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March 29, 2024, 03:53:19 pm

Author Topic: QCE Chemistry Questions Thread  (Read 31197 times)

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jasmine24

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Re: QCE Chemistry Questions Thread
« Reply #45 on: August 12, 2020, 04:41:50 pm »
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It's for the research investigation :) Would I have to mention that it is a theoretical model and would this be a limitation?


I'm assuming Jasmine that you're looking at this for SHE and/or IA3? Absolutely if you ever got something like that question on an exam or test without them providing the required information that would be very unreasonable.

1729

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Re: QCE Chemistry Questions Thread
« Reply #46 on: August 12, 2020, 04:59:24 pm »
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It's for the research investigation :) Would I have to mention that it is a theoretical model and would this be a limitation?
I'm not the best person to answer these kinds of questions as I am not familar with the QCE study design at all. But I don't think you really do need to mention that it is a theoretical model because it should be assumed. Just make sure the mention what you are assuming and that it is a "perfect world" if you aren't entirely sure. (for example, "neglecting the forces between atoms ...")

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #47 on: August 12, 2020, 05:15:08 pm »
+2
It's for the research investigation :) Would I have to mention that it is a theoretical model and would this be a limitation?


When asking that question something to ask yourself is:
 - How important is the accuracy of these models to the claims you are making?


You're very probably fine to just reference them properly :)

matthew hay

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Re: QCE Chemistry Questions Thread
« Reply #48 on: August 26, 2020, 06:41:54 pm »
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Hi there,

Do you or anyone know if there is an annotated syllabus? or even annotated Units 4 syllabus?? Please reply no if not (it saves me the extravagant search!

Thanks and kind regards, Matt  :D

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #49 on: August 26, 2020, 07:43:54 pm »
+1
Hi there,

Do you or anyone know if there is an annotated syllabus? or even annotated Units 4 syllabus?? Please reply no if not (it saves me the extravagant search!

Thanks and kind regards, Matt  :D

Hey,

I've upload a units 3&4 syllabus annotations file here.

Please feel free to let me know if you have any feedback on this & I hope you find it useful :)

A.Rose

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Re: QCE Chemistry Questions Thread
« Reply #50 on: August 29, 2020, 10:11:59 am »
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Hi!
I need some help understanding electrolysis! I am quite rusty since learning it last year.
I'm confused with this question (the answers in this paper are sometimes wrong so if you disagree with the answer it could be wrong but maybe not - I'm not sure). The answer is in the textbox.

I know galvanic cells and electrolytic cells are the same in terms of reduction happens at the cathode and oxidation at the anode and I know the anode of positive and cathode negative but this question has two different electrodes and I'm confused why the half questions are Zinc and Nickel - what happens at the aluminium electrode? Any explanations for how electrolysis works, in general, will be much appreciated!

Thank you!

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #51 on: August 29, 2020, 12:35:47 pm »
+3
Hi!
I need some help understanding electrolysis! I am quite rusty since learning it last year.
I'm confused with this question (the answers in this paper are sometimes wrong so if you disagree with the answer it could be wrong but maybe not - I'm not sure). The answer is in the textbox.

I know galvanic cells and electrolytic cells are the same in terms of reduction happens at the cathode and oxidation at the anode and I know the anode of positive and cathode negative but this question has two different electrodes and I'm confused why the half questions are Zinc and Nickel - what happens at the aluminium electrode? Any explanations for how electrolysis works, in general, will be much appreciated!

Thank you!

For this question, you're going to want to look at page 10 of your data booklet. In an electrochemical cell - regardless of if it's galvanic or electrolytic - the chemicals that react are ALWAYS going to be the ones that give the BIGGEST difference (this will end up being the BIGGEST Ecell). So, first thing you need to do is figure out what chemicals you're working with. In this case, you have:

Al(s)
Ni(s)
Zn2+ (aq)
NO3- (aq)

So, look for ANY equations in the electrochemical series that contain these chemicals and ions. There's also a very sneaky one to be aware of - all of this is happening in an aqueous solution, so water could be involved, too! So you need to consider that reaction, too. In an exam, you could just highlight these - but I'm going to have to write them out, d'oh. They are:

