VCE Chemistry Question Thread

[DBA-144]

I have another question about dissolving substances. So I believe that if the energy released from breaking existing bonds between solute and solvent is similar (or does it HAVE to be equal?) to the energy released when the new bonds between these substances form, the substances will dissolve. However, in your post on the chem thread, you said that we need to weigh up the entropy benefit against any heat released (by this I assume you mean the enthalpy of the reaction?)

So, can you please clarify what you mean by this and how and what we would need to consider for the substances would dissolve? Under which circumstances will the substances dissolve then? So, does it depend if the entropy of the reaction is greater than that of the heat realeased?

Furthermore, I believe my confusion actually stems from not quite understanding quite what entropy refers to. After some googling, it does seem to me that the entropy is the randomness, or the disorder, of a system. I am also aware of the 2nd law of thermodynamics and that this relates to entropy but am not quite sure how? 

Another question I have is if energy is used to break bonds and then energy is released when new bonds are formed, wouldnt the enegy be stroed in the bonds? However, I now think that this is not the case, that energy is not stored in bonds. Thus, where is this energy stored and what form is it in? Is it chemical potential energy? Sorry for the wierd and probably very stupid question, but I cant quite visualise energy, as its not really tangible, unlike atoms, which I pretend are just cloloured tiny polystyrene balls. 

Why is chem so hard!

Thanks to anyone who answers my questions.

[lzxnl]

I have another question about dissolving substances. So I believe that if the energy released from breaking existing bonds between solute and solvent is similar (or does it HAVE to be equal?) to the energy released when the new bonds between these substances form, the substances will dissolve. However, in your post on the chem thread, you said that we need to weigh up the entropy benefit against any heat released (by this I assume you mean the enthalpy of the reaction?)

So, can you please clarify what you mean by this and how and what we would need to consider for the substances would dissolve? Under which circumstances will the substances dissolve then? So, does it depend if the entropy of the reaction is greater than that of the heat realeased?

Furthermore, I believe my confusion actually stems from not quite understanding quite what entropy refers to. After some googling, it does seem to me that the entropy is the randomness, or the disorder, of a system. I am also aware of the 2nd law of thermodynamics and that this relates to entropy but am not quite sure how? 

Another question I have is if energy is used to break bonds and then energy is released when new bonds are formed, wouldnt the enegy be stroed in the bonds? However, I now think that this is not the case, that energy is not stored in bonds. Thus, where is this energy stored and what form is it in? Is it chemical potential energy? Sorry for the wierd and probably very stupid question, but I cant quite visualise energy, as its not really tangible, unlike atoms, which I pretend are just cloloured tiny polystyrene balls. 

Why is chem so hard!

Thanks to anyone who answers my questions.

Why is this so hard? Because this is the realm of physical chemistry, which overlaps with physics. So you need to be good at both! Fortunately, VCE chemistry does not ask for any understanding of dissolving, beyond 'like dissolves like', which as we've established doesn't account for why some ionic substances don't dissolve.

So, entropy is formally defined in terms of how many ways the energy can be rearranged (microstates). Maximising the entropy can be thought of as dispersing the entropy over the largest possible volume, which is certainly achieved by letting dissolved particles swim around in the entire solvent, as opposed to being stuck in a thin layer above or below the solvent (think oil on water).

Now, most chemical processes don't go to completion. They instead reach an equilibrium, where the forwards process and the backwards process balance each other. This is what incomplete dissolving means. There are ways to predict the extent to which something will dissolve, one of which is the Gibbs free energy G, defined as G = H - TS. H is the enthalpy, which tells you how much heat is given off by the reaction. T is the temperature, S is the entropy. So, this quantity decreases if a chemical process is spontaneous, and you can think of it as a balance between H and S.

As for the second law of thermodynamics, it is entirely probabilistic. I'll give you a snapshot of it. Let's suppose that you have a 1 L container with 1000 identical 1 mL cubes. Suppose you fill one cube with 1 mole of helium gas. Then, the probability of finding one helium atom in the 1 mL cube is 1/1000. The probability of finding all 6.02*10^23 atoms in the 1 mL cube is then (1/1000)^(6.02*10^23), which is tiny. It's exceedingly unlikely to find the helium all in the one cube; the helium is much more likely to expand into the other cubes as well. So, the second law just says that energy wants to be dispersed because that's much more likely.

