Login

Welcome, Guest. Please login or register.

March 28, 2024, 09:33:41 pm

Author Topic: VCE Chemistry Question Thread  (Read 2313037 times)  Share 

0 Members and 12 Guests are viewing this topic.

Erutepa

  • VIC MVP - 2019
  • Forum Leader
  • ****
  • Posts: 721
  • evenin'
  • Respect: +775
Re: VCE Chemistry Question Thread
« Reply #8595 on: May 28, 2020, 05:36:55 pm »
+3
Hey there,
Question from textbook:
Lithium metal is prepared by electrolysis of a molten mixture of lithium chloride and potassium chloride.
a) Write a half-equation for the reaction occurring at the cathode in this cell
The answer uses Lithium for the reaction at the cathode, however in the presence of potassium chloride, isnt potassium a greater oxidant?
The reason why this occurs is that the electrochemical series is only really for comparing standard electrode potentials for aqueous solution (1M). When dealing with molten solutions, things don't neccasarily follow the exact same trend, hence you have lithium ions being preferentially reduced over the potassium ions.
Qualifications
 > Have counted to 227
 > Can draw really good spiders
 > 2 Poet points
 > 6.5 insanipi points
 > 1 Bri MT point

thatdumbstudent

  • Forum Regular
  • **
  • Posts: 83
  • Respect: +8
Re: VCE Chemistry Question Thread
« Reply #8596 on: May 28, 2020, 08:25:22 pm »
0
Explain why electrolysis sometimes yields different products from those predicted using the electrochemical series?

jammol7

  • Trailblazer
  • *
  • Posts: 45
  • Respect: +18
Re: VCE Chemistry Question Thread
« Reply #8597 on: May 28, 2020, 08:34:56 pm »
+6
Explain why electrolysis sometimes yields different products from those predicted using the electrochemical series?

Non standard conditions, perhaps?
VCE 2019-21
Lit 38 | Methods 47 | Bio 49 | Chem 50 |  PE 43 | Drama 38 | Sport & Rec 36

Moonblossom

  • Adventurer
  • *
  • Posts: 22
  • Respect: 0
Re: VCE Chemistry Question Thread
« Reply #8598 on: May 31, 2020, 03:47:01 pm »
0
Hi, I'm a little unsure what pressure and temperature does to an equilibrium system. My understanding is that when temperature increases, the endothermic reaction is favoured, pressure decreases and concentration decreases as well so the reaction favours the side of the reaction with less particles.
If, for example, the reaction is 2NO2 <-> N2O4 and volume was decreased, would the NO2 side be favoured (?) and if yes, would the reaction favour the forward reaction?

Also, is there a difference between writing K and Kc?

Sorry, one more thing! I know temperature is the only one that can change the equilibrium constant but when the concentration is doubled, K is squared. Doesn't this count as changing the equilibrium constant?

Thank you!

Geoo

  • MOTM: DEC 19
  • Victorian Moderator
  • Forum Leader
  • *****
  • Posts: 596
  • Class of 2020
  • Respect: +685
Re: VCE Chemistry Question Thread
« Reply #8599 on: May 31, 2020, 05:14:37 pm »
+4
Hi, I'm a little unsure what pressure and temperature does to an equilibrium system. My understanding is that when temperature increases, the endothermic reaction is favoured, pressure decreases and concentration decreases as well so the reaction favours the side of the reaction with less particles.
If, for example, the reaction is 2NO2 <-> N2O4 and volume was decreased, would the NO2 side be favoured (?) and if yes, would the reaction favour the forward reaction?

Also, is there a difference between writing K and Kc?

Sorry, one more thing! I know temperature is the only one that can change the equilibrium constant but when the concentration is doubled, K is squared. Doesn't this count as changing the equilibrium constant?

