WeiWeiMan
Well-Known Member
Pre sure it ain’t mentioned
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prelim content in HSC chem exams (2 Viewers)
- Thread starter momothemonkey
- Start date
You don't need partial pressures to answer that question in the sense of a detailed explanation, calculation or formula written version of it that's taught in 1st year university chemistry where you learn about Ptotal = P1 + P2 + P3 + .... + Pn. It's more you need to memorise a certain phrase or concept around the things when added to an equilibrium don't cause it to shift which are:
1. Catalyst - it just speeds up the how fast the equilibrium is reached since it increases rate of reaction but it doesn't cause a shift
2. Solids - when added to a saturated solution have no effect cause you already reached the max solubility so you can't dissolve any more to change the concentration of things involved in a reaction
3. Inert gases - like helium and argon. It increases overall pressure of the system but it doesn't change the partial pressures (or simply put the amount of the reactants/products)
That last 3rd thing is basically the key point they want you to write about.
An answer to this question would be along these lines:
- Mention that adding an inert gas increases overall pressure of the system but it doesn't change the amounts of the NO2 and N2O4 present and the volume of the vessel doesn't change either since it says in the question its a fixed volume vessel(you can use the words partial pressures here if you want to but it wouldn't lead to you losing marks if you didn't because saying it doesn't change amounts of NO2 and N2O4 is essentially the same thing)
- Link this then to this means there is no shift in the position of the equilibrium based on Le Chatelier's principle since there is no disturbance and therefore no resulting change in the NO2 and N2O4 concentrations
- Now link it to temperature in that because the equilibrium was not disturbed the rate of the exothermic forward reaction = rate of the endothermic reverse reaction. That means the system is in dynamic equilibrium where changes are happening on a microscopic level but there's no total change to things like temperature.
- You can mention things like K constant doesn't change as well if you want
I do agree though with what was posted earlier about it's a bit strange this question is 4 marks. As someone who is an experienced tutor if I was making this question I would make it max 3 marks.
1. Catalyst - it just speeds up the how fast the equilibrium is reached since it increases rate of reaction but it doesn't cause a shift
2. Solids - when added to a saturated solution have no effect cause you already reached the max solubility so you can't dissolve any more to change the concentration of things involved in a reaction
3. Inert gases - like helium and argon. It increases overall pressure of the system but it doesn't change the partial pressures (or simply put the amount of the reactants/products)
That last 3rd thing is basically the key point they want you to write about.
An answer to this question would be along these lines:
- Mention that adding an inert gas increases overall pressure of the system but it doesn't change the amounts of the NO2 and N2O4 present and the volume of the vessel doesn't change either since it says in the question its a fixed volume vessel(you can use the words partial pressures here if you want to but it wouldn't lead to you losing marks if you didn't because saying it doesn't change amounts of NO2 and N2O4 is essentially the same thing)
- Link this then to this means there is no shift in the position of the equilibrium based on Le Chatelier's principle since there is no disturbance and therefore no resulting change in the NO2 and N2O4 concentrations
- Now link it to temperature in that because the equilibrium was not disturbed the rate of the exothermic forward reaction = rate of the endothermic reverse reaction. That means the system is in dynamic equilibrium where changes are happening on a microscopic level but there's no total change to things like temperature.
- You can mention things like K constant doesn't change as well if you want
I do agree though with what was posted earlier about it's a bit strange this question is 4 marks. As someone who is an experienced tutor if I was making this question I would make it max 3 marks.
might not be super relevant, but I thought you could add a more soluble solid to a solution to force more of the less soluble out (ie salt in saponification)
however this also contradicts my understanding of precipitation titration and what dissolves in general as the less soluble substance would ppt first (since there is an equilibrium, shouldn’t the amount of solid decrease over time when another more soluble substance is added therefore, as the solid is in a saturation equilibrium?)
not sure how it ties in to lcp/collision theory qs but I’ve also been told to memorise these things
They won't assess that type of thing in HSC cause it's outside the scope of the syllabus and way too advanced for the type of understanding they want students during the HSC to gain as the main stuff is you should know how equilibriums shift when you change certain factors, calculate K constants and related quantities like pH and pKa.
What you are describing is a term called 'salting out'. It's used in a lab practical type setting for instance in organic chemistry research labs when we do a liquid-liquid separation using a separatory funnel.
Say for instance you have a compound that is partly soluble in both water and an organic solvent but you want it to only be soluble in the organic solvent so that when the two liquid layers separate, your compound will only be soluble in the organic solvent layer. That is normally achieved by adding a bunch of saturated salt solution like NaCl or NH4Cl which then pretty much saturates the entire water layer and so the compound then ends up dissolving fully into the organic layer. Now when you drain off the water layer you can collect the organic layer which contains your compound.
In terms of your precipitation titration question normally when you do a titration the more soluble substance will be quite a low concentration. Think for example when you do a titration with acid-base in module 6 we are not using 2 mol/L HCl or NaOH solutions, they are typically 0.10 mol/L roughly. Same thing applies for the precipitation titrations the concentrations of the solutions are so low that the more soluble substance won't ever reach it's Ksp saturation point so the thing that ends up precitating is just the very insoluble compound like a BaSO4. If that stuff started happening then the whole basis around how we want the precipitation titration to work would fall apart and so the calculations and values we get would be inaccurate.
Thanks, I am interested in the separations and reactions, I think it’s a shame that hsc just gives them to us says memorise and we aren’t expected to know much about the why. But I expect markers won’t love me if I put oos content in my responses
The new syllabus is actually quite better than the old one pre-2018 as it used to be way more memorising. A majority of the syllabus back then was similar to the sections about biofuels/ethanol and polymers where you literally had to memorise paragraphs about advantages/disadvantages of each fuel type and then the uses related to properties for the many different polymers like polystyrene, LDPE, HDPE etc.
The issue they have is because there is a lot of content in different areas they are limited to only telling you the main aspects and then not delving deeper into the why something is happening which is fair enough because if they go into detail on everything then the syllabus will be double the length that it is. For instance in the module 8 with the different spectroscopy techniques such as IR, mass spec and NMR you are just required to know the basics and then apply that to find the structure of some simple molecules which is good.
The reason they are not going into the fundamentals behind those techniques is because it is quite complicated and too much for a high school level. You need to understand physics concepts to be able to do that. That's where if you do the degree or subjects in chemistry/physics in university you will learn about those things there. In uni we spend mutliple courses over years learning about those things so it's impossible for them to do that in HSC.
yeah I agree I think mod 8 is alright as is. I guess I was more thinking mod 7? Like remove polymers and give us more depth on organic rxns (the polymers section is all fake chem anyway, at least what we are expected to know for a lot of the stuff particularly with nylon)
RZ1234
🏳️⚧️🏳️🌈🍉
these are marks for unintelligent people like me to secure.
Everyone’s intelligent (arguably, maybe not some people/politicians that I shouldn’t mention) don’t say that that