November 15, 2019, 02:12:00 pm

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#### EllingtonFeint

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##### Re: VCE Chemistry Question Thread
« Reply #8190 on: October 16, 2019, 09:42:20 pm »
0
Oh! I forgot to add in attachments of a question (which is part of the above topics) that was causing lots of confusion.
Here it is... (attached, with the answer too)
I have no idea what is going on and would be so appreciative of some help/explanation
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#### JR_StudyEd

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##### Re: VCE Chemistry Question Thread
« Reply #8191 on: October 16, 2019, 09:46:44 pm »
-2
-Do we need to memorise Faraday's laws?
Well, the relevant study design dot point says: the application of stoichiometry and Faraday’s Laws to determine amounts of product, current or time for a particular electrolytic process. So you wouldn't need to memorise it as such. Just apply the knowledge of it.
And the two Faraday equations are in the data booklet! Yay! (They're on page 5 I believe, under chemical relationships, electric charge and number of moles of electrons)

Quote
-Is the cathode positive in the secondary cell??
Yes. The only time it would be negative is if it's an electrolytic cell.

At the moment, that's all I've got. Hope that helped!
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#### Erutepa

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##### Re: VCE Chemistry Question Thread
« Reply #8192 on: October 16, 2019, 09:49:22 pm »
+3
Hello,
More questions; sorry !!
-Do we need to memorise Faraday's laws?
-Is the cathode positive in the secondary cell??
-Actually could sombody please just give me a super quick summary of secondary cells (probably also fuel cells) and which way the recharges and discharges (i.e.just something super basic like the top equation, oxidation (or is it reduction now???) goes L to R on the electrochem series).

Sorry, I am just so unutterably confused about this topic    and I've watched a bunch of youtube videos and looked through my summary notes and books but I don't get it yet. I think that I've got the Galvanic cells down but the rest of this is just a tangled mess in my head. We had this random period of time where our chemistry teacher left and our classes were covered by CRTs for a while before we got a new teacher and I guess this is one of those topics that suffered immensely

1) No you don't need to memorise faraday's law - its in the data booklet as:
$n(e^-)=\frac{Q}{F}$
where Q is the magnitude of charge (product of current and time) and F is faraday's constant (also in the data booklet as 96500)

2) In any electrochemical cell during discharge:
- Anode=oxidation=negative
- Cathode=reduction=positive
In any electrochemical cell during recharge
- Anode=oxidation=positive
- Cathode=reduction=negative

3) I have to go to bed now, so can't give a summary of it here right now. I did give a brief explanation that might help here: https://atarnotes.com/forum/index.php?topic=188141.msg1137087#new
You might also want to have a look around the notes section of ATAR notes - past chemistry lectures slides have pretty good summaries in them which may help and you might even be able to find summary notes made by others.
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#### EllingtonFeint

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##### Re: VCE Chemistry Question Thread
« Reply #8193 on: October 17, 2019, 06:11:27 pm »
0
Hey,
Does anyone have any tips on how to remember the reaction pathways (like substitution reactions and stuff) as well as all the reactants needed (KMnO7 2- etc. (??))

I'm struggling to fit it all in my brain :/
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#### colline

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##### Re: VCE Chemistry Question Thread
« Reply #8194 on: October 17, 2019, 06:23:09 pm »
+5
Hey,
Does anyone have any tips on how to remember the reaction pathways (like substitution reactions and stuff) as well as all the reactants needed (KMnO7 2- etc. (??))

I'm struggling to fit it all in my brain :/
I drew up a reaction pathways diagram and stuck it to my bathroom wall so I spend a good chunk of my time staring at it until it becomes engrained in my mind.

That, and I guess just grinding through questions until everything eventually becomes muscle memory.

Sorry if that’s crap advice but I don’t link there’s much of a cheat way to this.
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#### EllingtonFeint

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##### Re: VCE Chemistry Question Thread
« Reply #8195 on: October 17, 2019, 08:25:02 pm »
0
Hey,
I'm confused about how to do part C of this question.
The solutions say to use the n=V/Vm... what exactly is Vm and how do I find it??

*Also, another question. Sorry guys!
For the second pic, the one showing the mass spectrum, how am I possibly able to tell what the molecular formula of the parent molecule is? Is there some kinda rule I'm missing? Cos surely, (I mean I know this is so like way off but whatevs) you could have CH47N??

Thanks!
« Last Edit: October 17, 2019, 08:53:42 pm by EllingtonFeint »
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#### insanipi

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##### Re: VCE Chemistry Question Thread
« Reply #8196 on: October 17, 2019, 09:29:28 pm »
+5
Hey,
I'm confused about how to do part C of this question.
The solutions say to use the n=V/Vm... what exactly is Vm and how do I find it??
Hey!
$\text{V}_m$ is the volume of a gas at 1 mol, and is known as the molar volume!
This can be calculated by rearranging the ideal gas law- $\text{PV} = \text{nRT}$, solving for $\text{V}$. Assuming standard lab conditions- 100 kPa, 298.15K, and that we're calculating the volume for 1 mol, you can substitute in the values and then use this value to use $\text{n}(\ce{O2})=\frac{\text{V}}{\text{V}_m}$ to find the moles of $\ce{O2}$!