Thank you!
Hello!
I think you have a few things mixed up. Generally one thing is changed at a time, e.g temperature or volume. When talking about the effect on the equilibrium position e.g a temperature change, in terms of equilibrium that won't change the pressure and concentration. It will change the pressure in a rate of reaction, not equilibrium. So view all these factors as separate things, except for volume which I have shown down below, and in terms on concentration graphs.
So i'll start with pressure.
Pressure in terms of VCE when volume is changed.
If you have an increase in volume there is now "more space" for particles to "move around," hence less pressure.
If you have a decrease in volume, there is now "less space" for particles so they bump into each other more, hence higher pressure.

How does this effect an equilibrium system:
If we take the equation you have given there are two particles on the left hand side of the equation (2NO2), and one particle on the right, (1N2O2).
If the volume decreases, pressure increases, so the equilibrium position will shift to favour the side with the least number of particles, to lower the pressure. So it would be the N2O4 side that is favoured not NO2, since N2O4 has the lower amount of particles compared to NO2 (which has two). You would say that the forward reaction is favoured.

If volume increases, pressure decreases, so the equilibrium will shift to favour the side with the most number of particles to increase the pressure. So it would be the NO2 side, since that has 2 particles. Look at the coefficients to see the number of particles. The reverse reaction is favoured.

BTW forward reaction means the going right ----> the reverse reaction is going left/ the other way <-------

For temperature:

To make life easy, view temperature as just another reactant or product:
If it is exothermic, add energy onto the product side: *this reaction is endothermic but for the sake of getting a point across think of it as exothermic for the example:
2NO2 <-> N2O4 + energy
If you increase the energy of an exothermic reaction it will favour the reverse reaction, (more NO2).
If you decrease the energy of an exothermic reaction, it will favour the forward reaction, (more N2O4)
If it is endothermic, add energy to the reactant side:
energy + 2NO2 <-> N2O4
If you increase the energy of an endothermic reaction it will favour the forward reaction, (more N2O4).
If you decrease the energy of an endothermic reaction, it will favour the reverse reaction, (more NO2)

Temperature has no effect on volume. So separate temperature, pressure and concentration, I think you are lumping them all together, and mixing up rate of reaction and equilibrium!
There is no difference between K and Kc.
I don't think concentration is the right word. But, it you increase the coefficients e.g 4NO2 <-> 2N2O4, you would square the K constant. The reason this doesn't change the equilibrium constant is because the equilibrium constant is a ratio. Even if you add more of the reactants and products, it's still the same ratio.
Hope this helps.


2020: VCE 93.2
2022: BSci/Arts (Chemistry/Pharmacology and French)@Monash

Moonblossom

  • Adventurer
  • *
  • Posts: 22
  • Respect: 0
Re: VCE Chemistry Question Thread
« Reply #8600 on: May 31, 2020, 06:53:56 pm »
0
Hello!
I think you have a few things mixed up. Generally one thing is changed at a time, e.g temperature or volume. When talking about the effect on the equilibrium position e.g a temperature change, in terms of equilibrium that won't change the pressure and concentration. It will change the pressure in a rate of reaction, not equilibrium. So view all these factors as separate things, except for volume which I have shown down below, and in terms on concentration graphs.
So i'll start with pressure.
Pressure in terms of VCE when volume is changed.
If you have an increase in volume there is now "more space" for particles to "move around," hence less pressure.
If you have a decrease in volume, there is now "less space" for particles so they bump into each other more, hence higher pressure.

How does this effect an equilibrium system:
If we take the equation you have given there are two particles on the left hand side of the equation (2NO2), and one particle on the right, (1N2O2).
If the volume decreases, pressure increases, so the equilibrium position will shift to favour the side with the least number of particles, to lower the pressure. So it would be the N2O4 side that is favoured not NO2, since N2O4 has the lower amount of particles compared to NO2 (which has two). You would say that the forward reaction is favoured.

If volume increases, pressure decreases, so the equilibrium will shift to favour the side with the most number of particles to increase the pressure. So it would be the NO2 side, since that has 2 particles. Look at the coefficients to see the number of particles. The reverse reaction is favoured.

