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October 30, 2020, 06:37:30 pm

Author Topic: QCE Chemistry Questions Thread  (Read 5796 times)

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snr.mmorris4.19

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Re: QCE Chemistry Questions Thread
« Reply #60 on: September 12, 2020, 07:38:21 pm »
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I think you're numbering or drawing one of these two wrong. See the attached - structures + names for each (ignore the (E) part if you don't know what that is - it's just another method of identifying cis/trans stereoisomers, and not something you need to know. I generated these using a computer program and wasn't sure how to turn the E off, sorry!) along with the chemical formula. As you can see, the chemical formula for each is exactly the same.
thankyou!

snr.mmorris4.19

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Re: QCE Chemistry Questions Thread
« Reply #61 on: September 13, 2020, 08:17:21 am »
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What are they referring to when the question asks, calculate the concentration in the cleaning solution of a titration? What is the cleaning solution?

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #62 on: September 13, 2020, 10:12:06 am »
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What are they referring to when the question asks, calculate the concentration in the cleaning solution of a titration? What is the cleaning solution?

Could you please share the whole question?

I'm not sure how to answer this without more information. 

cunglee0805

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Re: QCE Chemistry Questions Thread
« Reply #63 on: September 19, 2020, 05:36:38 pm »
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hi!! I had a question about standard electrode potentials. In the data booklet the all the reactions are reduction reactions, so If you reverse an equation to write the oxidation reaction do you have to change the sign (+/-) of the E value ?

fun_jirachi

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Re: QCE Chemistry Questions Thread
« Reply #64 on: September 19, 2020, 06:45:54 pm »
+3
hi!! I had a question about standard electrode potentials. In the data booklet the all the reactions are reduction reactions, so If you reverse an equation to write the oxidation reaction do you have to change the sign (+/-) of the E value ?

Yes, you have to change the sign :)
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cunglee0805

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Re: QCE Chemistry Questions Thread
« Reply #65 on: September 20, 2020, 12:11:01 pm »
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Yes, you have to change the sign :)

thanks so much! I had another question when asked to explain what is occurring in a system  at a molecular and atomic level as it approaches equilibrium/when equilibrium is established what points do we need to cover?

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #66 on: September 20, 2020, 06:07:27 pm »
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thanks so much! I had another question when asked to explain what is occurring in a system  at a molecular and atomic level as it approaches equilibrium/when equilibrium is established what points do we need to cover?

Good question! What do you think is all the points?
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cunglee0805

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Re: QCE Chemistry Questions Thread
« Reply #67 on: September 20, 2020, 09:13:36 pm »
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Good question! What do you think is all the points?

hmm I'm not quite sure but I know that with equilibriums bonds between reactants and products are constantly being broken and reformed. also that the rates of the forward and reverse reaction are equal but I really don't know how to distinguish between molecular level and atomic level

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #68 on: September 20, 2020, 10:33:20 pm »
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hmm I'm not quite sure but I know that with equilibriums bonds between reactants and products are constantly being broken and reformed. also that the rates of the forward and reverse reaction are equal but I really don't know how to distinguish between molecular level and atomic level

Right, so your issue isn't talking about things on a molecular level, but rather to differentiate between an atomic and a molecular level. I'm gonna be honest, at a guess, I feel like this is so complex that you probably won't be asked about this - but with no previous exams to inform us of whether the QCAA will discuss this or not, we don't have the luxury of saying, "this is beyond the scope of QCE - don't worry".

So, let's talk worst-case scenario. You seem quite comfortable with the molecular description of things - the rates of the forward reaction and reverse reaction are equal. I'm gonna use a case example that you should be familiar with - the self-ionization of water:



Alright, so from this, we know that if we only have one free proton and one free hydroxide ion, we have 1,000,000,000,000 (1 quadrillion, holy fuck) molecules of water. But this is only a snapshot, and what's actually happening is that the rate at which one of those quadrillion water molecules breaks apart, the free proton and hydroxide ion are also recombining into water. This is what you described on a molecular level, but for some not-yet-decided reaction - I just picked a specific example.

What about on the atomic level? Well, if a molecular description talks about the things that are smaller than a molecule (bonds between atoms in the molecule), then an atomic level descriptions talks about the things that are smaller than atoms - protons, neutrons, and electrons. It turns out that protons and neutrons don't actually do much in chemical reactions, only the electrons do, so that's what we should talk about.

Well, on the atomic level, you have an oxygen bound to two hydrogens. Over time, the bond between one of the hydrogens is changing - the electrons that are being shared will move towards the oxygen, so that the oxygen now has 7 electrons solely in its valence shell, instead of 6. At the same time, an oxygen attached to a hydroxide ion has 7 electrons solely in its valence shell - to which a free proton will approach it, and slowly one of those electrons will enter the first energy level of the free proton, and so now the oxygen shares two electrons, not just 1, and only has 6 electrons solely in its valence shell.

Make sense? An atomic description would discuss the electrons within atoms, just as a molecular description would discuss the bonds within molecules. Tbh, in an exam, I would hope that either would be expected, and that either would be accepted, as long as you get the key point across that whatever description you use, the key point that the rate at which these events happen is happening is the same, is used. But, without prior evidence, it's a bit hard to make this statement if you want to be 100% safe - though it is worth noting that usually these assessment authorities WANT you to do well, and so normally err on the side of the reason.


