VCE Chemistry Question Thread

[Bri MT]

The general approach for these types of questions is:
1. Get the number of mols from the mass using n = m/M
2. Use the balanced chemical equation to find out the number of mols of the thing you want to learn about
3. Convert from number of mols to mass using nM = m

You find M using the periodic table.


To give it a non-chemistry analogy: let's say this person bakes a dozen muffins for every 3 dozen bananas they buy. I.e. 3Banana + otherStuff -> muffin + wasteProducts
If you're told the weight of a dozen bananas and you know what the scales read when all the bananas going into the recipe were on there, you can figure out how many  dozens of muffins will be made.

A mol, like a dozen, is a number telling us how many things we have. 6.02 * 10^23 is much bigger than 12 but it's still just a number.

[Moonblossom]

Hey! The first thing to realize here is that 3 M is the concentration of HCl, not potassium dichromate. So it's not as simple as just chucking your values into n=cV. I've attached my working on paper, but I'll walk through the steps (hopefully you don't mind a huge spoiler).

Spoiler

To begin, let's work backwards. The only useful value we really have for this part of the question is the concentration of hydrogen chromate ions, which we can convert into moles since there's a volume of 500mL. However, how would we relate that to dichromate ions? Through a chemical equation, of course! Hmmmmm...but there isn't one. Thankfully, the question somewhat hints that we can create our OWN chemical equation, through "hydrolisation". Not too sure what this type of chemical reaction is, but surely it has to do with water?? Thus, I just tried to balance it kind of like a redox reaction (treating potassium ions as spectator ions), and that's how I got that second chemical equation. We already know that we can calculate the numbers of moles of hydrogen chromate ions, so we can just do some simple stoichiometry using that new equation to find the number of moles of dichromate ions. We can then infer that the number of moles of potassium dichromate is the same as that of dichromate ions. So, now that we have n(potassium dichromate), we can just use a little m=nM magic to get the mass of potassium dichromate, 0.605 g. You already know how to calculate the molar mass, so you're good there 

[a weaponized ikea chair]

Is it true that protons fuse together in nuclear fusion such as in the sun to form photons?

Also, how does plasma form?

[Erutepa]

Is it true that protons fuse together in nuclear fusion such as in the sun to form photons?

Also, how does plasma form?

Hey
Fusion is the process by which two nuclei combine (or fuse) to form one or more different nuclei. This can sometimes involve the fusion of two protons - or two hydrogen nuclei - to form a deuterium nuclei, however fusion also occurs with nuclei containing neutrons. This process of fusion does indeed occur in stars and can produce photons, although other forms on energy are also released from the process.

Plasma is essentially a super-heated gass with high enough kinetic energy such that some electrons become unbound to their respective atoms, resulting in a ionized gas-like substance. There are multiple ways to produce plasma and If you are interested I encourage you to read about them, but essentially the production of plasma involves increases in the kinetic energy of a gas to the point where it ionizes.

As a note for anyone reading this - this most certainly is not relevant to VCE chemistry. Infact these questions are more relevant to Physics than Chemisty, so in future I would encourage you to ask these questions there 

[Chocolatepistachio]

For this question
What event caused the changes shown on the graph at:
i) The four minute mark
- Is it pressure has been increased

ii) The eight minute mark?
-  Would it be the concentration of CO increased

At the 12 minute mark is it the temperature decreased

[Erutepa]

For this question
What event caused the changes shown on the graph at:
i) The four minute mark
- Is it pressure has been increased

ii) The eight minute mark?
-  Would it be the concentration of CO increased

At the 12 minute mark is it the temperature decreased

at the 4 minute mark you will notice that all gasses increase in concentration. This could happen in two ways: either amounts of each gass were added to the system, or the volume of the vessel containing the gasses was decreases ( c = m/L thus by decreasing L you increase c). Looking at the graph you will notice that all concentrations increase by 50% - this is consistent with a reduction of the container volume by a third (the new container is two thirds the volume of the initial container). This indicates that we probably are observing this reduction in vessel volume.
The reason why we wouldn't say the pressure has increased is that is not whats causing the change, instead the pressure would increase as a result of the container being reduced in volume.

