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March 28, 2024, 07:48:43 pm

Author Topic: VCE Chemistry Question Thread  (Read 2312969 times)  Share 

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keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8985 on: January 12, 2021, 11:58:23 am »
+4
ok thank youu

For this question how do you know its SN1 or SN2 for b

Woah - who's asking you this? This here be some uni-level chemistry. And not just any level, this isn't taught until 2nd year organic chemistry at Monash. Some places likely teach it in first year, but definitely not VCE level (unless I'm reading the study design wrong). I wouldn't worry about being able to answer this (goes double for anybody reading this and scared by the terminology because they'd never seen it before)

The quick way to identify which is which is to know that SN1 reactions can only occur if there is a stable intermediate for the reaction to progress through. If the intermediate isn't going to be stable, then the reaction will be SN2. Remember, the first step in SN1 is the leaving group leaving - for both molecules, this would be the bromide. If the bromide leaves the first molecule, you're left with a primary carbocation - not particularly stable, and so an SN1 mechanism is very unlikely, and it's more likely to undergo SN2 - like you've suggested. For the second reaction, however, the leaving of the bromide creates a tertiary carbocation - much more stable, so an SN1 mechanism is much more likely.

Another thing you can look at is sterics - or how bulky the neighbouring groups are. This rule is a little more complicated than the carbocation rule (note: looking at the stability of the intermediate isn't always going to involve carbocations, it's just that in this instance, that is what we've ended up looking at) I've used above. If you look at the first molecule, look at the carbon that the SN2 reaction would happen to. It's the one connected to the bromide. For an SN2 reaction to happen, the nucleophile needs to be able to attack that carbon while the leaving group (the bromide) is still attached. Because the molecule is nice and linear, there should be enough physical space available for the nucleophile to attack (try imagining the molecules in 3D and how you'd expect them to twist around and behave). However, for the second molecule, do you see that "V"-shaped group that's attached to the same carbon as the leaving group? That thing will likely get in the way of an SN2 reaction - whereas an SN1 reaction will have more room, because when the nucleophile comes in, the leaving group is already gone. As a result, we would say that the SN2 pathway is sterically hindered, and so you'd expect to see an SN1 mechanism as a result.


An important thing to remember - these reactions aren't necessarily "this mechanism, not that mechanism" - both can and will happen. If you've drawn a mechanism that makes chemical sense, then the reaction can proceed that way. The question is which mechanism is more likely. For example, in the case of the first molecule, the SN2 mechanism is much more likely, but the SN1 mechanism can still occur. It's highly likely if you were to watch the reaction play out, 99% of the molecules will undergo an SN2 mechanism, but 1% might still undergo an SN1 mechanism. When it comes to mechanisms, there are no hard-and-fast rules, because there's not even necessarily a correct answer at the end - just what we think is most likely. As a result, you might make a mechanism that is consistent with all the rules you know, but still get the question "wrong" because one of the steps is more likely to go a different way than you thought it would. Try not to be discouraged - it's just  a matter of practice makes perfect, and you'll eventually learn to realise all the different pathways a molecule can take.

redset8

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Re: VCE Chemistry Question Thread
« Reply #8986 on: January 13, 2021, 04:26:00 pm »
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Hey, just a quick question. For such a question as "write a thermochemical equation for the complete combustion of methane", is the water generally written as \(\ce{H2O(l)}\) or \(\ce{H2O(g)}\). The answers have it as liquid. My logic is it is the lower energy one???

SS1314

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Re: VCE Chemistry Question Thread
« Reply #8987 on: January 13, 2021, 04:50:37 pm »
+1
Hey, just a quick question. For such a question as "write a thermochemical equation for the complete combustion of methane", is the water generally written as \(\ce{H2O(l)}\) or \(\ce{H2O(g)}\). The answers have it as liquid. My logic is it is the lower energy one???

From my understanding, water is always a liquid as it is assumed that the experiment is performed in SLC (standard lab conditions).
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redset8

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Re: VCE Chemistry Question Thread
« Reply #8988 on: January 13, 2021, 05:39:33 pm »
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From my understanding, water is always a liquid as it is assumed that the experiment is performed in SLC (standard lab conditions).

Ah ok thanks heaps.

LE-0130

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Re: VCE Chemistry Question Thread
« Reply #8989 on: January 21, 2021, 11:47:21 am »
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Hi!
Current stuck on this question, seems rlly basic, but for some reason I can't do it, bcuz I'm not quite sure what the chemical equation would look like. :P Anyways, any help or advice would be greatly appreciated! Thanks :)

Question:
"How many faradays of charge are required to produce:
(d) 1 mole of hydrogen molecules (H2) from sodium chloride solution?"

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8990 on: January 21, 2021, 12:18:14 pm »
+7
Hi!
Current stuck on this question, seems rlly basic, but for some reason I can't do it, bcuz I'm not quite sure what the chemical equation would look like. :P Anyways, any help or advice would be greatly appreciated! Thanks :)

Question:
"How many faradays of charge are required to produce:
(d) 1 mole of hydrogen molecules (H2) from sodium chloride solution?"
For this questions we are considering an electrolytic cell with solution of sodium chloride (meaning water will be present).
Knowing that the chemical species present consists of Na+, Cl- and H20, we can now consider the electrochemical series. At the anode, an oxidation reaction will occur - the two potential reactions listed in the series is the oxidation of chloride ions to chlorine gas, and the oxidation of water to hydrogen peroxide and H+. Remember that the reaction that requires the least electrode potential (volts) to proceed will occur first, since the oxidation of these chloride ions has a lower electrode potential requirement (needs input of 1.36v at slc) this is the reaction that will be occuring at the anode.
For the cathode, the two possible reduction reactions we see in the electrochemical series is the recuction of sodium ions to sodium metal, and the reduction of water to hydroxide ions and hydrogen gas. Applying the same rule that the reaction that requires the least electrode potential (volts) to proceed will occur first, we can see that the reduction of water requires the least electrode potential and thus will occur at the cathode.
to summarise, here we will have the oxidation of chloride ions at the anode, and the reduction of water at the cathode.
Hopefully this will help you get through the rest of the question, but feel free to ask for help or to point out anything i poorly explained.
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LE-0130

