Login

Welcome, Guest. Please login or register.

March 29, 2024, 08:10:10 am

Author Topic: VCE Chemistry Question Thread  (Read 2313402 times)  Share 

0 Members and 2 Guests are viewing this topic.

coldairballoon

  • Trailblazer
  • *
  • Posts: 49
  • Class of 2020
  • Respect: +24
Re: VCE Chemistry Question Thread
« Reply #8460 on: February 24, 2020, 09:14:46 pm »
+1
Something I still don't get about galvanic cells is how the reaction can happen? Like, I understand what's happening, I just don't really understand why. How is the redox reaction able to occur if the anode and cathode are only connected by wire and aren't actually in contact with each other; is it kind of like a siphon where the ions from the solution surrounding the cathode pull electrons from the cathode and the electrons from the wire/anode just move along accordingly and end up causing the anode to oxidise as a result? Or am I just getting it completely wrong, I don't know. My teacher isn't being super helpful about this lol 'you just have to accept that that's what happens'
[2019] Biology - 50, Methods - 44
[2020] Chemistry - ?, English Language - ?, Latin - ?, Specialist - ?

bi-and-ready-to-cry

  • Adventurer
  • *
  • Posts: 5
  • Respect: +1
Re: VCE Chemistry Question Thread
« Reply #8461 on: February 24, 2020, 09:32:39 pm »
+2
Benjamin has purchased a new camp stove that runs on propanol and decides to test it out.
He adds 146.25g of fuel to the stove and heats up 2L of water for 3 minutes, during which time the temperature of the water rises from 20.2 degrees to 94.6 degrees (celsius). When Benjamin reweighs the fuel there is 99.13g remaining.

Calculate the heat of combustion (KJ/g) for the propanol.

Okay so first I figured out how much energy was used (using the q=mcAT formula)
Therefore Q = (2000g) *(4.18)*(74.4)
= 622 KJ

Next step all I did was

622 / 47.12g

47.12g as that is how much was used up in the reaction.

=13.2 KJ G-2

Just wondering if I did this correctly.
thanks

yeah that seems right, double check your units though. You wrote kj g^-2 not -1

yeshelly

  • Fresh Poster
  • *
  • Posts: 4
  • Respect: 0
Re: VCE Chemistry Question Thread
« Reply #8462 on: February 24, 2020, 09:38:08 pm »
0
Whats's the difference between surface tension and interface tension?
« Last Edit: February 24, 2020, 09:49:21 pm by yeshelly »
2019 - Biology 1/2
2020 - Biology 3/4 , Chemistry 1/2 , P.E. 1/2, English Language 1/2 , Methods 1/2 , Specialist 1/2
2021 - Chemistry 3/4 , P.E. 3/4, English Language 3/4 , Methods 3/4 , Specialist 3/4

Ionic Doc

  • Forum Leader
  • ****
  • Posts: 571
  • Respect: +212
Re: VCE Chemistry Question Thread
« Reply #8463 on: February 25, 2020, 07:11:52 am »
0
yeah that seems right, double check your units though. You wrote kj g^-2 not -1

Thanks, yeah typo error I'll fix that up.
2019/2020 - Psychology | Biology | Chemistry | Methods | Further | English
2021 - Science @ Melbourne University

jnlfs2010

  • Forum Regular
  • **
  • Posts: 87
  • Class of 2020
  • Respect: +10
Re: VCE Chemistry Question Thread
« Reply #8464 on: February 25, 2020, 05:18:09 pm »
0
Hey, this is an MC question from the 2018 Chemistry exam. I don't really understand the question and the answer like I don't know how to start to achieve a correct thought process to the answer. What does it mean when air is present, and where is it present?

Hello, can someone help me out?

Thanks!
VCE:
2019: Accounting [45]
2020: English [37],  Economics [42], Specialist Mathematics [42], Mathematical Methods [43], Chemistry [44]
ATAR: 98.80

2021-2023: Bachelor of Biomedicine [The University of Melbourne]

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 4719
  • invest in wellbeing so it can invest in you
  • Respect: +3677
Re: VCE Chemistry Question Thread
« Reply #8465 on: February 25, 2020, 05:43:09 pm »
+3
Hello, can someone help me out?

Thanks!

The question is saying that if air is able to react then the following reactions occur (it's not important for you to know where air is reacting). You then look at the provided half reactions (diagram not needed at all) to see where oxidation and reduction is occuring.

remember:
 Oxidation is loss of electrons / increase in oxidation number
 Reduction is gain of electrons / decrease in oxidation number

Hope this helps!

Evolio

  • MOTM: MAY 20
  • Forum Leader
  • ****
  • Posts: 604
  • Respect: +485
Re: VCE Chemistry Question Thread
« Reply #8466 on: February 25, 2020, 07:35:06 pm »
0
Quote
Key to remember electrostatic attraction here! Electrons are attracted to positive hence if there was no salt bridge the cathode would become negative (from lack of cations) and thus electrons would be repelled from going there. Consequently this would mean electrons would fail to flow from the anode across the wire to the cathode, hence no longer produce electricity!!!!!
Ohhh, thank you! That makes so much more sense!  ;DSo, overall are there more cations in the salt bridge to balance out the negative charge at the cathode?

