VCE Chemistry Question Thread

[zsteve]

Hey guys, hoping someone can explain why the mono-hydrate shouldn't worry me as much as it is rn. And I have looked at the examiner reports but I still can't make sense of it 

Mono-hydrate means that water has been incorporated into the ionic structure of CaC2O4 so that for each CaC2O4, you also have one H2O.

Once you have free Ca2+ ions in solution (thanks to the acid), addition of ammonium oxalate would precipitate CaC2O4.H2O, the H2O is present because the environment is moist.

If you wanted to get rid of the H2O and just have plain CaC2O4, you could heat the precipitate and evaporate the water.

Be aware that you can also have stuff like CaC2O4.(1/2)H2O (one H2O for every 2 CaC2O4) or even CaC2O4.(1/3)H2O (1 H2O for every 3 CaC2O4).

Because we are dealing with ionic compounds, there is no 'molecular structure' to talk about, and we are fine.

[blacksanta62]

Mono-hydrate means that water has been incorporated into the ionic structure of CaC2O4 so that for each CaC2O4, you also have one H2O.

Once you have free Ca2+ ions in solution (thanks to the acid), addition of ammonium oxalate would precipitate CaC2O4.H2O, the H2O is present because the environment is moist.

If you wanted to get rid of the H2O and just have plain CaC2O4, you could heat the precipitate and evaporate the water.

Be aware that you can also have stuff like CaC2O4.(1/2)H2O (one H2O for every 2 CaC2O4) or even CaC2O4.(1/3)H2O (1 H2O for every 3 CaC2O4).

Because we are dealing with ionic compounds, there is no 'molecular structure' to talk about, and we are fine.

Thanks zsteve

Could you help with the equation and show some important steps? It's one mark so the you can do it the quick way if you want

[jyce]

Thanks zsteve

Could you help with the equation and show some important steps? It's one mark so the you can do it the quick way if you want

Ordinarily, the net ionic equation for the precipitation reaction would be: Ca²⁺(aq) + C₂O₄^2-(aq) -> CaC₂O₄(s)
But since it's monohydrate, you just need to add 1 water molecule on both sides: Ca²⁺(aq) + C₂O₄^2-(aq) + H₂O(l) -> CaC₂O₄.H₂O(s)

EDIT: Are you familiar with how to write a net ionic equation for a precipitation reaction?

[The Usual Student]

Hey what key words would I need to get full marks for this practice sac question: Explain why increasing the volume of a solution does not increase the reaction rate whilst increasing the concentration does?
The reaction was CaCO3 + HCL and the HCL was the solution we were altering concentration for

Like I know the answer but I fail to express it in words, and I don't really know what is important to mention

[keltingmeith]

Hey what key words would I need to get full marks for this practice sac question: Explain why increasing the volume of a solution does not increase the reaction rate whilst increasing the concentration does?
The reaction was CaCO3 + HCL and the HCL was the solution we were altering concentration for

Like I know the answer but I fail to express it in words, and I don't really know what is important to mention

Well, why don't you try to put it into words and we can critique it for you?

(also, somewhat picky sidenote: remember, it's HCl, not HCL!)

[The Usual Student]

Well, why don't you try to put it into words and we can critique it for you?

(also, somewhat picky sidenote: remember, it's HCl, not HCL!)

woops! yep

So from my understanding
-  increasing the volume of the HCl increases the number of particles of HCl, however, this does not increase the rate of reaction as the likelihood of a particle of HCl colliding with a surface particle of CaCO3 does not increase as the volume that these particles occupy has also increased whilst when the concentration of HCl increases for the same volume of HCl, the number of particles of HCl in the fixed volume increases which leads to a higher likelihood of collisions and higher rate of reaction. (exam answer)

please tell me if i got the concept wrong since in writing this as i realized my understanding isn't as clear as i thought,

essentially i was tying to say that a higher concentration leads to more collisions since the particles are more concentrated in the solution whilst simply adding more volume still leaves the particles spaced out and hence doesn't increase the reaction rate at all. ( my informal answer)

[blacksanta62]

Ordinarily, the net ionic equation for the precipitation reaction would be: Ca²⁺(aq) + C₂O₄^2-(aq) -> CaC₂O₄(s)
But since it's monohydrate, you just need to add 1 water molecule on both sides: Ca²⁺(aq) + C₂O₄^2-(aq) + H₂O(l) -> CaC₂O₄.H₂O(s)

EDIT: Are you familiar with how to write a net ionic equation for a precipitation reaction?

Yep, confident with writing precipitation equations Thought there was more to the question than that (adding water to both sides plus a . before the water

Thank you

[Elizawei]

Quick question guys,
is it 3-bromo-2-methyl-pentane or is it 2-methyl-3-bromopentane? I've been taught that nomenclature goes by alphabetical order, but some commercial exams answers have it written the other way

Thanks in advance

[Syndicate]

Quick question guys,
is it 3-bromo-2-methyl-pentane or is it 2-methyl-3-bromopentane? I've been taught that nomenclature goes by alphabetical order, but some commercial exams answers have it written the other way

Thanks in advance

Yes it goes by the alphabetical order; which means 3-bromo 2-methylpentane is correct.

[Elizawei]

Yes it goes by the alphabetical order; which means 3-bromo 2-methylpentane is correct.

Thanks!

[Maz]

Hey,
could you please help me?
I have a fuel cell test on Monday and I was just wondering if you had any idea what kind of questions they generally ask. It is on the hydrogen-oxygen and methanol fuel cell specifically

Also,
Would you happen to know if there are any variations in electrolytes used (if any) and chemical reasons for these variations?

I'd really appreciate any ideas
Thankyou

[blacksanta62]

To what degree would I need to understand transciption and translation for VCE chemistry? We haven't covered it in detail in biology yet so I'm a bit blank. Should I just get my head around it now since I'm gong to have to deal with it in biology anyway and will it be as in depth as biology. The Heinemann book just has a little snippet for it.

Thanks

Edit: Will there be short answer questions on forensic applications of DNA? Or will VCAA generally ask multiple choice questions? With that said will a rough understanding of the applications be enough for me to move on? Thanks

[jyce]

To what degree would I need to understand transciption and translation for VCE chemistry? We haven't covered it in detail in biology yet so I'm a bit blank. Should I just get my head around it now since I'm gong to have to deal with it in biology anyway and will it be as in depth as biology. The Heinemann book just has a little snippet for it.

Thanks

Edit: Will there be short answer questions on forensic applications of DNA? Or will VCAA generally ask multiple choice questions? With that said will a rough understanding of the applications be enough for me to move on? Thanks

You don't need to know about transcription or translation for VCE Chemistry, and DNA forensics was removed from the SS in 2013.

[blacksanta62]

You don't need to know about transcription or translation for VCE Chemistry, and DNA forensics was removed from the SS in 2013.

Thanks for the reply Jyce
So would recommend reading and summarising them if they're not going to be examined??

[jyce]

Thanks for the reply Jyce
So would recommend reading and summarising them if they're not going to be examined??

No, I would not recommend it - unless you want to learn about those concepts out of pure interest.