For your last question, if energy is used to break and reform bonds, any excess energy goes into what is commonly called heat, or random thermal motion of the surrounding particles. Energy is required to break bonds and energy is released when bonds are formed, yes.

For visualising energy, think of it as the ability to exert a force over a distance. Indeed, potential energy is often defined in terms of the amount of energy required to get the atoms to that position. So, if there is a lot of energy in a bond, like a very compressed spring, that means a lot of force was required over some distance to compress the spring.

Feel free to ask if you have any more questions!

[DBA-144]

Ok. Cool so another question I have is if ethene and ethene molecules were to react, why would the bonds break? Again, I understand this is a pretty simple question, but I am struggling to make sense of it. So, if we have ethene molecules, which are 'happy', as all of the atoms have a complete octet, why would they react to form polyethene? Of course, the carbon carbon double bond is stronger than the single bond, then wouldnt this lead us to believe that the bonds that have previously formed are stronger than the new single bonds that form? Does this have something to do with how the new dispersion forces are much stronger, because the molecule chape changes from trigonal planar, to a long straight hydrocarbon chain? 

Similarly, with the topic of spontaneity, how could we possibly have these sorts of molecules reacting without adding energy? That is, if a reaction is endothermic (where energy in reactants is greater than products), how do these reactions start if there are molecules which are in their typically 'satisfied' state, having a stable octet or are metallic or ionic substances which have a balanced charge, how do these molecules react without energy to start the reaction and break existing bonds,etc.? 
 
I know these questions are pretty simple or obvious ones, but I cant quite make sense of them. And I think I finally have a solid understanding of entropy, to some small extent anyways.

So just like heat wants to be transferred over a larger region, the helium atoms in the beaker example Izxnl gave previously want to be spread out. Thus, there is only one arrangement that we find all of the atoms in that 1 mol of helium atoms in that 1ml cube Izxnl talked about. Hence, it is much more likely that we find the atoms spread out. This is a  statistical phenomenon, and the meaning of this phrase would be that it is more likely we find these atoms spread out across the beaker rather than staying in that one big clump of helium atoms. Am I on the right track here?

Also, is this statistical phenomenon similar to what the schrodinger model of electron configuration says, that we are more likely to find those electrons in those orbitals, rather than what the Bohr model treats the electrons as; simply orbiting around a fixed radius around the nucleus?

Can we extend entropy to the Schrodinger model? And is my understanding of entropy so far sound and is it sufficient for VCE? do we even do entropy in vce chem or physics

Another: I am still not quite satisfied with this little bit: so if we need to weigh up entropy gain and any heat released, at what point would the substance dissolve and not dissolve? I take it that if the entropy gain and the heat released is balanced, then we would see the substances dissolving, bonds would break, etc. but if the entropy gain is more than that of the heat released, I take it that the substance would also dissolve. Thus, if the heat release is greater than the entropy gain, or that the entropy gain is not 'enough', the substance would not dissolve? 

Just trying to get my head around all of this stuff.
Thanks. Any help is appreciated.

[Yertle the Turtle]

snip

So the thing to remember is that even though the bond strength of a double bond is greater than that of a single bond, the bond is more rigid and therefore more brittle. A single bond can rotate easily, and therefore it holds together, while the sheer strength of a double bond causes it to be more reactive, as it more readily splits into less volatile bonds, such as the C-C single bond chain. In general there are orders of preference that molecules follow, different ways that they can be formed, that all work, but work better and therefore in preference in certain ways. You can see this with the electrochemical series, as many ionic compounds can work together, but due to their different electrochemical charges, they will bond by preference into different compounds. I don't think that entropy is covered in any kind of depth in the VCE courses, and in general it is wisest at VCE level to just accept some things. I know this is hard, as I spent all of last year trying to get explanations, but otherwise you start to understand too much to be of value in the exams, and you can really overcomplicate things that are really very simple at this level. Hope this helps!

