but don't know how much and how to structure it. For reference, VCAA sample exam q4b.
i also have issues with this. so i tend to state the observation and say hence.
Two peaks indicate two different hydrogen
environments. hence a ch2 group is bonded to a ch3 group in the molecule.

3 peaks indicate only 3 different carbon environments
for the five carbon atoms. Hence some carbon atoms must be in
the same environment.
this can be quite challenging to word so just write on as every feature as much as you can. i think mentioning and briefly explaining your observation should be enough. however im the type of student to overwrite in these ambiguous situations so i understand why you struggle with wording. just remember the answer is never as hard as you think.

MOD EDIT: merged posts.
« Last Edit: November 10, 2018, 10:38:01 pm by insanipi »

Lear

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Re: VCE Chemistry Question Thread
« Reply #7579 on: November 10, 2018, 10:11:33 pm »
+1

and about the personal message, i thought it would make it easier for people to get my name. but not to easy  :)

Freddie Hg..Hg...Mercury...Ohhhhhhh

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hums_student

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Re: VCE Chemistry Question Thread
« Reply #7580 on: November 10, 2018, 10:25:17 pm »
+1
Dude are you sure? states were always emphasised as being seriously important in ensuring full marks for an equation. again i think we need someone with a more confident answer, it seems our teachers are teaching conflicting information. (this is why i think vcaa should write the textbooks, turn those study design points into full pages of info and sell it!)
I'm 99.8% sure that states are not required for electrolysis reactions. Most resources I found online do not include states either.
One exception, some chem website in the uk (not chemguide.uk, something else, can't remember) have (aq) turned to (l) however (s) stayed the same.
Like I said, I think VCAA accepts both.

Also, @Freddie, possible profile pic for you?

PS. Lear = King Lear = Shakespeare  ;D at least that's what I think it means...
« Last Edit: November 10, 2018, 10:27:53 pm by Lsjnzy13 »
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Freddie Hg

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Re: VCE Chemistry Question Thread
« Reply #7581 on: November 10, 2018, 10:27:39 pm »
+1
I'm 99.8% sure that states are not required for electrolysis reactions. Most resources I found online do not include states either.
One exception, some chem website in the uk (not chemguide.uk, something else, can't remember) have (aq) turned to (l) however (s) stayed the same.
Like I said, I think VCAA accepts both.

Also, @Freddie, possible profile pic for you?

PS. Lear = King Lear = Shakespeare  ;D
ill take your word on the states and lears name. im also gonna take that dp to  :P

sweetiepi

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Re: VCE Chemistry Question Thread
« Reply #7582 on: November 10, 2018, 10:40:51 pm »
+1
ill take your word on the states and lears name. im also gonna take that dp to  :P
Please keep this Q&A thread on topic.

Also I have merged several posts together, please refrain from posting multiple times in a row.
Happy studies y'all :)
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2020: Bachelor of Pharmaceutical Science (Honours) Read my uni journey here!

Azim.m

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Re: VCE Chemistry Question Thread
« Reply #7583 on: November 10, 2018, 11:01:33 pm »
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The answer has to be chlorine gas right?

Freddie Hg

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Re: VCE Chemistry Question Thread
« Reply #7584 on: November 10, 2018, 11:07:58 pm »
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The answer has to be chlorine gas right?
it says solution. therefore water is present in the reaction vessel
water is a stronger reducing agent than chlorine ions
therefore water is oxidised (oxygen is actually being oxidised) and o2 gas is produced at the anode

Azim.m

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Re: VCE Chemistry Question Thread
« Reply #7585 on: November 10, 2018, 11:09:54 pm »
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it says solution. therefore water is present in the reaction vessel
water is a stronger reducing agent than chlorine ions
therefore water is oxidised (oxygen is actually being oxidised) and o2 gas is produced at the anode
But the concentration of KCL is 4M, it’s not in standard conditions

Freddie Hg

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Re: VCE Chemistry Question Thread
« Reply #7586 on: November 10, 2018, 11:13:04 pm »
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But the concentration of KCL is 4M, it’s not in standard conditions
it says SLC. which is in my opinion, a clearance to ignore the fact that 4 molar might impact the prediction of the reaction. its also telling the students that they can go ahead and use the electrochemical series as its valid.
i understand slc is 1 molar and can be confusing. but you are provided with two contradictory pieces of information.
« Last Edit: November 11, 2018, 05:51:44 am by Freddie Hg »

Lear

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Re: VCE Chemistry Question Thread
« Reply #7587 on: November 10, 2018, 11:27:59 pm »
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I do not believe knowledge of the chlorine exception is expected of VCE students.
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hums_student

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Re: VCE Chemistry Question Thread
« Reply #7588 on: November 11, 2018, 10:15:12 am »
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Re: 2018 NHT question above (too many people for me to quote), a concentration of 4M doesn't mean it's not at SLC. SLC just means that it's carried out at 25 degrees Celsius at 100kPa. The two pieces of information do not contradict each other.

The question states to only use the electrochemical series to predict what gas would be formed, therefore you have to disregard the fact that the solution is of a higher concentration than 1M.

The very next question states that chlorine gas is also formed, I think that's a clear red flag that the answer to the first question cannot be chlorine gas. In the next question you're expected to say that as it's not performed under standard conditions, chlorine gas might be preferentially oxidised.
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Freddie Hg

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Re: VCE Chemistry Question Thread
« Reply #7589 on: November 11, 2018, 10:23:37 am »
+1
Re: 2018 NHT question above (too many people for me to quote), a concentration of 4M doesn't mean it's not at SLC. SLC just means that it's carried out at 25 degrees Celsius at 100kPa. The two pieces of information do not contradict each other.

The question states to only use the electrochemical series to predict what gas would be formed, therefore you have to disregard the fact that the solution is of a higher concentration than 1M.

The very next question states that chlorine gas is also formed, I think that's a clear red flag that the answer to the first question cannot be chlorine gas. In the next question you're expected to say that as it's not performed under standard conditions, chlorine gas might be preferentially oxidised.
SLC does mean 1 molar concentration tho? btw when i did it i first did chlorine gas then changed my answer when i saw the next question state that chlorine gas has been formed. as vcaa would not give the answer to the previous question in the stem.

edit: SLC only refers to temperature and pressure and not molar concentration. did a quick google search.
« Last Edit: November 11, 2018, 10:27:07 am by Freddie Hg »