HSC Chemistry Question Thread

[J.B]

Hi,
I have just learn't about Le Chatelier's Principle and discussed this in relation to the equation H2O(l) + CO2(g) ↔ H2CO2(aq).
I understand that this reaction is exothermic so heat is produced.
I also understand therefore when increasing the temperature this will shift the equilibrium to the left to 'counteract' the extra heat. And I understand that when increasing the pressure this will shift the equilibrium to the right as it will result in less stress due to there only being one molecule colliding with the wall of the container.
But I don't understand that as increasing temperature is a way of increasing pressure, how come when looked at separately they favour different directions of the reaction?
Thanks.

[jakesilove]

Hi,
I have just learn't about Le Chatelier's Principle and discussed this in relation to the equation H2O(l) + CO2(g) ↔ H2CO2(aq).
I understand that this reaction is exothermic so heat is produced.
I also understand therefore when increasing the temperature this will shift the equilibrium to the left to 'counteract' the extra heat. And I understand that when increasing the pressure this will shift the equilibrium to the right as it will result in less stress due to there only being one molecule colliding with the wall of the container.
But I don't understand that as increasing temperature is a way of increasing pressure, how come when looked at separately they favour different directions of the reaction?
Thanks.

Hey! You've picked up a solid flaw in the way that Le Chatelier's principle is taught; there is a lot of back-and-forward going on that the syllabus glosses over. So, my main answer is that you're sort of right. Let's take it step by step.

First, we increase the temperature. The reaction is exothermic, and so releases energy upon moving forward. The equilibrium will want to DECREASE it's temperature, to minimise the change, and thus move to the left (towards the reactants).

Okay, so we've moved the equilibrium to the left. There are now more reactants than there used to be; what does that mean? Well, the reactants contain a gas, so there is MORE gas than there used to be. As such, the pressure will have increased.

The HSC stops there. However, you've asked a very deep, very important question; then what?

Remember, the equilibrium shifts to MINIMISE the change, but not completely switch the system back to it's original state. You can imagine it like a percentage; there is a temperature change of x%, but the equilibrium will only shift y%, where y is much smaller than x.

Back to the question at hand. Well, the pressure has increased; so what? Well, if pressure increases, the equilibrium will shift to the side with fewer moles of gas. In this case, that's the products; thus the equilibrium will shift to the right (towards the products). However, it will only do so z% (where z is smaller than y, and much smaller than x. If this x,y,z thing is confusing, ignore it; think of it like a dampening effect with each successive equilibrium shift). So, yes, pressure increased, but the equilibrium shifted to minimise that increase!

Why stop there? By shifting to the right this second time, we've increased the concentration of products. The equilibrium doesn't like that; it wants to minimise the change in concentration, and shift back towards the left.

And then back towards the right.
And then back towards the left.
And the back again.
And again.

What you've picked up on is the endless nature of this interaction. We call it equilibrium because it doesn't stop, it merely stabilises. It's a deep, complicated solution to the seemingly obvious flaw that you pointed out. I hope I made sense of it, and that you can understand it.

This is beyond the syllabus, but I think it helps you to internalise what is physically going on when an equilibrium is introduced to a change. Thank you for your brilliant question!

[J.B]

Thank you, this clears things up a lot!

[1tankengine]

Hey, I just had a practice exam covering Mod 1 and I am struggling with cell notation and cell potentials, I always mix up the direction of the potentials and I was wondering if there was an easy way to remember how to work these problems. Thanks 

[RuiAce]

Hey, I just had a practice exam covering Mod 1 and I am struggling with cell notation and cell potentials, I always mix up the direction of the potentials and I was wondering if there was an easy way to remember how to work these problems. Thanks 

The direction of the potentials depends on how high up in the standard reductions potential they are. Note that you're given the reduction potentials on your data sheet.

Given any cell, you should be able to identify the species (e.g. Mg|MgCl₂). Once you've identified the species, it is always the one further up that needs to be flipped. The one getting flipped is always undergoing oxidation, whereas the one not flipped is always undergoing reduction.

And the mnemonic RED-CAT AN-OX is all you need to determine which electrode is going on. Reduction at Cathode, Anode is Oxidation.

[Aaron12038488]

I'm doing preliminary Chem, and I find the techniques really confusing for some reason. Should I basically memorise all of these techniques, which techniques do I have to know, and how much info for each technique. Any help would be appreciated.

Also how do I write a scientific report. My teacher said it was different from junior science. Thx

Moderator action: Posts merged. At times like this, please resort to the 'modify' function to avoid repeat-posting.

[RuiAce]

I'm doing preliminary Chem, and I find the techniques really confusing for some reason. Should I basically memorise all of these techniques, which techniques do I have to know, and how much info for each technique. Any help would be appreciated.

What do you mean by "technique"

Technique could mean anything in prelim. E.g separation of mixtures techniques, techniques in calculation questions

Also how do I write a scientific report. My teacher said it was different from junior science. Thx

I question your teacher. What?

Because in Yr 11/12 science I still stuck with:

Aim
(Hypothesis - Maybe.)
Apparatus
Safety
Method
Results
Discussion
Conclusion <- Although, I've rarely needed one of these before.

[Aaron12038488]

I mean seperation of mixtures which you indicated.

[RuiAce]

As far as I recall the mixture separation techniques can be identified by considering what it is you have. There might be some inaccuracy in the content but it should give an indication of where to go.