[lzxnl]

Ok. Cool so another question I have is if ethene and ethene molecules were to react, why would the bonds break? Again, I understand this is a pretty simple question, but I am struggling to make sense of it. So, if we have ethene molecules, which are 'happy', as all of the atoms have a complete octet, why would they react to form polyethene? Of course, the carbon carbon double bond is stronger than the single bond, then wouldnt this lead us to believe that the bonds that have previously formed are stronger than the new single bonds that form? Does this have something to do with how the new dispersion forces are much stronger, because the molecule chape changes from trigonal planar, to a long straight hydrocarbon chain?

A double bond is indeed stronger than a single bond, but remember that a double bond is broken to form two single bonds, and a double bond is weaker than two single bonds. You need to think of a double bond as two bonds. Indeed, the third bond in a triple bond is weaker than the second bond in a double bond.

Simple explanation: bonding twice to the same atom is awkward because, as mentioned above, you're not going to be able to rotate as easily. Imagine holding hands AND legs with another person. The second bond is thus readily broken.

Complicated explanation: involves at least bond hybridisation or molecular orbital theory which I don't wish to confuse you further with.

Indeed, in polymerisation, you have a catalyst which binds to the double bond too (won't go into how; complicated) to weaken the bond further, making polymerisation easier.

Finally, dispersion forces are negligible if anything other force is present. Only consider them if you've exhausted every other possible force. It's like whether you assume that five year old kid next door shot your mum. Only assume that kid was responsible if no one else could have done it and you have video footage. Extreme examples get the point across easier.


Similarly, with the topic of spontaneity, how could we possibly have these sorts of molecules reacting without adding energy? That is, if a reaction is endothermic (where energy in reactants is greater than products), how do these reactions start if there are molecules which are in their typically 'satisfied' state, having a stable octet or are metallic or ionic substances which have a balanced charge, how do these molecules react without energy to start the reaction and break existing bonds,etc.? 


Well, your reactants aren't at absolute zero, right? So they have a temperature, which means that they exhibit random thermal motion. They can get the energy required to react from this random motion and collisions between particles. This is how nearly any spontaneous reaction with no external energy input works btw; every chemical reaction that reforms bonds requires energy to firstly break bonds, even ones as weak as F-F bonds.
 


I know these questions are pretty simple or obvious ones, but I cant quite make sense of them. And I think I finally have a solid understanding of entropy, to some small extent anyways.
I don't actually expect many VCE students to give you a response to these, as entropy isn't covered in VCE and statistical physics isn't taught until second year uni, so they're not trivial

So just like heat wants to be transferred over a larger region, the helium atoms in the beaker example Izxnl gave previously want to be spread out. Thus, there is only one arrangement that we find all of the atoms in that 1 mol of helium atoms in that 1ml cube Izxnl talked about.
I'm not going to complicate issues here much further, but you'll learn in due course that even in the 1 mL cube, there are squillions of ways you can rearrange the energy; it's just a LOT less than in the 1 L cube in total. Don't get too caught up on the 1 cube

Hence, it is much more likely that we find the atoms spread out. This is a  statistical phenomenon, and the meaning of this phrase would be that it is more likely we find these atoms spread out across the beaker rather than staying in that one big clump of helium atoms. Am I on the right track here?
Yup. You got it.

Also, is this statistical phenomenon similar to what the schrodinger model of electron configuration says, that we are more likely to find those electrons in those orbitals, rather than what the Bohr model treats the electrons as; simply orbiting around a fixed radius around the nucleus?
No. The Bohr model fails not because of statistical physics. You need to think of quantum mechanics as being separate from statistical physics (to begin with; you can mix them, and at a deep enough level all physics is inter-related, except maybe general relativity and quantum mechanics but let's not go there). Rather, the Bohr model fails because it does not take into account electron-electron repulsions. Indeed, if you solve the Schrodinger equation for the hydrogen atom, which DOESN'T have electron-electron repulsions due to only having one electron, you get exactly the same results as you do as in the Bohr model.


Can we extend entropy to the Schrodinger model? And is my understanding of entropy so far sound and is it sufficient for VCE? do we even do entropy in vce chem or physics

Entropy, in general, is defined in a way such that you could easily incorporate it into some quantum statistical description of matter, so yes to the first question. No, I'm not going to explain how that works, you've got enough to think about

Is your understanding sufficient for VCE? You don't need to know about entropy for VCE to begin with; below I've listed what you actually need from what I've said for VCE. Don't be surprised.