BTW forward reaction means the going right ----> the reverse reaction is going left/ the other way <-------

For temperature:

To make life easy, view temperature as just another reactant or product:
If it is exothermic, add energy onto the product side: *this reaction is endothermic but for the sake of getting a point across think of it as exothermic for the example:
2NO2 <-> N2O4 + energy
If you increase the energy of an exothermic reaction it will favour the reverse reaction, (more NO2).
If you decrease the energy of an exothermic reaction, it will favour the forward reaction, (more N2O4)
If it is endothermic, add energy to the reactant side:
energy + 2NO2 <-> N2O4
If you increase the energy of an endothermic reaction it will favour the forward reaction, (more N2O4).
If you decrease the energy of an endothermic reaction, it will favour the reverse reaction, (more NO2)

Temperature has no effect on volume. So separate temperature, pressure and concentration, I think you are lumping them all together, and mixing up rate of reaction and equilibrium!
There is no difference between K and Kc.
I don't think concentration is the right word. But, it you increase the coefficients e.g 4NO2 <-> 2N2O4, you would square the K constant. The reason this doesn't change the equilibrium constant is because the equilibrium constant is a ratio. Even if you add more of the reactants and products, it's still the same ratio.
Hope this helps.
Hi! Thank you so much for that explanation. It's cleared up all of things. Sorry I did mean to type in volume not temperature initially, I don't really know what happened. But the explanation for temperature was also really helpful and helped clear a couple of other things up so thank you for including all of that.

Just for extra clarification, increased volume means decreased pressure (means decreased concentration?) meaning the side more more particles will be favoured?

Geoo

  • MOTM: DEC 19
  • Victorian Moderator
  • Forum Leader
  • *****
  • Posts: 596
  • Class of 2020
  • Respect: +685
Re: VCE Chemistry Question Thread
« Reply #8601 on: May 31, 2020, 07:08:55 pm »
+2
Hi! Thank you so much for that explanation. It's cleared up all of things. Sorry I did mean to type in volume not temperature initially, I don't really know what happened. But the explanation for temperature was also really helpful and helped clear a couple of other things up so thank you for including all of that.

Just for extra clarification, increased volume means decreased pressure (means decreased concentration?) meaning the side more more particles will be favoured?
Yes. If you decrease the pressure (via the increase in volume), or a solution is diluted (concentration goes down), then the side with more particles will be favoured! This is because the equilibrium system wants to get back to equilibrium, so it will do whatever it can to counteract a change. So, if the pressure decreases, the system will increase the pressure by favouring the side of the reaction with more particles.
Also talking about concentration refers to more aqueous solutions instead of gas solutions. Concentration isn't really used in gas scenarios.
2020: VCE 93.2
2022: BSci/Arts (Chemistry/Pharmacology and French)@Monash

amelia_nap

  • Adventurer
  • *
  • Posts: 5
  • Respect: 0
Re: VCE Chemistry Question Thread
« Reply #8602 on: June 03, 2020, 01:06:25 pm »
0
Hi everyone, :D
I know this may be a hard question to answer, but I was wondering if it is a good idea to do both vce chemistry and vce biology?
I feel i am quite good at science but is it too much work to do both subjects, considering they both rely heavily on memory? :-\

colline

  • MOTM: NOV 19
  • Forum Obsessive
  • ***
  • Posts: 341
  • ♡ 2 Timothy 1:7 ♡
  • Respect: +512
Re: VCE Chemistry Question Thread
« Reply #8603 on: June 03, 2020, 01:19:22 pm »
+4
Hi everyone, :D
I know this may be a hard question to answer, but I was wondering if it is a good idea to do both vce chemistry and vce biology?
I feel i am quite good at science but is it too much work to do both subjects, considering they both rely heavily on memory? :-\
A lot of people do both chemistry and biology in VCE. There are some overlapping topics between the two subjects, which helps when you're studying both subjects together.

You obviously need to memorise content and theory but on the whole, both subjects focus more on understanding and applying what you've learnt.

If you are worried about the workload, maybe consider doing one as an early 3/4 if you are allowed. Many people do biology 3/4 in year 11.