EDIT: also highly welcome people's collective input on this last paragraph - maybe they've heard rumours, or have their own thoughts on whether this level of detail is too much, or whether it's reasonable to expect the QCAA to be this nitpicky or not. My thought is it's not and you don't need to be ready to answer on this level of nuance, but discussion is great for this kind of thing!
« Last Edit: September 20, 2020, 10:40:13 pm by keltingmeith »
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Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #69 on: September 21, 2020, 12:59:09 pm »
+1
My input into the discussion:

The QCAA syllabus dot points in question here are:
appreciate that observable changes in chemical reactions and physical
changes can be described and explained at an atomic and molecular level
symbolise equilibrium equations by using ⇌ in balanced chemical equations
understand that, over time, physical changes and reversible chemical
reactions reach a state of dynamic equilibrium in a closed system, with the
relative concentrations of products and reactants defining the position of equilibrium

I think it's important to note here that these are two separate dot points rather than a statement about applying atomic understanding to equilibrium. Additionally, the use of "appreciate" as the verb suggests to me that a high level of detail would not be expected.

So yeah, I think keltingmeith's description is great for your understanding but I highly doubt that it would be needed.

If you really want to chase this up, I recommend looking at WACE given that they have these dot points:
 observable changes in chemical reactions and physical changes can be described and explained at an
atomic and molecular level
 over time, in a closed system, reversible physical and chemical changes may reach a state of dynamic
equilibrium, with the relative concentrations of products and reactants defining the position of
equilibrium

laurahhh

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Re: QCE Chemistry Questions Thread
« Reply #70 on: September 22, 2020, 02:57:45 pm »
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Hello! I have been stuck on this question for about an half hour and I literally don't know how the textbook got the answer.

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #71 on: September 22, 2020, 03:00:55 pm »
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Hello! I have been stuck on this question for about an half hour and I literally don't know how the textbook got the answer.

Okay, so what have you tried? I could just give you the answer, but it'd be a great bonus if I could also tell you where you went wrong in your approach ;)
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laurahhh

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Re: QCE Chemistry Questions Thread
« Reply #72 on: September 22, 2020, 03:16:48 pm »
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Okay, so what have you tried? I could just give you the answer, but it'd be a great bonus if I could also tell you where you went wrong in your approach ;)

Ah well, I tried to work backwards from the answer and I'm no where close. So it's easy to say that I haven't done anything right.  :-\

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #73 on: September 22, 2020, 03:47:02 pm »
+2
Ah well, I tried to work backwards from the answer and I'm no where close. So it's easy to say that I haven't done anything right.  :-\

Yeah, so like, answers a crutch - you don't really learn if relying on answers or worked solutions. Think of doing your QCE like you were running a race. Let's say you go out all the time and practice. Every day. But all you ever do is walk. Then, it comes time to do that race - your body is used to being outside, it's used to moving a lot, but the moment you start to run, it has no idea what to do, because it's too used to walking. Sure, you'll probably do better than if you didn't do all that walking, but will you do as well as you possible can? Always try just doing a question first - even if you don't know where to start, just try something. You'll learn a lot more that way ;)

As for this one, we have a lot of numbers that are potentially just there to throw us off. So let's break everything up into steps:

1. Sodium carbonate solution was made. [Na2CO3] is now known. Numbers were 5.267g of sodium carbonate in 250mL of water.

2. Sodium carbonate was titrated with unknown HCl. [HCl] is now known. Numbers were 10 mL of sodium carbonate solution with 21.3 mL of hydrochloric acid solution.

3. HCl was titrated with unknown Ba(OH)2. [Ba(OH)2] is now known. Numbers were 25 mL of barium hydroxide and 27.1 mL of hydrochloric acid.

So, let's do a together. So, if we look at our steps, we need to look at step 2. The easiest way to do this is to start with an equation we KNOW we have to use, then figure out what we do and don't have in that equation, and work backwards. So, to figure out the concentration of HCl, we use the equation:



We know V, but not n. Okay, so we need to find n - we can use the fact that 10mL of the carbonate solution was used to react with the HCl. Okay, since they react by the chemical equation:



We can use the mathematical equation:



Okay, so we need the amount of mol of sodium carbonate. Well, to find that, we HAVE to use the information in step 2. Remember - if you want the mol from a step, you ALWAYS have to use the mol from the same step. So that would mean the mol from the 10mL, so we use the equation:



But we don't know the concentration for sodium carbonate!! So, the question is, can we use the same concentration from step 1 in step 2? The answer is yes - you can never use information from a later step, but you can use information from a previous step if it does not change. The concentration of sodium carbonate does not change from step 1 to step 2, so we can use it. This means we use the equation:



I'm just gonna call those n1 and V1 to make my life easier. Okay, so we know the volume is 250mL - but we don't know n! So, we have to find n. Ah, but we do know that sodium carbonate was weighed, so we can use the equation:



And finally, we have all of those numbers! So, we calculate n1:



And we can finally go through all of our previous equations:









which gives us the final molarity of HCl - 0.0467.

So I'm sure this felt like a lot of maths to do all at once, but remember - all we did is figure out what equation we had to use, and then just went backwards one at a time - which should be a little less daunting if you go through this process yourself. Why don't you now try b and c, and see if you can do those yourself?
« Last Edit: September 22, 2020, 03:48:53 pm by keltingmeith »
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laurahhh

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Re: QCE Chemistry Questions Thread
« Reply #74 on: September 22, 2020, 08:16:03 pm »
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Yeah, so like, answers a crutch - you don't really learn if relying on answers or worked solutions. Think of doing your QCE like you were running a race. Let's say you go out all the time and practice. Every day. But all you ever do is walk. Then, it comes time to do that race - your body is used to being outside, it's used to moving a lot, but the moment you start to run, it has no idea what to do, because it's too used to walking. Sure, you'll probably do better than if you didn't do all that walking, but will you do as well as you possible can? Always try just doing a question first - even if you don't know where to start, just try something. You'll learn a lot more that way ;)



Thank you! :)
I got halfway there so I guess I'm not too bad. But the answer from the textbook said the molarity of HCl was 0.5M. Which is why I'm very confused.