Another: I am still not quite satisfied with this little bit: so if we need to weigh up entropy gain and any heat released, at what point would the substance dissolve and not dissolve? I take it that if the entropy gain and the heat released is balanced, then we would see the substances dissolving, bonds would break, etc. but if the entropy gain is more than that of the heat released, I take it that the substance would also dissolve. Thus, if the heat release is greater than the entropy gain, or that the entropy gain is not 'enough', the substance would not dissolve? 

Strictly speaking, entropy and heat are different quantities, so you can't compare them directly. Indeed, entropy becomes more important at higher temperatures. What you've touched upon is related to chemical equilibrium. You're right; at some stage, if the entropy gain does not overcome the energy consumption of the reaction, the reaction essentially stops going forwards. It's a tug of war, and you don't need to understand this in much detail anyway.

Just trying to get my head around all of this stuff.
Thanks. Any help is appreciated.

You ask good questions, but unfortunately most of them are completely irrelevant to the course
This is what you should know from what you've asked:
1. Esters are mostly nonpolar
2. Double bonds react fairly easy; you can react ethene with more things than you can ethane, and ethane often requires UV light whereas ethene doesn't
3. Things don't dissolve to completeness
4. Double bonds can be polymerised
5. Normally, like dissolves like, although there are exceptions you must know for ionic compounds that don't, like barium sulfate, most metal hydroxides, lead (II) sulfate etc
6. What sets off a reaction?

That is it. Below is what you don't need for VCE (looks like most of what you've asked, eh? Not exhaustive)
1. Why things dissolve and why endothermic processes even occur
2. Anything related to entropy. At all.
3. Why double bonds are weaker than two single bonds
4. Schrodinger's equation; the Bohr model appears in 3/4 Physics

Good luck!

[redpanda83]

Someone help!
Do i need to memorise the names of certain co-enzymes and their properties for unit 4?
Or just general properties and how they behave would do?
I am just lazy and thought its too much hassle to memorise those
So? Do i have to? is it important?
thanks!

[Lear]

No you don’t. You just need to understand how they work in general and be able to apply this knowledge to questions where they might talk about a specific coenzyme.

[redpanda83]

No you don’t. You just need to understand how they work in general and be able to apply this knowledge to questions where they might talk about a specific coenzyme.

Thanks! thats good for me ahaha

[Quinapalus]

Someone help!
Do i need to memorise the names of certain co-enzymes and their properties for unit 4?
Or just general properties and how they behave would do?
I am just lazy and thought its too much hassle to memorise those
So? Do i have to? is it important?
thanks!

There is quite a good definition of a coenzyme and its function straight from the study design.

Quote: "the principles of the action of coenzymes (often derived from vitamins) as organic molecules that bind to the active site of an enzyme during catalysis, thereby changing the surface shape and hence the binding properties of the active site to enable function as intermediate carriers of electrons and/or groups of atoms (no specific cases required)."

From https://www.vcaa.vic.edu.au/Documents/vce/chemistry/ChemistrySD-2016.pdf

[redpanda83]

There is quite a good definition of a coenzyme and its function straight from the study design.

Quote: "the principles of the action of coenzymes (often derived from vitamins) as organic molecules that bind to the active site of an enzyme during catalysis, thereby changing the surface shape and hence the binding properties of the active site to enable function as intermediate carriers of electrons and/or groups of atoms (no specific cases required)."

From https://www.vcaa.vic.edu.au/Documents/vce/chemistry/ChemistrySD-2016.pdf

Thanks! I was just looking at the study design.
At the bottom of study design they state this " the cross-study key science skills are examinable."
My question is, Is it a good idea to go through biology intersection with chem from bio books? or is it fine if i stick to heinemann chem and then practice exams
I am not doing vce bio, so just wondering if i should go through that.

[Quinapalus]

Thanks! I was just looking at the study design.
At the bottom of study design they state this " the cross-study key science skills are examinable."
My question is, Is it a good idea to go through biology intersection with chem from bio books? or is it fine if i stick to heinemann chem and then practice exams
I am not doing vce bio, so just wondering if i should go through that.