VCE: Literature [50] Methods [50] Further [48] Chemistry [40] Biology [33]
2022: Bachelor of Science (Mathematical Economics) @ ANU

Geoo

  • MOTM: DEC 19
  • Victorian Moderator
  • Forum Leader
  • *****
  • Posts: 596
  • Class of 2020
  • Respect: +685
Re: VCE Chemistry Question Thread
« Reply #8604 on: June 04, 2020, 07:47:36 pm »
0
What would be some strengths and limitations in a experiment?

What could be considered a strength or a limitation?
2020: VCE 93.2
2022: BSci/Arts (Chemistry/Pharmacology and French)@Monash

sweetiepi

  • National Moderator
  • ATAR Notes Legend
  • *****
  • Posts: 4767
  • "A Bit of Chaos" (she/they)
  • Respect: +3589
Re: VCE Chemistry Question Thread
« Reply #8605 on: June 04, 2020, 08:01:24 pm »
+6
What would be some strengths and limitations in a experiment?

What could be considered a strength or a limitation?
This often depends on the experiment itself. This could be related to chemicals or the methods you use! :)

For an chemical (kinda) example lets say we could use reagent A or regent B for mixed into media for cells.
Reagent A has a high toxicity to cells (limitation), but reacts faster in cells (strength); whereas reagent B has a lower toxicity to cells (strength), but takes ages to react in cells.

For method- shaking vs tilting in a north-south-east-west fashion could be something to use. Shaking ensures that everything is well dispersed (strength), but cells do not adhered well when shaken (weakness); whereas tilting can sometimes unevenly disperses (weakness), but cells can adhere super well (strength).

Hope this helps a little! :)
2017-2019: Bachelor of Pharmaceutical Science (Formulation Science)
2020: Bachelor of Pharmaceutical Science (Honours) Read my uni journey here!

keltingmeith

  • Honorary Moderator
  • Great Wonder of ATAR Notes
  • *******
  • Posts: 5493
  • he/him - they is also fine
  • Respect: +1292
Re: VCE Chemistry Question Thread
« Reply #8606 on: June 05, 2020, 05:28:09 pm »
+5
What would be some strengths and limitations in a experiment?

What could be considered a strength or a limitation?

Here's some examples stolen from first year chemistry experiments (note: their theory ain't that different to yours)

Experiment is heating hydrated copper sulphate to determine how much of the compound is water by mole (i.e., determine x in CuSO4.xH2O). All you do is weigh copper sulphate, heat it until its colour stops changing, weigh it again, and do maths based on how much weight is lost.

Pro: Methodology is simple

Con: Some of the copper sulphate may convert to copper oxide if you heat it too hard. However, if you don't heat it enough, not all the water will be removed.

Way to fix the experiment: Add a dessicating, or drying, agent to help remove the water - but make sure it's added in a way so that it isn't mixed in with the copper and is easily removed.

Next, how about an experiment where you try to make an ester, but there are 10 steps involved:
1. Add chemicals
2. Mix things in a bowl
3. Transfer things to another bowl
4. Mix things in bowl with more chemicals
5. Transfer things to another bowl
6. Filter
7. Add precipitate to another bowl, add in more chemicals
8. Mix things
9. Filter
10. Ya done

Con: This is so many steps. The more steps you have, the less efficient things are. Particularly when multiple reactions (note: every time I say mix things, that's a separate reaction) happen at once. This particular transformation involves 4 reactions - if all of those reactions have a 95% yield (note: this is normally a REALLY good yield, see below), then your yield is now only going to be (0.95)^4=81%, which is still good, but not great any more.

Pro: potentially this is low cost method. Let's say that no matter what, each step is 95% yield. This includes the 4 steps from above, but also a theoretical reaction with only one step. If that one step reaction costs $1000, then you're LOSING 0.05*$1000 = $50 per product. What if the 4 step method only costs $200? Even though that yield ends up being 81%, you only lose 0.19*$400 = $36 per product.