At the 8 minute mark the concentration of CO is indeed increased - but the question needs to know what changed caused this. The sudden increase in CO concentration we observe in the graph can either be from a decrease in volume of container or the addition of  CO to the container. Since only CO increases immediately, it cannot be a decrease in container volume and thus CO must have been added into the container. We can further confirm this by applying le chatelier's principle and seeing if the other gasses change consistent with this proposed addition of in CO. If we were to add CO to the system - increase in the concentration of CO, the the system would partially oppose this change by favouring the reaction that consumes CO - in this case the forward reaction. Consequently the concentration of Cl2 would decrease and the concentration of COCl2 would increase. Both of these changes are observed and further support the idea that CO was added to the system.

You are indeed correct about the change in temperature at the 12 minute mark.

Just for those first two questions you seemed to identify roughly what was happening, but you weren't specific about what was causing the change. 

[ArtyDreams]

Heyyyy!! Got a quick question regarding HNMR splitting, because I tend to see things everywhere and get confused.

1. CH₃CH₂CH₂CH₂OH
Now I need to find the splitting pattern of the FIRST Hydrogen in that list. Is it a sextet because it has 5 hydrogens neighbouring it? Will VCAA ask a question like this?

2. CH₃CH₂OCH₂CH₃ (2 H environments)
Now I need to find the splitting pattern of the Hydrogens with the CH2s (the bolded ones)
Is it a ONE SEPTET because theres 6 hydrogens in the neighbouring environment?
Or is it ONE QUARTET because ONE H is adjacent to 3 hydrogens??

I just get confused because I see different things in some exams than VCAA has stipulated, and wondering how simple VCAA wants it to be. I'm confused.

Thank you!!

[keltingmeith]

Heyyyy!! Got a quick question regarding HNMR splitting, because I tend to see things everywhere and get confused.

1. CH₃CH₂CH₂CH₂OH
Now I need to find the splitting pattern of the FIRST Hydrogen in that list. Is it a sextet because it has 5 hydrogens neighbouring it? Will VCAA ask a question like this?

2. CH₃CH₂OCH₂CH₃ (2 H environments)
Now I need to find the splitting pattern of the Hydrogens with the CH2s (the bolded ones)
Is it a ONE SEPTET because theres 6 hydrogens in the neighbouring environment?
Or is it ONE QUARTET because ONE H is adjacent to 3 hydrogens??

I just get confused because I see different things in some exams than VCAA has stipulated, and wondering how simple VCAA wants it to be. I'm confused.

Thank you!!

1. The first hydrogen is not a sextet, nor are their 5 hydrogens neighbouring it. This is true for the second hydrogen - note that the first hydrogen in that formula is bolded here:

CH₃CH₂CH₂CH₂OH

This hydrogen has four nearby hydrogens it will "see". The first two, however, are attached to the same carbon - they're equivalent, and do not affect splitting. The other two hydrogens are attached to the neighbouring carbon, and so they do affect splitting. Using the n+1 rule, you expect to see a triplet. For this next example, I've bolded the second hydrogen, because that's the one we're talking about, and underlined the hydrogens in neighbouring environments that it will "see" and that will affect splitting:

CH₃CH₂CH₂CH₂OH

In this case, you would expect to see a sextet. Can you see why?

2. Diethyl ether! Smelly compound, kinda blech. You can see its NMR here. As you can see, the NMR only has two peaks (hopefully not surprising) - one is a triplet (belonging to the two CH₃s), and the other a quartet. Where there's a plane of symmetry, you can assume that what you see is only reflective of one side of the molecule - which is one reason why mass spec and IR data is so important in interpreting structure.

As for will VCAA ask these kinds of questions (for you to predict the splitting) - I'm not sure. The study design says you do have to be aware of multiplicity and n+1 rule, so those questions aren't ruled out. However, I did a search in the last 3 exams (the only ones relevant to this study design), and none of them mentioned splitting. I also believe that those questions would be too tricky - splitting is slightly more complicated than you're lead to believe in VCE, and I think VCAA is aware of people doing extension getting confused, so it's likely just easier for them to request you be able to interpret NMR in terms of splitting, but not ask you any questions related to it, just so they can ask you general structural questions.

