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AS Chemistry- helping each other out!

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Has anyone done the qualitative coursework on silver nitrate and identifying 4 compounds and anyone done quantitive one on enthalpy? Need help and any pointers would be great thanks:smile:


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Anyone care to explain why the solubility of group 2 hydroxides increase as you go down the period?
The enthalpy change for hydration as you go down the group for hydroxides becomes more negative (more interactions between ions and water molecules).


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Original post by ThatMadClown
The enthalpy change for hydration as you go down the group for hydroxides becomes more negative (more interactions between ions and water molecules).


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I have another query. It is to do with halogens and displacement reactions. Can you just check what I am thinking is right?

Which of the following mixtures would react.

i( Br2(aq)+2NaCl(aq)
ii Cl2(aq) + 2NaI(aq)


The answer is the second one. The reason for this is because chlorine is more reactive than iodine and so displaces it. It is more reactive due to being more electronegative. Is there anything else I need to consider with this? It says something about chlorine being a better oxidation agent - care to explain this part?
Just to clarify something it may have finally clicked. Is an oxidising agent something that takes electrons from a species so itself gets reduced. An a reducing agent is something that gives electrons to another species so it gets oxidised.
Original post by Super199
I have another query. It is to do with halogens and displacement reactions. Can you just check what I am thinking is right?

Which of the following mixtures would react.

i( Br2(aq)+2NaCl(aq)
ii Cl2(aq) + 2NaI(aq)


The answer is the second one. The reason for this is because chlorine is more reactive than iodine and so displaces it. It is more reactive due to being more electronegative. Is there anything else I need to consider with this? It says something about chlorine being a better oxidation agent - care to explain this part?


Suree, the halogens are oxidising agents so they get reduced. When you go down the halogens the atoms get bigger (in terms of shielding) so you know that Cl is a smaller atom than I which means that it'll be more electronegative because the incoming electron will have a stronger overall electrostatic attraction (because of the protons in the nucleus, less shielding). It's a better oxidising agent because it's smaller and so can attract an electron more strongly.

Hope this helps :smile:


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Original post by ThatMadClown
Suree, the halogens are oxidising agents so they get reduced. When you go down the halogens the atoms get bigger (in terms of shielding) so you know that Cl is a smaller atom than I which means that it'll be more electronegative because the incoming electron will have a stronger overall electrostatic attraction (because of the protons in the nucleus, less shielding). It's a better oxidising agent because it's smaller and so can attract an electron more strongly.

Hope this helps :smile:


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Yeah understood! Thanks :smile:
Original post by afghan123
I'm sure you'll be fine dw. We had a prep lesson, and it left me more confused, oh well, we have one more try to really go for it


I hope so, my teacher hasn't done anything for us she's absolutely hopeless, yeah that's true. I'd better get to revising then lol! Good luck
Original post by Georgiam247
I hope so, my teacher hasn't done anything for us she's absolutely hopeless, yeah that's true. I'd better get to revising then lol! Good luck


Haha yh, it's an enthalphy one, I really need this one to go well so I can get an A. Good luck on yours too!
Original post by Disney0702
Hi there!

I am an A2 Chemistry student.

I got an A last year at AS level.

In the March MOCKs I got a U grade for Chemistry but that was because I lacked understanding at that time.

To help me understand better I bought the CGP text book which helped contribute to my A grade.

I recommend you get it if you really want that high A grade.

CGP AS Chemistry

SAMPLE PAGES

I also recommend you do lots of past papers so help strengthen your exam technique.



Thanks for the advice as in the January mocks I also got a u and some topics I completely understand whilst some of my knowledge is a bit shaky. Can I just ask what exam board did you do
Hey guys I have a problem with calculating% error..

So when you use the formula % error = maximum error / quantity measured x 100%, say I have a measuring cylinder graduated in a 1cm3 division, the maximum error is +- 0.5cm3 right? So when substituting it into the equation, is the figure I put in for maximum error 0.5cm3 or 1cm3? Do you have to double it because the 0.5cm3 error could go either way? I'm soooo confused

Ive seen examples where its doubled and other examples where it isn't, what am I missing?
Original post by Georgiam247
Hey guys I have a problem with calculating% error..