Way to fix the experiment: Use a different method. But note, if you're going to play this card, it is 100% better to offer an alternative. Your alternative doesn't have to worry about costs or anything - but if you've said something takes too many steps, tell me how to do it in less steps. Maybe you made ethyl butanoate. You know a faster way to make ethyl butanoate that takes less than 10 steps? Mix ethanol and butanoic acid with some sulfuric acid. It may sound scary to suggest a whole new synthesis - but you won't be asked to suggest one if you haven't learned about it

Now, here's a list of cons that I personally wouldn't accept - but bear in mind that I'm now 7 years out of high school, so it's worth taking these to your teacher to see if they will accept these responses:

1. Everyone will perceive colour changes differently - yes, but this is a philosophical argument. Not only is red, green, and blue all different enough that it doesn't matter if we see them slightly differently, but the colour change will typically happen when less than a drop is added. So it doesn't matter if you can't tell if this is "blue enough" yet, because that situation will never actually *happen*.

2. We need to repeat the experiment more times. Like, this is obvious - if we could do the experiment more times, we would. BUT, that doesn't make this experiment pointless. If an experiment is flawed, we're not going to do it 4 times before deciding if it's flawed - that's a waste of time. We're going to design the experiment to do once, then check if we can find any flaws. Then we'll run the experiment, look at the result, and ask again if there are any flaws. We'll worry about doing it more times AFTER we've figured out if that experiment is okay.

3. Solution wasn't mixed well enough. Unless they've given you an experiment that explictly says "solution wasn't mixed", then they did mix it. If this is an experiment you did yourself, then why DIDN'T you mix it? But also: do you need to mix it? If all you're doing is drying copper sulphate for example, mixing the lump of solid isn't going to do anything. Mixing is only going to help if something is dissolved into solution (see collision theory - if the solid is in a lump, you're only going to get collisions on the outside of the solid lumps, which is less than 1% of the molecules in that lump)

4. The reaction only has a 95% yield. Yes, it's not 100% - but that doesn't matter. You will lose some material in transfer (note: this isn't a con either, unless your reaction has 20 steps or something when it could be done in 1. Transfer loss is usually only about 1%, even in 10 step reactions), so it's expected not to get 100%. In fact, 95% is REALLY, REALLY good.

Can't think of any more at the moment - but if I do before you come back, I'll add them.

Chocolatepistachio

  • Science Games: Silver
  • Forum Obsessive
  • ***
  • Posts: 470
  • Respect: +51
Re: VCE Chemistry Question Thread
« Reply #8607 on: June 06, 2020, 02:13:08 am »
0
Hello

Couldn’t really find any information about this

The Bayer company started selling aspirin in 1899 and cyclotides  are current research. What is similar about their origins?

What are some of the differences between getting a pharmaceutical drug approved in the 1800s compared to now?

If someone could help that would be great
Thanks!


sweetiepi

  • National Moderator
  • ATAR Notes Legend
  • *****
  • Posts: 4767
  • "A Bit of Chaos" (she/they)
  • Respect: +3589
Re: VCE Chemistry Question Thread
« Reply #8608 on: June 06, 2020, 09:11:40 am »
+5
Hello

Couldn’t really find any information about this

The Bayer company started selling aspirin in 1899 and cyclotides  are current research. What is similar about their origins?

What are some of the differences between getting a pharmaceutical drug approved in the 1800s compared to now?

If someone could help that would be great
Thanks!


Hi!
Both aspirin and cyclotides are derived from plants.

Pharmaceutics in the 1800s was a really wild ride, I suggest you read more about it! (Here's one source that is helpful) One of the main differences is that the TGA (or FDA in the US) have put in place many guidelines and rules for drug development and approval- which includes the need for clinical trials. Such strict rules weren't in place in the 1800s!

Hope this helps a little! :)
« Last Edit: June 06, 2020, 09:13:55 am by insanipi »
2017-2019: Bachelor of Pharmaceutical Science (Formulation Science)
2020: Bachelor of Pharmaceutical Science (Honours) Read my uni journey here!

Chocolatepistachio

  • Science Games: Silver
  • Forum Obsessive
  • ***
  • Posts: 470
  • Respect: +51
Re: VCE Chemistry Question Thread
« Reply #8609 on: June 06, 2020, 12:16:36 pm »
0


why does an increase in co2 concentration cause an increase in plant growth up until a certain point