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Re: VCE Chemistry Question Thread
« Reply #8991 on: January 21, 2021, 12:33:28 pm »
+1
For this questions we are considering an electrolytic cell with solution of sodium chloride (meaning water will be present).
Knowing that the chemical species present consists of Na+, Cl- and H20, we can now consider the electrochemical series. At the anode, an oxidation reaction will occur - the two potential reactions listed in the series is the oxidation of chloride ions to chlorine gas, and the oxidation of water to hydrogen peroxide and H+. Remember that the reaction that requires the least electrode potential (volts) to proceed will occur first, since the oxidation of these chloride ions has a lower electrode potential requirement (needs input of 1.36v at slc) this is the reaction that will be occuring at the anode.
For the cathode, the two possible reduction reactions we see in the electrochemical series is the recuction of sodium ions to sodium metal, and the reduction of water to hydroxide ions and hydrogen gas. Applying the same rule that the reaction that requires the least electrode potential (volts) to proceed will occur first, we can see that the reduction of water requires the least electrode potential and thus will occur at the cathode.
to summarise, here we will have the oxidation of chloride ions at the anode, and the reduction of water at the cathode.
Hopefully this will help you get through the rest of the question, but feel free to ask for help or to point out anything i poorly explained.
Thank you! It makes so much more sense now!

Ruchir

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Re: VCE Chemistry Question Thread
« Reply #8992 on: January 22, 2021, 10:47:17 am »
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Hello I am in year 11. In the Heinmann textbook do we cover all chapters this year?
Or do we miss a few of them?

Sine

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Re: VCE Chemistry Question Thread
« Reply #8993 on: January 22, 2021, 01:01:58 pm »
+4
Hello I am in year 11. In the Heinmann textbook do we cover all chapters this year?
Or do we miss a few of them?
Really depends on the school. Some teachers solely rely on textbooks whilst others just use it as one of many resources.

Personally, we covered a lot of the content in the textbook but definitely not all. Textbooks are typically bloated with a lot of irrelevant content.

Ruchir

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Re: VCE Chemistry Question Thread
« Reply #8994 on: January 22, 2021, 02:22:16 pm »
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Really depends on the school. Some teachers solely rely on textbooks whilst others just use it as one of many resources.

Personally, we covered a lot of the content in the textbook but definitely not all. Textbooks are typically bloated with a lot of irrelevant content.

Thanks fo the answer :D
 if the textbook has 20 chapters how many (approx)do you think will be covered in school if my school only uses the textbook?
« Last Edit: January 22, 2021, 02:24:48 pm by Ruchir »

Sine

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Re: VCE Chemistry Question Thread
« Reply #8995 on: January 22, 2021, 04:17:50 pm »
+3
Thanks fo the answer :D
 if the textbook has 20 chapters how many (approx)do you think will be covered in school if my school only uses the textbook?
I'm not too sure about the current edition of the Heinemann textbook so wouldn't really be able to give you an accurate answer.

If your school has given you an outline of the topics you are covering this year you might be able to see where everything fits and what chapters will be relevant to you.

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #8996 on: January 22, 2021, 09:16:19 pm »
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Do we need to know specific numbers for the energy content of fuels? Also, do we need to worry about energy efficiency? It's not on the study design but I see some resources mention it.

 Edit: I was also wondering what the difference is between petrol and liquified petroleum gas? The Heinemann textbook treats them as if they are two separate things but which are we supposed to know about?
« Last Edit: January 22, 2021, 09:28:38 pm by miyukiaura »
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Sine

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Re: VCE Chemistry Question Thread
« Reply #8997 on: January 22, 2021, 09:31:21 pm »
+2
Do we need to know specific numbers for the energy content of fuels? Also, do we need to worry about energy efficiency? It's not on the study design but I see some resources mention it.
Have you had a look at the databook? They should be in there.

As for energy efficiency, I think you should be covering it.

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #8998 on: January 22, 2021, 10:48:50 pm »
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Have you had a look at the databook? They should be in there.

As for energy efficiency, I think you should be covering it.
Yeah, I've seen the databook but it doesn't have the figures for coal or crude oil. Thanks anyway :)
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Re: VCE Chemistry Question Thread
« Reply #8999 on: January 22, 2021, 11:22:17 pm »
+3
Do we need to know specific numbers for the energy content of fuels? Also, do we need to worry about energy efficiency? It's not on the study design but I see some resources mention it.

 Edit: I was also wondering what the difference is between petrol and liquified petroleum gas? The Heinemann textbook treats them as if they are two separate things but which are we supposed to know about?

I don't think you need to know specific numbers per se for coal or crude oil. You do need to know roughly the energy content of those fuels however in order to compare them; often you'll see questions asking you to compare the energy content of fossil fuels vs biofuels, coal vs petrol or petrodiesel vs biodiesel.
Knowing the energy efficiency of different methods of energy production is also something that often gets questioned, for example the energy efficiency of a coal or gas-fired powerplant vs a fuel cell, so having a rough grasp of those percentages is often helpful.
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