EDIT: Just did a more thorough search - there have been questions in previous years, but the only one to ask you to predict the pattern was in 2015 - AND it was multiple choice, making the answer a little more obvious. In 2014, you were asked to interpret a splitting pattern, in 2012 you were asked to answer a question based on a splitting pattern, in 2010 you were asked to ask a question based on a splitting pattern, then asked to draw a group based on a splitting pattern, and then nothing else up to 2008. I stopped checking there, because I figure anything pre-2008 isn't entirely relevant (although the 2008 exam 2 assessor's report was amended in 2016, so clearly VCAA are still answering questions based on them!)

But yeah, this just adds weight to my theory - asking you to determine a splitting pattern from scratch is a little too difficult. Not because what you were taught is hard, but because the actual truth of the matter is, and they don't want to disadvantage people who know more about splitting than they need to. Hence the only time it was asked to be predicted, it was a 2015 question, it was multiple choice, and likely why we've seen NO splitting questions since 2015.

[ArtyDreams]

1. The first hydrogen is not a sextet, nor are their 5 hydrogens neighbouring it. This is true for the second hydrogen - note that the first hydrogen in that formula is bolded here:

CH3CH2CH2CH2OH

This hydrogen has four nearby hydrogens it will "see". The first two, however, are attached to the same carbon - they're equivalent, and do not affect splitting. The other two hydrogens are attached to the neighbouring carbon, and so they do affect splitting. Using the n+1 rule, you expect to see a triplet. For this next example, I've bolded the second hydrogen, because that's the one we're talking about, and underlined the hydrogens in neighbouring environments that it will "see" and that will affect splitting:

CH3CH2CH2CH2OH

In this case, you would expect to see a sextet. Can you see why?

Oops. Silly me, I did mean to type the second hydrogen. I understand now. Thank you for answering it!!

2. Diethyl ether! Smelly compound, kinda blech. You can see its NMR here. As you can see, the NMR only has two peaks (hopefully not surprising) - one is a triplet (belonging to the two CH3s), and the other a quartet. Where there's a plane of symmetry, you can assume that what you see is only reflective of one side of the molecule - which is one reason why mass spec and IR data is so important in interpreting structure.

Right, awesome! I keep getting confused with all this even though I apply the n+1 rule. I find the company exams make it harder than VCAA so I always see contradicting things.

As for will VCAA ask these kinds of questions (for you to predict the splitting) - I'm not sure. The study design says you do have to be aware of multiplicity and n+1 rule, so those questions aren't ruled out. However, I did a search in the last 3 exams (the only ones relevant to this study design), and none of them mentioned splitting. I also believe that those questions would be too tricky - splitting is slightly more complicated than you're lead to believe in VCE, and I think VCAA is aware of people doing extension getting confused, so it's likely just easier for them to request you be able to interpret NMR in terms of splitting, but not ask you any questions related to it, just so they can ask you general structural questions.

Okay this makes me relieved. Thank you so much keltingmeith, the help is much appreciated!

[Chocolatepistachio]

Hello
I need help with this question would this be right

Thanks

[p0kem0n21]

Hello
I need help with this question would this be right

Thanks

Hey! I'm going to start with part b because it's much shorter to answer. You're correct in stating that catalysts do not affect the position of equilibrium; however, you do not explain WHY this is the case. I know you wrote "Adding a catalyst...of the equilibrium.", but you're basically writing "No, adding a catalyst will not change the position of the equilibrium because adding a catalyst does not change the position...". Hopefully you see that it's not a real reason. Anyways, the actual reason is because catalysts increase the forward and reverse reactions to the same extent, meaning that there is no net increase in either reactants or products.

