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September 22, 2019, 07:58:30 am

Author Topic: VCE Chemistry Question Thread  (Read 934535 times)  Share 

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EllyJayne

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Re: VCE Chemistry Question Thread
« Reply #8025 on: May 23, 2019, 02:25:51 pm »
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Does anyone have any ideas or websites for the practical investigation sac thats done in unit 4? our school is doing it in unit 3 and I have no idea what to do.
Thanks !

Matthew_Whelan

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Re: VCE Chemistry Question Thread
« Reply #8026 on: May 24, 2019, 05:54:05 pm »
+1
Does anyone have any ideas or websites for the practical investigation sac thats done in unit 4? our school is doing it in unit 3 and I have no idea what to do.
Thanks !

Hi there, just a fellow year 12 struggler.
A list of suggested ideas can be found on the VCAA website. The site is currently down but anything regarding food processes, vitamins, equilibrium, electrochem, etc. If its relevant to the course then it should be fine. Your textbook may provide ideas too.
I suggest something you find interesting and know really well.
Good luck!
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Matthew_Whelan

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Re: VCE Chemistry Question Thread
« Reply #8027 on: May 24, 2019, 09:33:18 pm »
+2
Why is temperature the only thing that changes an equilibrium constant?
Hi there!
When variables such as pressure, concentration etc are changed, the equilibrium will shift, but when reestablished, the equilibrium constant will remain the same when calculated.
As for temperature; heat can be a reactant or product (depending on if it is exo-/endo-), thus being a 'player' in the system. When temperature is increased or decreased in an equilibrium system, the concentrations of reactants/products will be different when equilibrium is reestablished, therefore, the K constant will change.
Hopefully that makes sense to you and isnt just chem jargon regurgitated.  ;)
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Ionic Doc

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Re: VCE Chemistry Question Thread
« Reply #8028 on: May 25, 2019, 10:46:23 pm »
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hey everyone just doing some hydrocarbon revision ( chem 1/2) when I stumbled across this question

Explain the following;

Compounds containing a quadruple carbon–carbon bond do not exist


can anyone explain why? don't carbons have 4 valence electrons each, so when both carbons share electrons (covalent bond)  they should both have a total of 8 electrons, so than why cant a quadruple bond be formed?

edit

another question
C4H8 reacts readily with bromine. Give the balanced equation for the reaction and name any product(s).

I WROTE C4H8 + Br2 - - - - - - > C4H8Br2 

is this correct,
thnx
thnx in advance  :)
« Last Edit: May 25, 2019, 11:11:45 pm by Ionic Doc »
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Matthew_Whelan

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Re: VCE Chemistry Question Thread
« Reply #8029 on: May 25, 2019, 11:11:37 pm »
+4
hey everyone just doing some hydrocarbon revision ( chem 1/2) when I stumbled across this question

Explain the following;

Compounds containing a quadruple carbon–carbon bond do not exist


can anyone explain why? don't carbons have 4 valence electrons each, so when both carbons share electrons (covalent bond)  they should both have a total of 8 electrons, so than why cant a quadruple bond be formed?

thnx in advance  :)
Hi, I found this site which explains; it seems quite complicated,  i presume this is just inquisitive :)
http://www.madsci.org/posts/archives/2000-10/971992131.Ch.r.html

Hi all
I was just wondering if anybody has any idea about the effects of temperature change and concentration change on the rate of REVERSE reaction before equilibrium is established? Or is it only catalysts as the amount of activation energy for both reverse and forward reactions lower???
Hello there,
Equilibrium occurs in a system, as you'd know the rate of forward and reverse reactions are the same. When the system is perturbed or stressed, the equilibrium changes. For instance, if you add more reactants, the forward reaction will occur to 'use up' the excess reactants to reestablish equilibrium. When product is increased in a system, or reactants are removed, the reverse reaction will occur to undo this stress. The rate of the reverse reaction will increase instantly but slows down as it approaches equilibrium. Temperature is the same, although is dependent on whether the reaction is endothermic or exothermic.
I hope I answered your question appropriately, there are some great sites that explain equilibrium well; it took me a while to get my head around it tbh.
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peachxmh

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Re: VCE Chemistry Question Thread
« Reply #8030 on: May 27, 2019, 05:12:18 pm »
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Hello hello,

For a recent SAC, one of the questions I got wrong was about the difference in cloud point between biodiesel and petrodiesel. I thought that biodiesel had a higher cloud point than petrodiesel from a table included in my chemistry textbook (see image attached). Wouldn't this then mean that the temperature at which biodiesel forms crystals is higher than that of petrodiesel?