Solid + Solid = sieving, centrifugation etc.
Liquid + Undissolved solid - filtration
Liquid + Dissolved solid - evaporation, distillation etc.
Liquid + Liquid - distillation etc.

And then you have other things like using magnets which are for special cases where there's a magnetic metal involved.

Although, at the end of the day you could still argue it is most certainly memorisation.

[bsdfjnlkasn]

Why is it that an increase in pressure results in a decrease of concentration in that same substance? Is there a physical way (idk maybe using an analogue) that we can imagine this, and then a mathematical expression that verifies the relationship of the two? I've just kind of learnt as a rule of thumb when applying LCP but would prefer a more concrete understanding if that's ok.

Any help would be greatly appreciated!!

[RuiAce]

Why is it that an increase in pressure results in a decrease of concentration in that same substance? Is there a physical way (idk maybe using an analogue) that we can imagine this, and then a mathematical expression that verifies the relationship of the two? I've just kind of learnt as a rule of thumb when applying LCP but would prefer a more concrete understanding if that's ok.

Any help would be greatly appreciated!!

Better give an example. Because depending on the equation, by LCP an increase in pressure just shifts the equilibrium to the side with fewer moles of gas and aqueous substances. Which means whilst it's decreased on one end, it's increased on the other end.

The mathematical expression is taught in the industrial chemistry option - the equilibrium constant.

[chexmixrocks]

So isotopes become radioactive usually when the proton:neutron ratio is more than 1:1.5 right? So why is Cobalt-60 and Technetium-99 radioactive?

[RuiAce]

So isotopes become radioactive usually when the proton:neutron ratio is more than 1:1.5 right? So why is Cobalt-60 and Technetium-99 radioactive?

Explanation of radioactive decay not related to the n-p ratio is beyond the HSC course.

Technetium is a special case. It's the lightest element that has no stable radioisotopes. Took them centuries to just discover that element. Link for further reading at your own leisure.

Cobalt-59 is the stable one in the cobalt bunch. It may also be worth mentioning that Nickel-60 is stable, but Nickel-59 is not.

The n:p ratio gives an indicator as to which are generally stable. The rule is not strictly binding to the stable ones. It's more binding towards which are unstable, if anything.

[bsdfjnlkasn]

Better give an example. Because depending on the equation, by LCP an increase in pressure just shifts the equilibrium to the side with fewer moles of gas and aqueous substances. Which means whilst it's decreased on one end, it's increased on the other end.

The mathematical expression is taught in the industrial chemistry option - the equilibrium constant.

Hey Rui,

I understand what happens due to an increase in pressure according to LCP but the confusion arises when I try to connect how an increase in pressure leads to a decrease in concentration. Maybe for the example, H₂O + CO₂ --> H₂CO₃

could you run through with me what happens if the pressure of CO₂ is increased? Like other than applying the blanket rule: the equilibrium shifts to the side of fewer gas moles. Would this increase in pressure, increase the concentration of CO₂ which will then force the reaction forward? So is it safe to assume the opposite of what I think i've been taught in the first line? That being, an increase in pressure results in a decrease in concentration?

Sorry for inundating you with questions, I really appreciate the help

[RuiAce]

Hey Rui,

I understand what happens due to an increase in pressure according to LCP but the confusion arises when I try to connect how an increase in pressure leads to a decrease in concentration. Maybe for the example, H₂O + CO₂ --> H₂CO₃

could you run through with me what happens if the pressure of CO₂ is increased? Like other than applying the blanket rule: the equilibrium shifts to the side of fewer gas moles. Would this increase in pressure, increase the concentration of CO₂ which will then force the reaction forward? So is it safe to assume the opposite of what I think i've been taught in the first line? That being, an increase in pressure results in a decrease in concentration?

Sorry for inundating you with questions, I really appreciate the help

When we talk about pressure, we talk about pressure on the entire system. If we're just increasing the quantity of one thing, by consequence we just increase the concentration of that one substance. Always make sure that we do not mix up the terminology here.

If we are adding more CO₂ in, then we are making the concentration of CO₂ greater. (Does this increase the 'pressure'? Yes, because we increased the concentration of a gas. But that's not what we mean by 'increasing the pressure'.)
Because there's too much CO₂ on side, by LCP the system wants to minimise this disturbance by using up some of the CO₂. Hence the equilibrium shifts to the right.

Also, remember that your equilibrium arrows are REVERSIBLE, not one-sided arrows.

When we talk about increasing the "pressure" in a broad context, we're talking about increasing the concentration of EVERYTHING. We increase the concentration of CO₂, but ALSO the concentration of H₂O and H₂CO₃.

Of course, in this example, the ONLY gas is CO₂. If we increase the pressure, the only thing that we're going to be concerned in is CO₂.

But in general, because we put too much gases in there, the equilibrium shifts to the side with the LEAST moles of gas. By doing this, the system eliminates moles of gas so that essentially, it's as if the system is a bit freed up (like making a room less stuffy). So the equilibrium also shifts to the right.

(Btw, forget that I said aqueous in my earlier post. I have no clue what I was thinking - probably juggled too many things at once.)
___________________________________

For this example, because CO₂ was the only gas, the ultimate effect was that the concentration of CO₂ DID go down due to an increase in pressure.

But if you take 2 NO₂ <--> N₂O₄, where both substances are gases and on products and reactants, you can't just make these assumptions. Concentration and pressure should not be tied with each other for the sake of LCP analysis.


P.S. Do point out if something doesn't make sense. I literally woke up and just started typing an answer so I was a bit tired at the time, and I haven't checked what i wrote.