As for your graph, I can spot a few errors. Beginning with the most important one, you did not draw a curve to represent the ammonia (NH₃) concentration. Indeed, it can be a bit difficult (and often impossible) to determine the unknown concentration of another chemical in the substance, but there is a greater emphasis on how the reaction system changes. The only thing which you could definitively draw at first is that at t=0, the concentration of ammonia is 0.
Another issue is that you haven't labelled the curves with whichever substance's concentration you're modelling (i.e. just from the graph, we don't know which curve is modelling H₂ concentration or N₂ concentration). I'm not too sure whether it's a requirement to draw, but it makes the graphs much easier to follow.

Now, onto the science of the graph    Your drawing from time=0 to time=t₁ is correct; the concentrations of the reactants would decrease while the concentration of the product formed (ammonia) would increase since there is no product initially. However, your graph is incorrect from t₁ to c₁. In this interval, the reaction would be in equilibrium, so there would be no net changes in concentration of any of the substances (this would be represented by a horizontal line). The "favoured" part just means that reactants will have a higher concentration than product(s) in this interval.

From c1 to t2 (cbs with subscripts anymore  ), you drew the reactants as increasing in concentration. However, since the temperature is decreased, the reaction system will attempt to oppose this change by raising the reaction system's temperature by shifting towards the side which releases energy (i.e. favouring the exothermic reaction). Since the forward reaction is exothermic, it would be expected that more ammonia would be produced, whilst reactants would be consumed (and thus their concentration would actually decrease instead).

t2 to c2 is just equilibrium. Horizontal lines. You got this.

The pressure increase at c2 can be viewed as decreasing the volume whilst retaining all molecules, so an instantaneous increase in the concentrations of ALL substances would be observed (this would be represented by a finite vertical line at c2). However, after this instantaneous increase, we would view the concentration of N₂ to increase even more, whilst the concentrations of the reactants would decrease slightly (but these increases/decreases are not as great in magnitude as the instantaneous increase at c2, since the changes are only partially opposed). This increase in products and decrease in reactants can be explained by Le Chatelier's principle: the increase in pressure will be partially opposed by the reaction system by decreasing pressure. This is achieved by reducing the number of molecules in the system, which can be achieved by shifting towards the side with fewer molecules, which is the products side in this case.

Finally, after all that mayhem, there's equilibrium past t3. Horizontal lines

I've attached my own graph here. Note that the relativity of the concentrations may not be super correct; also, c2 is admittedly a bit messy, but hopefully you see the 'vertical' increase in concentration for all substances.

[zhouzhennan]

Does hydrogen gas count as a greenhouse gas? Would the hydrogen gas produced from water electrolysis count as not contributing to greenhouse emission? Thanks!!

[needvcehelppls]

What is the point of having a standard solution? I understand all the calculations involved with volumetric analysis etc but standard solutions & primary standards never seem to come up.

Thanks in advance!

[keltingmeith]

Does hydrogen gas count as a greenhouse gas? Would the hydrogen gas produced from water electrolysis count as not contributing to greenhouse emission? Thanks!!

Hydrogen gas is not a greenhouse gas.

What is the point of having a standard solution? I understand all the calculations involved with volumetric analysis etc but standard solutions & primary standards never seem to come up.

Thanks in advance!

Standard solutions and primary standards actually come up all the time, you just don't realise they've come up because the name "primary standard" or "standard solution" is rarely used. Eg, if the question was, "1 mL of 1.00 M hydrochloric acid is diluted to 10 mL, and then titrated against 20 mL of sodium hydroxide", then the hydrochloric acid used is from a standard solution. Or maybe the question was, "2.00g of sodium hydroxide was weighed and dissolved in 50.00 mL of water. 10 mL of this solution was then titrated against 24.35 mL of hydrochloric acid", then the sodium hydroxide used was a primary standard.

Primary standards are things of known mass that you can use to compare against other chemicals, and a standard solution is the same thing but it's a solution of known concentration, and you MUST use one or the other in a volumetric analysis, or you won't get an accurate concentration of what you're titrating against in your experiment. It is highly unlikely you'll be asked to define what these are, but you absolutely are dealing with them in every exam there's a volumetric question.

[ArtyDreams]

Are we expected to have units on a Kc calculation - such as the M-1 etc ? Do we get penalised if we don't - my teacher has been and I'm just curious if the same thing applies with VCAA.