Al (s) ---> Al3+ (aq) + 3e-,                                                              Eo=+1.68
2H2O (l) + 2e- ---> H2 (g) + 2OH- (aq),                                         Eo=+0.83
Zn2+ (aq) + 2e- ---> Zn (s),                                                            Eo=-0.76
Ni (s) ---> Ni2+ (aq) + 2e-,                                                             Eo=-0.24

And looking further down, there are no nitrate equations - so we don't need to worry about it. Now, to make this easier to read, I'm going to display the data from most negative E to most positive E - the same way it's displayed in the data booklet:

Zn2+ (aq) + 2e- ---> Zn (s),                                                            Eo=-0.76
Ni (s) ---> Ni2+ (aq) + 2e-,                                                             Eo=-0.24
2H2O (l) + 2e- ---> H2 (g) + 2OH- (aq),                                         Eo=+0.83
Al (s) ---> Al3+ (aq) + 3e-,                                                              Eo=+1.68

So, looking at this, what will make the biggest difference? Ionic zinc and solid aluminium! So, we calculate the Ecell and we get +0.92. Well, that's not going to work for an electrolytic cell - the Ecell needs to be negative! Okay, what if we try with water - then we get an Ecell of 0.07. Still problematic... That means, the only electrolytic cell that will work is the one with solid nickel and ionic zinc - which has an Ecell of -1.00 (huh, neat).

Basically, you can think of it as this - if the zinc reacted with the water or aluminium, then we'd be CREATING electricity. However, electrolytic cells don't create electricity - they consume it. So, the moment you start putting electricity into the system, the aluminium isn't going to want to react anymore (kinda like in equilibrium - you've changed the system by feeding it electricity, so the system won't want to produce electricity anymore).



Now, I know what you're thinking. "That's very long and tedious, I don't have the time to do this in an exam!" - well, I've got good news for you - we can cheat a little bit. Essentially, this method boils down to there's a difference between general reactivity, and electrochemical reactivity. In fact, electrochemical reactivity correlates exactly with the electrochemical series. When it's written the way it is in the QCE data booklet, the left side goes up by decreasing oxidant strength, and the right side goes up by increasing reductant strength. In any electrolytic cell, the weakest oxidant will react with the weakest reductant. In this case, the weakest oxidant (at the top of the left) is ionic zinc, and the weakest reductant (at the bottom on the right) is solid nickel.


EDIT: After doing all of this, I've realised that I read the equation for water wrong - so in fact, the answer should have nickel reacting with water. I imagine the textbook writers simply forgot they were doing everything in water, hence why they didn't include that equation - it's an easy mistake to make!

matthew hay

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Re: QCE Chemistry Questions Thread
« Reply #52 on: September 06, 2020, 04:32:16 pm »
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Hi all,

Can someone explain chromatography and electrophoresis of proteins is, or refer me to a post if there is one?

I've read my text-book, looked at videos, read the ATAR Notes course-book and still don't understand it. From what I've grasped, the textbook talks about size-exclusion chromatography and ATAR notes course-book talks about chromatography in general. I misunderstand what's actually happening, and it feels like the more it read into it, the more I'm confused about the topic. Also, do we need to understand the formula found in the attachments section below?

There's definitely new words involved in this topic which only adds to the confusion. Also how deep do we go into this topic, like what's the objective - the syllabus is kinda vague?

I get that it must be a hassle to respond, so whoever does end up responding Thank You :)
« Last Edit: September 06, 2020, 04:41:57 pm by matthew hay »

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #53 on: September 06, 2020, 05:13:43 pm »
+3
Hey,

You should've done chromatography in year 11, did your school cover this?

The basic idea of chromatography is this: you have a stationary phase (stationary: not moving) and a mobile phase (mobile: moving). If something would really rather hang out with the mobile phase, it'll only lag behind the mobile phase by a little bit. On the other hand, if it's not that sure whether it would rather hang out with the mobile phase or the stationary phase, it'll move slower. This means that when you try this with multiple different things, they'll separate out based on how good they are at sticking with the mobile phase rather than the stationary phase.