The question asked whether petrodiesel or biodiesel was more suitable in Antarctica. I thought that biodiesel was better as it would only form crystals at higher temperatures compared to petrodiesel. (I understand why crystal formation is a bad quality in a fuel). I did some research as well however many sites say that biodiesel is bad in low temperatures because of its crystal formation - why is this the case if biodiesel has a higher cloud point? Pls help thanks! ;D

P.S. Just to clarify that I'm not going crazy, cloud point is the lowest temperature at which crystals form in a fuel, am I correct?
« Last Edit: May 27, 2019, 05:19:55 pm by peachxmh »
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electrickxss

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Re: VCE Chemistry Question Thread
« Reply #8031 on: May 27, 2019, 05:38:08 pm »
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hi,
I totally get you, dude. it makes sense that since petrodiesel has a lower cloud point, it would more easily form crystals in lower temps...
I may be completely wrong here, but maybe you are right, but the answer they were looking for had more to do with other factors such as polarity (cause biodiesel is polar and Antarctica is surrounded with frozen water) or something similar? i dont really get why they marked it wrong either, your answer seems pretty right to me :/

Owlbird83

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Re: VCE Chemistry Question Thread
« Reply #8032 on: May 29, 2019, 07:33:08 pm »
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Hello hello,

For a recent SAC, one of the questions I got wrong was about the difference in cloud point between biodiesel and petrodiesel. I thought that biodiesel had a higher cloud point than petrodiesel from a table included in my chemistry textbook (see image attached). Wouldn't this then mean that the temperature at which biodiesel forms crystals is higher than that of petrodiesel?

The question asked whether petrodiesel or biodiesel was more suitable in Antarctica. I thought that biodiesel was better as it would only form crystals at higher temperatures compared to petrodiesel. (I understand why crystal formation is a bad quality in a fuel). I did some research as well however many sites say that biodiesel is bad in low temperatures because of its crystal formation - why is this the case if biodiesel has a higher cloud point? Pls help thanks! ;D

P.S. Just to clarify that I'm not going crazy, cloud point is the lowest temperature at which crystals form in a fuel, am I correct?

Biodiesel has a higher cloud point than petrodiesel, so crystals begin to form at a higher temperature (so it is not just for the high temp at which they form, any temp lower than that crystals will still be there). Therefore it is bad in cold temperatures because the temperature the crystals start to form at is higher. Petrodiesel's cloud point is lower so that means the temperature it begins to form crystals at is lower. So basically petrodiesel can withstand a decreasing temperature longer than the biodiesel before the crystals start to form. So petrodiesel will be better in Antarctica.
I hope this makes sense and helps!

Also, someone please correct me if I'm wrong  :D
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jasheel

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Re: VCE Chemistry Question Thread
« Reply #8033 on: May 30, 2019, 06:43:46 pm »
+4
hey everyone just doing some hydrocarbon revision ( chem 1/2) when I stumbled across this question

Explain the following;

Compounds containing a quadruple carbon–carbon bond do not exist


can anyone explain why? don't carbons have 4 valence electrons each, so when both carbons share electrons (covalent bond)  they should both have a total of 8 electrons, so than why cant a quadruple bond be formed?

edit

another question
C4H8 reacts readily with bromine. Give the balanced equation for the reaction and name any product(s).

I WROTE C4H8 + Br2 - - - - - - > C4H8Br2 

is this correct,
thnx
thnx in advance  :)

The first question is outside the scope of the chemistry course. It does fit into the Lewis model for molecules, but not other models, and is highly unstable.

For the second question your answer C4H8Br2 has too many bonds on each carbon if you draw it out!

When an alkane reacts with a halogen (Br2), one of the hydrogens in the alkane will be replaced with the Br.