Definitely do not need extra biology information! The chemistry course is large enough as it is ! It is biochemistry, not chembiology?! 

Prepare your notes (only once, don't rewrite, only summarise once, or waste of time that could be used for exams/questions). Do reference the study design to check all your knowledge, do the practise exams, and a good teacher does really help if you don't understand something.

I think 'key science skills' aren't knowledge, but more general ideas/principles such as variables (independent, dependent, controlled), understanding the correct scientific procedure for a scientific poster, etc.

[redpanda83]

Definitely do not need extra biology information! The chemistry course is large enough as it is ! It is biochemistry, not chembiology?! 

Prepare your notes (only once, don't rewrite, only summarise once, or waste of time that could be used for exams/questions). Do reference the study design to check all your knowledge, do the practise exams, and a good teacher does really help if you don't understand something.

I think 'key science skills' aren't knowledge, but more general ideas/principles such as variables (independent, dependent, controlled), understanding the correct scientific procedure for a scientific poster, etc.

Oh that stuff is easy then! ahaha chem course isnt that big though xD if i can finish it in one month. but i love it!!!!
I am nearly done with the course, i will start typing up the good copy of notes as soon as i am done with the course. 
For exams - which publishers are good indication for end of year exams?
Thanks

[Lear]

It’s impressive that you’ve almost finished the course already. I was pretty much in the same position as you. You’re setting yourself up for great success! Just make sure you chill out a little over the holidays and enjoy your freedom while it lasts. You have a long and exhausting year coming up!

For resources, I’d highly HIGHLY recommend the Exampro Chemistry book. Yes the questions are sometimes above the VCE level but I found it superrrr useful for solidifying concepts. They had a very similar question in the book to the Vitamin C coenzyme question in the 2018 exam. I don’t think I would’ve been able to answer it without doing the similar one before. If you’re aiming above 45 definitely check it out.

Other than that, I pretty much didn’t use any commercial question resources. I found the likes of TSSM, Insight and Lisachem to be pretty useless and repetitive. Neap May be worth checking out.

I’d recommend just focusing on VCAA exams. I literally did all of  2000-2017 exams including the NHT exams. These were 100x better than any commercial company’s resource. Some questions from 2000-2005 are very interesting.

[redpanda83]

It’s impressive that you’ve almost finished the course already. I was pretty much in the same position as you. You’re setting yourself up for great success! Just make sure you chill out a little over the holidays and enjoy your freedom while it lasts. You have a long and exhausting year coming up!

For resources, I’d highly HIGHLY recommend the Exampro Chemistry book. Yes the questions are sometimes above the VCE level but I found it superrrr useful for solidifying concepts. They had a very similar question in the book to the Vitamin C coenzyme question in the 2018 exam. I don’t think I would’ve been able to answer it without doing the similar one before. If you’re aiming above 45 definitely check it out.

Other than that, I pretty much didn’t use any commercial question resources. I found the likes of TSSM, Insight and Lisachem to be pretty useless and repetitive. Neap May be worth checking out.

I’d recommend just focusing on VCAA exams. I literally did all of  2000-2017 exams including the NHT exams. These were 100x better than any commercial company’s resource. Some questions from 2000-2005 are very interesting.

Thanks so much!~
I didnt know about exampro, i will definitely try the book out! Its a shame they dont seem to have physics one.
I have finished physics, methods, further maths coruse ~   
just around 50ish pages for chem course.
I am having lots of break and also sometimes going out with friends! I only study when i really feel like as thats only when i am most productive.
I will def check those other commercial resources out
Thanks!

[Quinapalus]

Thanks so much!~
I didnt know about exampro, i will definitely try the book out! Its a shame they dont seem to have physics one.
I have finished physics, methods, further maths coruse ~   
just around 50ish pages for chem course.
I am having lots of break and also sometimes going out with friends! I only study when i really feel like as thats only when i am most productive.
I will def check those other commercial resources out
Thanks!

Sounds great! Take confidence in the work you have done, don't burn out, and don't forget to pay at least some attention on your English subject  (Top 4, am I right!)
Remember you are free regardless of whether you have exam responsibilities or not!