Quote
For the VCAA 2010 exam, I think as it is an electrolytic cell rather than just a galvanic, lithium ions would need to move back to anode so that the cell has enough lithium in the anode to discharge. both equations involve lithium ions as well so the ions would be constantly moving from when they are created in oxidation to when they are consumed in reduction.
Sorry if this doesn't make sense, majority of chem knowledge went poof after my exam last year XD
Oh ok, so would the lithium ions and the ions in the electrolyte be moving to the cells?

coldairballoon

  • Trailblazer
  • *
  • Posts: 49
  • Class of 2020
  • Respect: +24
Re: VCE Chemistry Question Thread
« Reply #8467 on: February 26, 2020, 09:14:32 pm »
0
Redox reactions don't necessarily need to occur at the same place, right? Just at the same time?
Like, in a Galvanic cell or something, the oxidizing agent doesn't have to be in direct contact with the reducing agent.
[2019] Biology - 50, Methods - 44
[2020] Chemistry - ?, English Language - ?, Latin - ?, Specialist - ?

keltingmeith

  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 5493
  • he/him - they is also fine
  • Respect: +1292
Re: VCE Chemistry Question Thread
« Reply #8468 on: February 27, 2020, 06:48:58 am »
+6
Ohhh, thank you! That makes so much more sense!  ;DSo, overall are there more cations in the salt bridge to balance out the negative charge at the cathode?

Not necessarily - there might be more ions in the salt bridge, there might be more in the anode, or there might be more in the cathode. The key is just that while there ions in all three of these places, the reaction can continue. When one of these places runs out of ions, then the reaction will stop.

Oh ok, so would the lithium ions and the ions in the electrolyte be moving to the cells?

If there were ions in the electrolyte, probably. Remember, an electrolyte is just a solution that is conductive. While this is usually done by using salty water, remember that it doesn't have to be done this way. And in fact, if you read the question in full, you'll notice it says that the electrolyte isn't a saline solution - it's molten lithium chloride! So in this case, lithium ions ARE the ions in the electrolyte.

Redox reactions don't necessarily need to occur at the same place, right? Just at the same time?
Like, in a Galvanic cell or something, the oxidizing agent doesn't have to be in direct contact with the reducing agent.

Correct. Google "galvanic cell two beakers" for lots of examples of cells where the two are not in direct contact. Remember, though, that there DOES need to be a salt bridge connecting them, or else the reaction can't happen.

Evolio

  • MOTM: MAY 20
  • Forum Leader
  • ****
  • Posts: 604
  • Respect: +485
Re: VCE Chemistry Question Thread
« Reply #8469 on: February 27, 2020, 08:52:15 pm »
0
Ahh, I have a better understanding now!
Thank you, keltingmeith!  ;D

whys

  • VIC MVP - 2020
  • Forum Leader
  • ****
  • Posts: 722
  • Respect: +916
Re: VCE Chemistry Question Thread
« Reply #8470 on: February 27, 2020, 10:39:54 pm »
0
Are we allowed to write on the Data Book during the chem exam?
Thank you!
psych [50] bio [50]
2021-2025: BMedSci/MD @ Monash

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 4719
  • invest in wellbeing so it can invest in you
  • Respect: +3677
Re: VCE Chemistry Question Thread
« Reply #8471 on: February 27, 2020, 10:47:30 pm »
+4
Are we allowed to write on the Data Book during the chem exam?
Thank you!

Yep :)

Obviously don't answer questions there tho

BitcoinEagle

  • Trailblazer
  • *
  • Posts: 29
  • You should've bought Bitcoin in 2008.
  • Respect: +3
Re: VCE Chemistry Question Thread
« Reply #8472 on: March 09, 2020, 01:34:31 pm »
+1
To what extent are we meant to know about the transesterification reaction of biodiesel and the products that evenuate?
Would we have to answer a question like this:

Write the semi-structural formula for biodiesel produced from oil predominantly containing  ;myristic acid.


and if so how? :D
2019: Methods {47} Bio {47}
2020: Spesh {44}, Physics {46}, Chemistry {46}, English {50}, Economics {47}

ATAR: 99.95

Proudest achievement: Doing Unit 1 Lang alone in sem 1 2019, Unit 2 Lit alone in sem 2 and unit 3/4 English alone in 2020 and somehow pulling out a rank 1. (It's a long story)

angrybiscuit

  • Trendsetter
  • **
  • Posts: 196
  • Respect: +204
Re: VCE Chemistry Question Thread
« Reply #8473 on: March 09, 2020, 10:23:53 pm »
+4
To what extent are we meant to know about the transesterification reaction of biodiesel and the products that evenuate?

You have to know that:
triglyceride + alcohol (generally ethanol or methanol) ---> fatty esters + glycerol



The triglyceride will have that backbone as shown above but the 'R' groups can be differing fatty acids or can all be the same.

Would we have to answer a question like this:
Write the semi-structural formula for biodiesel produced from oil predominantly containing  ;myristic acid.

and if so how? :D
You can be asked to answer questions such as that. To answer that the question will have to specify what alcohol it reacts with. The resulting product is simply an esterification reaction between the myristic acid and an alcohol.
« Last Edit: March 10, 2020, 08:39:34 pm by angrybiscuit »
somewhere, something incredible is waiting to be known.
carl sagan

Snow Leopard

  • MOTM: SEP 19
  • Forum Leader
  • ****
  • Posts: 574
  • Respect: +196
Re: VCE Chemistry Question Thread
« Reply #8474 on: March 10, 2020, 09:13:36 pm »
0
Hey,

Just a Yr 11 unit 1/2 chem student here. I have a chem sac this week including stuff on moles.
Is there an ideal way which we should be setting up our scientific calculator cos I have a feeling that mine isn't set properly cos it isn't giving me the right answers.

For example, the textbook says that if you enter 60x Avogadro's number, then you should get 3.06 x 10^25 but my calculator gives me 3.16 x 10^25.

Thanks in advance!