VCE Chemistry Question Thread

[IndefatigableLover]

Do you think it's worth trying to start unit 4 as well. The first 5 chapter of Heinemann are basically a review of 1/2 aren't they?

I think understanding bits of Unit 4 AOS1 wouldn't hurt since technically speaking, it would make sense for topics of Equilibrium to be taught first in Chemistry I reckon Though the first five chapters of Heinemann are a review of 1&2, it is still crucial to understand it all and what comes with it (such as percentage conversions).

Ideally, you'd want to finish Unit 4 AOS1 before Term 3 starts so you can start doing old Chemistry exams (2006-2012) since the old SD encompasses the current Unit 3 and Unit 4 AOS1 into it which is nice revision leaving you with plenty of time to learn Unit 4 AOS2.

[lzxnl]

I think understanding bits of Unit 4 AOS1 wouldn't hurt since technically speaking, it would make sense for topics of Equilibrium to be taught first in Chemistry I reckon Though the first five chapters of Heinemann are a review of 1&2, it is still crucial to understand it all and what comes with it (such as percentage conversions).

Ideally, you'd want to finish Unit 4 AOS1 before Term 3 starts so you can start doing old Chemistry exams (2006-2012) since the old SD encompasses the current Unit 3 and Unit 4 AOS1 into it which is nice revision leaving you with plenty of time to learn Unit 4 AOS2.

I wouldn't look at chemical equilibrium without knowing about the basics of chemical calculations like concentrations and stoichiometry though.

[keltingmeith]

Fun fact: from 2017, equilibrium will be taught in unit 3. In fact, it's all back to front as to what we have now - unit 3 AoS1 is electrochem, fuels, calorimetry, etc. and unit 3 AoS2 is equilibrium, rates, electrolysis and rechargeable batteries.

[lzxnl]

Fun fact: from 2017, equilibrium will be taught in unit 3. In fact, it's all back to front as to what we have now - unit 3 AoS1 is electrochem, fuels, calorimetry, etc. and unit 3 AoS2 is equilibrium, rates, electrolysis and rechargeable batteries.

What about units 1/2? If that order is the same, then I guess students will learn electrochem when it's still 'fresh' from year 11.

But how can you teach electrochem without equilibrium? I can't believe VCAA doesn't teach it that way. It would make so much more sense to students (I feel) if you tell them that strongest oxidant/strongest reductant idea works because that has the highest equilibrium constant.

[Biology24123]

Is the stationary phase always polar in TLC and are the mobile and stationary phase always opposite charge (I would assume so)

[Sine]

Is the stationary phase always polar in TLC and are the mobile and stationary phase always opposite charge (I would assume so)

normal phase chromatography, the stationary phase is polar and mobile phase is non-polar.

reverse phase chromatography, the stationary phase is non-polar and mobile phase is polar.

Yes I would think so mobile phase and stationary phase would always have to have opposite charge; how else would a mixture be separated.

[keltingmeith]

Your stationary and mobile phase CAN have the same polarity, particularly if you want to separate compounds based on something other than polarity. I don't think this is touched on much in VCE, however.

[Biology24123]

Your stationary and mobile phase CAN have the same polarity, particularly if you want to separate compounds based on something other than polarity. I don't think this is touched on much in VCE, however.

ok thanks

[HasibA]

Write the ionic half equations and the balanced overall ionic
equation for the reaction in which:

manganese dioxide (MnO2) reacts with concentrated
hydrochloric acid to form chlorine gas and a solution
containing manganese(II ) ions.

For this question, i correctly got the MnO2 half-equation , but the other half-equation was wrong. The suggested answers said that the other half-equation was 2Cl–(aq) ->  Cl2(g) + 2e–. My question is, why wasn't it HCL on the left hand side, why was it CL-? is it because HCL is an acid and can donate its Hydrogen? or maybe i am completely wrong? thanks friends

[zsteve]

If you wrote HCl, I believe it would also be correct, but you'd get H+ on the right hand side in addition.
The reason we use Cl- is because HCl is a strong acid, so 99% of the HCl molecules in the solution become Cl- and H+ once they're in water. Hence we can basically assume that all HCl has become Cl- and use Cl- only.

[keltingmeith]

My question is, why wasn't it HCL on the left hand side, why was it CL-? is it because HCL is an acid and can donate its Hydrogen? or maybe i am completely wrong? thanks friends

This is going to sound so nitpicky, but PLEASE write Cl and not CL. You WILL lose marks if you write CL instead of Cl, and whoever's marking your work will think of you pretty badly.

zsteve hit the nail on the head, look at his response for an answer to your question.

[Maz]

Can someone please help me with this question?
Write the reactions of the following substances with sodium hydroxide...
1.HS^-
2.HSO3^-
3.HCOOH
4. H2NCH(CH3)COOH (alanine amnio acid)
and can you please tell me how to principally do these type of questions so i can do the others please?
Thankyou in advance

[keltingmeith]

Can someone please help me with this question?
Write the reactions of the following substances with sodium hydroxide...
1.HS^-
2.HSO3^-
3.HCOOH
4. H2NCH(CH3)COOH (alanine amnio acid)
and can you please tell me how to principally do these type of questions so i can do the others please?
Thankyou in advance

Mmmm, 4 is a bit of a weird one, but it basically comes down to this.

You have some molecule, and you're reacting it with sodium hydroxide. Sodium hydroxide is a strong base, and will predominately act as a base. So, assuming it acts as a base, the thing it reacts with will be an acid (because that's how base reactions work - similarly, if you react something with an acid, then that thing will act as a base).

Let's use the HS- as a case example. Since NaOH is a base, HS- must act as an acid - an acid donates a proton, and a base accepts a proton, giving us:



This, however, is a very strange way of writing it. So, once the sodium hydroxide abstracts the proton, we assume that the water will be lost from the salt, giving us:



Which is the final answer (once you include states). At this point, you could also assume that the sodium and sulfide actually form an ionic salt together - however, since the LHS has a charge, the RHS should also have a charge, and it's easier to show this without combining the two. If you were to combine the two, though, remember that charges must also balance, so you'd write it like so:

[HasibA]

If you wrote HCl, I believe it would also be correct, but you'd get H+ on the right hand side in addition.
The reason we use Cl- is because HCl is a strong acid, so 99% of the HCl molecules in the solution become Cl- and H+ once they're in water. Hence we can basically assume that all HCl has become Cl- and use Cl- only.

thank you so much! really cleared things up!

This is going to sound so nitpicky, but PLEASE write Cl and not CL. You WILL lose marks if you write CL instead of Cl, and whoever's marking your work will think of you pretty badly.

zsteve hit the nail on the head, look at his response for an answer to your question.

No that's absolutely fine! 100% agree with you, it was a typo but i'm glad you've mentioned it ! Anything that will help me improve in chemistry is 100% welcome, thank you !

[Maz]

Thankyou...that method worked for all of them