So when you use the formula % error = maximum error / quantity measured x 100%, say I have a measuring cylinder graduated in a 1cm3 division, the maximum error is +- 0.5cm3 right? So when substituting it into the equation, is the figure I put in for maximum error 0.5cm3 or 1cm3? Do you have to double it because the 0.5cm3 error could go either way? I'm soooo confused

Ive seen examples where its doubled and other examples where it isn't, what am I missing?


The probable error is +/- 1cm^3 not 0.5.

This is left as it stands, not doubles. 1/value *100
Original post by Random1357
The probable error is +/- 1cm^3 not 0.5.

This is left as it stands, not doubles. 1/value *100




the reason I'm so confused is this question I came across in a past paper; it says "The 100cm3 measuring cylinder used to measure the water was calibrated to within 1cm3. Calculate the % error in the vol of CO2 collected" (the experiment it refers to was an acid/carbonate reaction where the water filled cylinder was upside down in a trough of water to record the gas blah blah.

So the measured vol of CO2 was 64cm3, so I did 0.5 / 64cm3 x 100% = 0.78% error but the answer turned out to be 1.56% error. I'm guessing it's because the max error wasn't 0.5 it was 1, but I have NO idea why. I'm so sorry for the massive essay haha
(edited 9 years ago)
Anyone care to help me with these questions. Is 1. The OH group2. Potassium dichromate which is K2Cr2O7.3. I dunno4. Does it make ethanal?5. OH again?6. Excess of oxygen or what? I dunno can someone help me7. Pentan-3-one8. Possibly because there are no more OH. Can someone explain this as well.
(edited 9 years ago)
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Original post by Super199
Anyone care to help me with these questions. Is 1. The OH group2. Potassium dichromate which is K2Cr2O7.3. I dunno4. Does it make ethanal?5. OH again?6. Excess of oxygen or what? I dunno can someone help me7. Pentan-3-one8. Possibly because there are no more OH. Can someone explain this as well.


1) can't see Q

2) and acid; it only works if acidification

3) oxidise to carboxylic acid.

4)that's right :smile:

5) OH to carboxylic acid...

6) excess oxidising agent and reflux for ages...

7) yep.

8) kinda... Ketones only have C=O or C-C bonds. Both are very resistant to being broken so no reaction under normal conditions...
(edited 9 years ago)
Original post by Random1357
1) can't see Q

2) and acid; it only works if acidification

3) oxidise to carboxylic acid.

4)that's right :smile:

5) OH to carboxylic acid...

6) excess oxidising agent and reflux for ages...

7) yep.

8) kinda... Ketones only have C=O or C-C bonds. Both are very resistant to being broken so no reaction under normal conditions...

Question 1 was : "What change of functional group occurs during the oxidation"
Also can you explain 3?
Original post by Super199
Question 1 was : "What change of functional group occurs during the oxidation"
Also can you explain 3?


Yeah, I mean which oxidation?

3) when you oxidise an alcohol you get an aldehyde or ketone.

However aldehyde can also be oxidised again to form a carboxylic acid by the same reagent so if you don't remove your product then this reaction will also occur.
Original post by Random1357
Yeah, I mean which oxidation?

3) when you oxidise an alcohol you get an aldehyde or ketone.

However aldehyde can also be oxidised again to form a carboxylic acid by the same reagent so if you don't remove your product then this reaction will also occur.


Partial oxidation
Original post by Super199
Partial oxidation


Of a primary alcohol?

Alcohol -> aldehyde
jesus christ unit 2 chemistry is impossible for me, we've been taught it so terribly and i have the mock tomorrow, i know nothing

in the unit 1 mock i got a C, 4 marks off a B, all i want at this point would be a C for both exams because I don't see myself getting above a B in chemistry because of this unit 2

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