I hope this helps!!!

#### Bri MT

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##### Re: VCE Chemistry Question Thread
« Reply #8197 on: October 17, 2019, 09:48:29 pm »
+4
*Also, another question. Sorry guys!
For the second pic, the one showing the mass spectrum, how am I possibly able to tell what the molecular formula of the parent molecule is? Is there some kinda rule I'm missing? Cos surely, (I mean I know this is so like way off but whatevs) you could have CH47N??

Thanks!

Except you know that you couldn't have CH47N as it doesn't make sense for that molecule to exist. Often you would use mass spec in conjunction with other info. Definitely easier if you can get the empirical formula first (e.g. CnH2n+2) and then use mass spec to solve for n
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#### colline

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##### Re: VCE Chemistry Question Thread
« Reply #8198 on: October 18, 2019, 10:35:54 am »
0
On the 2017 NHT examiners reports, they’ve expressed the molar heat of combustion (delta H_c) as a negative value. However I thought that
- Molar heat of combustion is always positive
- The enthalpy (delta H) can be either positive or negative

Is my understanding correct and did VCAA make a mistake, or is there something that I am not aware of?

See Q10c (top of page 11) of https://www.vcaa.vic.edu.au/Documents/exams/chemistry/2017/nht/chemistrynht_examrep17.pdf

Thanks.
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#### hums_student

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##### Re: VCE Chemistry Question Thread
« Reply #8199 on: October 18, 2019, 10:46:40 am »
+2
On the 2017 NHT examiners reports, they’ve expressed the molar heat of combustion (delta H_c) as a negative value. However I thought that
- Molar heat of combustion is always positive
- The enthalpy (delta H) can be either positive or negative

Is my understanding correct and did VCAA make a mistake, or is there something that I am not aware of?

Your understanding is correct, it is a mistake on VCAA's part. Delta Hc is always a positive value.
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#### EllingtonFeint

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##### Re: VCE Chemistry Question Thread
« Reply #8200 on: October 18, 2019, 11:11:32 am »
0
Hey,
-So on the 2018 exam there's a 3 mark q about vitamin C being a coenzyme.
I understand the first two marks being about the function of a coenzyme and the active site and stuff but the third mark says...

In this synthesis ascorbic acid is oxidised during oxidation and reformed during reduction steps; hence, acting as an electron carrier/acts as a reductant by donating electrons to the enzyme substrate complex to facilitate reaction.

This wasn't in my textbook. Is this something we're just expected to know?? Do ALL coenzymes behave like this?? I don't even really understand what half of it means

-Also could somebody please give me a concise short definition of geometric isomers please

Thank youuu

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#### colline

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##### Re: VCE Chemistry Question Thread
« Reply #8201 on: October 18, 2019, 02:17:07 pm »
+4
Your understanding is correct, it is a mistake on VCAA's part. Delta Hc is always a positive value.
Thanks!

Also could somebody please give me a concise short definition of geometric isomers please
- Only occurs if there is a DOUBLE bond between the carbons
- Does NOT apply when one or more of the carbons have 2 identical substituents (ie if 2 of the same atoms are on one side of the double bond)
- Distinction between 'cis' and 'trans' -- 'cis' means that the two are on the same side, 'trans' means they're on different sides. (see image)

Hope that clears it up a little! I'm not too sure about your other questions but hopefully someone else can help
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#### alanihale

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##### Re: VCE Chemistry Question Thread
« Reply #8202 on: October 18, 2019, 06:52:09 pm »
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Your understanding is correct, it is a mistake on VCAA's part. Delta Hc is always a positive value.
I thought it was that delta Hc always had to be negative as the heat energy is being released. Is that not it?
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#### hums_student

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##### Re: VCE Chemistry Question Thread
« Reply #8203 on: October 18, 2019, 07:42:13 pm »
+1
I thought it was that delta Hc always had to be negative as the heat energy is being released. Is that not it?

From what I remember haven’t done chem in a while lol, what you described is the enthalpy of the reaction, which can be both either positively (endothermic reaction) or negative (exothermic reaction). However, the molar heat of combustion is different, and is - by convention - expressed as an absolute value.
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#### alanihale

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##### Re: VCE Chemistry Question Thread
« Reply #8204 on: October 18, 2019, 09:01:34 pm »
0
From what I remember haven’t done chem in a while lol, what you described is the enthalpy of the reaction, which can be both either positively (endothermic reaction) or negative (exothermic reaction). However, the molar heat of combustion is different, and is - by convention - expressed as an absolute value.
Oooh. That makes more sense. Thank you!
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