Size based chromatography works on the principle of smaller proteins being more likely to fit in small spaces in the stationary phase and smaller molecules moving more slowly.


In electrophoresis molecules are attracted to one side due to their electrical charge & their mass slows them down. This means the bigger a molecule is and the lower its charge, the longer it will take to move. So, you apply the voltage (to attract your molecules) for a limited time and if everything has the same charge you know that the molecule that moved the furthest is the smallest, the one that moved 2nd furthest is 2nd smallest etc. 

That formula is not in the syllabus and the syllabus dot points do not lead me to believe the equation is required either.

Please let me know what made sense and what didn't from those explanations and then I can add more/clarify as appropriate :)

matthew hay

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Re: QCE Chemistry Questions Thread
« Reply #54 on: September 07, 2020, 02:01:44 pm »
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Hey,

You should've done chromatography in year 11, did your school cover this?

The basic idea of chromatography is this: you have a stationary phase (stationary: not moving) and a mobile phase (mobile: moving). If something would really rather hang out with the mobile phase, it'll only lag behind the mobile phase by a little bit. On the other hand, if it's not that sure whether it would rather hang out with the mobile phase or the stationary phase, it'll move slower. This means that when you try this with multiple different things, they'll separate out based on how good they are at sticking with the mobile phase rather than the stationary phase.

Size based chromatography works on the principle of smaller proteins being more likely to fit in small spaces in the stationary phase and smaller molecules moving more slowly.


In electrophoresis molecules are attracted to one side due to their electrical charge & their mass slows them down. This means the bigger a molecule is and the lower its charge, the longer it will take to move. So, you apply the voltage (to attract your molecules) for a limited time and if everything has the same charge you know that the molecule that moved the furthest is the smallest, the one that moved 2nd furthest is 2nd smallest etc. 

That formula is not in the syllabus and the syllabus dot points do not lead me to believe the equation is required either.

Please let me know what made sense and what didn't from those explanations and then I can add more/clarify as appropriate :)

Ahh, I have a clearer picture now, thank you! I have another question though, will we be expected to know how to use/understand the Retention factor equation? I haven't seen it in my textbook, but it is in the ATAR Notes coursebook.

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #55 on: September 08, 2020, 02:06:34 pm »
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Ahh, I have a clearer picture now, thank you! I have another question though, will we be expected to know how to use/understand the Retention factor equation? I haven't seen it in my textbook, but it is in the ATAR Notes coursebook.

No worries!

There are a couple of reasons I included it in the courseguide:
- calculating Rf values for chromatography is in the units 1&2 syllabus
- the equation can aid conceptual understanding of how chromatography works

I think it is unlikely that they will ask you to calculate Rf values but they would expect you to have learnt it in year 11.

Dumdumsim

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Re: QCE Chemistry Questions Thread
« Reply #56 on: September 08, 2020, 10:07:16 pm »
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Just wondering if there is the annotated solutions for QCAA chemistry sample exam.

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #57 on: September 09, 2020, 04:35:39 pm »
+1
Just wondering if there is the annotated solutions for QCAA chemistry sample exam.

Hey!

Closest thing I know of to that is this set of MCQ solutions I wrote up last year.

Hope this helps & if you have any questions about the answers I've suggested please feel free to ask :)

snr.mmorris4.19

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Re: QCE Chemistry Questions Thread
« Reply #58 on: September 12, 2020, 06:59:58 pm »
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how is 2-methylprop-1-ene an isomer of butene? Isn't the carbon number in the parent chain different? I'm so confused with this!

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #59 on: September 12, 2020, 07:07:47 pm »
+4
how is 2-methylprop-1-ene an isomer of butene? Isn't the carbon number in the parent chain different? I'm so confused with this!

I think you're numbering or drawing one of these two wrong. See the attached - structures + names for each (ignore the (E) part if you don't know what that is - it's just another method of identifying cis/trans stereoisomers, and not something you need to know. I generated these using a computer program and wasn't sure how to turn the E off, sorry!) along with the chemical formula. As you can see, the chemical formula for each is exactly the same.