In this case it would be: C4H8 + Br2 —> C4H7Br + HBr
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briv01

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Re: VCE Chemistry Question Thread
« Reply #8034 on: June 06, 2019, 10:41:12 pm »
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Hey guys,

I just had a doubt about electrolysis. Let’s say there’s a half cell that has Fe at it’s anode and an inert carbon cathode and an aqueous electrolyte ( AgCl ). Fe is the strongest reductant and will therefore get oxidised. However, since Ag ions are the strongest oxidant than Fe ions, wouldn’t it undergo reduction and the cathode be electroplated with Ag?

fiona_atarnotes

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Re: VCE Chemistry Question Thread
« Reply #8035 on: June 09, 2019, 03:22:00 pm »
+2
Hey guys,

I just had a doubt about electrolysis. Let’s say there’s a half cell that has Fe at it’s anode and an inert carbon cathode and an aqueous electrolyte ( AgCl ). Fe is the strongest reductant and will therefore get oxidised. However, since Ag ions are the strongest oxidant than Fe ions, wouldn’t it undergo reduction and the cathode be electroplated with Ag?

Hey! I'm not too sure about what you're confused about but everything that you've stated seems to be consistent and doesn't seem to be contradictory or confusing.
Your statement about Fe being the strongest reductant is technically correct but with electrolysis, it doesn't matter which one is the strongest reductant because you are choosing which species oxidises by connecting it to the positive terminal of a battery. Therefore, by choice, we are forcing the oxidation of Fe to Fe2+ ions (this is why Cl- doesn't oxidise to Cl2) and note that we can really use whatever cathode we like because in electrolytic cells, the cathode effectively remains inert. What we don't get to choose with electrolysis however is which species reduces (oxidant). By oxidising Fe to form Fe2+ ions, the possible oxidants in the cell are Fe2+, Ag+ and H2O (as we have an aqueous species). Assuming that conditions are at SLC, Ag+ is the strongest oxidant and therefore will reduce preferentially on the graphite cathode.

Hope this clears things up!

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briv01

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Re: VCE Chemistry Question Thread
« Reply #8036 on: June 10, 2019, 06:40:21 pm »
+3
As the enthalpy is greater than 0, we can conclude that it is an endothermic reaction. As such, increasing the temp would shift the equilibrium to the right ( endothermic uses up heat energy ), producing a higher yield of SrO

Decreasing the pressure will yield more SrO as the system will favour the side will more particles ( aka the right ) to compensate for the low pressure

The rate of reaction is not the same as the extent of reaction. The rate only tells us how fast the reaction will proceed whilst the extent tells us how far the reaction proceeds befire equilibrium is established  ( can be in either direction ). If the extent of the forward reaction is high, there will be a higher yield. So no, increasing the rate won’t increase the yield, only how fast the reaction proceeds

rani_b

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Re: VCE Chemistry Question Thread
« Reply #8037 on: June 11, 2019, 10:32:07 am »
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For the above reaction, which set of factors would produce the greatest yield of SrO(s)? Would it be high temperature/low pressure, or high temperature/high pressure?

Decreasing the pressure will yield more SrO as the system will favour the side will more particles ( aka the right ) to compensate for the low pressure

Doesn't changing the pressure only affect gaseous equilibrium?
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rani_b

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Re: VCE Chemistry Question Thread
« Reply #8038 on: June 11, 2019, 10:35:33 am »
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Hey guys!

So the data booklet now specifies that 1mL of water is 0.997 grams. I was watching one of the AtarNotes chem videos and the lecturer mentioned we should now always use that conversion, but it also says density of water at 25 degrees Celsius. Shouldn't this mean I only use this conversion when the question specifies the reaction is taking place in a room at 25 degrees?
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leonm19

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Re: VCE Chemistry Question Thread
« Reply #8039 on: June 11, 2019, 11:38:55 am »
+3
Hey guys!

So the data booklet now specifies that 1mL of water is 0.997 grams. I was watching one of the AtarNotes chem videos and the lecturer mentioned we should now always use that conversion, but it also says density of water at 25 degrees Celsius. Shouldn't this mean I only use this conversion when the question specifies the reaction is taking place in a room at 25 degrees?

You're right, it does say at 25 degrees on the data booklet, but from my experience I think it's okay to use it even if the temperature isn't 25 degrees. For example, I use it when calculating q = mc delta t if I need to find the mass of water for that equation, even though the temperature isn't constant. You might wanna double check with someone else though, I could be wrong.