The Student Room Group

WJEC - AS Chemistry Unit 1 [New Spec] 27th May 2016

Scroll to see replies

Original post by Elliottmanwaring
It was on the spec, iodide ions gain electrons forming iodine which turns to a red/brown solution. Then you'd explain why- basically liberation of Iodine but I cba to explain now.. Go on chemguide and it'll explain probably.



Seems like you knew your stuff for that **** exam I'm dreading results day for CH1
Cheers buddy! Just to give you the info- https://answers.yahoo.com/question/index?qid=20070509033546AARgz93 thats a link... 4th paragraph contains basically the reasoning behind what is happening in the electrolysis.
Original post by ChemSinceDaay...
Seems like you knew your stuff for that **** exam I'm dreading results day for CH1
Original post by RasputinReborn
Aren't iodide ions oxidised to form I2, because if they gain electrons, they'll form I2- ions.


That statement is partially correct. It is true that the iodide is oxidised therefore loses an electron to form a neutral, diatomic iodine molecule. The fact that they would otherwise form I2- is incorrect. They could possibly form I3- but not I2-. I'm no Chemist so don't ask why I just remember reading that the for mation of a dinegative iodide isn't apparent. Only a trinegative ion called.. Imaginatively as all scientists are.... Triiodide. It is actually a combination of three Iodine atoms. But yeah in answer to your question, I don't think you can get an I2- because if you were to do that you'd be adding an electron to the 6s sub-shell. The addition of 1 do an iodine atom forms just I-.
Original post by Elliottmanwaring
It was on the spec, iodide ions gain electrons forming iodine which turns to a red/brown solution. Then you'd explain why- basically liberation of Iodine but I cba to explain now.. Go on chemguide and it'll explain probably.


Isn't iodine insoluble? Isn't it bromine that would turn the solution a red/brown color? I said a grey solid would form on the anode.
Original post by RasputinReborn
But for the sake of the exam, were the marks for:

2I- -----> I2 +2e- (1)

Solution turns brown (1)

Don't know what the third mark is, but j said chlorine has a higher electron affinity so displaces iodine and stays in solution



Posted from TSR Mobile


As I said, I'm no pro but I would assume the marks are for:
1. Chlorine is a more powerful oxidising agent than Iodine so it liberates the iodine.
2. 2I- ---> I2 + 2e-
3. Solution turns dark red/brown colour and an explanation of basically what the equation is telling you in terms of movement of electrons.

That's what I put but I'm not sure if that is on the mark scheme.
Original post by phaggatron5001
Isn't iodine insoluble? Isn't it bromine that would turn the solution a red/brown color? I said a grey solid would form on the anode.


If I'm honest I'm going off what you guys are asking I cannot even remember the question? Bromine then, same goes, same group ect. Different result. Sorry you're right it was on Bromine I believe.
Reply 46
Sounds like I messed this exam up guys. God bless.
Original post by RasputinReborn
No it was sodium iodide and sodium chloride. I remember the question after asked for it s reaction with AgNO3 and then with aqueous ammonia, then there was a calculation asking for the percentage of sodium iodide in the mixture.


Oh yeah! Fair play beside the question of the Flouridated water, this was one if the hardest calculations. I definitely got both of those questions wrong!
Original post by Elliottmanwaring
As I said, I'm no pro but I would assume the marks are for:
1. Chlorine is a more powerful oxidising agent than Iodine so it liberates the iodine.
2. 2I- ---> I2 + 2e-
3. Solution turns dark red/brown colour and an explanation of basically what the equation is telling you in terms of movement of electrons.

That's what I put but I'm not sure if that is on the mark scheme.


After comparing answers most of the stuff I've done is wrong I think 😢
I'm most likely going to re sit CH1 but CH2 is what I prefer so yeh. Is anyone else in the same boat as me or?
Original post by RasputinReborn
Me and 99% of Wales. There were no definition questions, no equilibria questions (excluding Kc and the concentration calculation) and only 3 marks on ionisation energies. This was a maths exam that was harder than C1 and C2...
Posted from TSR Mobile

Yeh, but this is a new spec so we should've expected a very bad paper so I think that next year will be more better because they'll include definitions, Kc, equilibria etc. Also actual calculations that we've done in class. I wish we done AQA because WJEC are *********!
0.27 was your answer for the equilibrium question?
Original post by RasputinReborn
Me and 99% of Wales. There were no definition questions, no equilibria questions (excluding Kc and the concentration calculation) and only 3 marks on ionisation energies. This was a maths exam that was harder than C1 and C2...


Posted from TSR Mobile
Original post by RasputinReborn
I got something around 4, but I'm probably wrong 😂


Posted from TSR Mobile

Does anyone know what iodine clock reactions is?
Original post by Elliottmanwaring
0.27 was your answer for the equilibrium question?


I had 0.27 too but I don't really know how
Original post by RasputinReborn
I got something around 4, but I'm probably wrong 😂


Posted from TSR Mobile


I got 4. something!! I found this paper really hard, most definitely resisting chemistry and possibly biology, WJEC have made their AS exams disgusting this year.
hey guys, will i get marks deducted for answer in incorrect significant figures, not those extreme one like 7 or 8 figures after decimal but like 2 to 3? Im frustrated about that because my teachers never care about it in tests and my frds said it have to depends on the smallest s.f. given in the question. HELP!!!!
Original post by matthewyakgw
hey guys, will i get marks deducted for answer in incorrect significant figures, not those extreme one like 7 or 8 figures after decimal but like 2 to 3? Im frustrated about that because my teachers never care about it in tests and my frds said it have to depends on the smallest s.f. given in the question. HELP!!!!


Well if in the question it states to specific sf then u must give it in that form. However if it's not stated then u don't have to because examiners will consider ecf (error carried forward) so they should give u the mark if reasonable.

Anyways how did u find the exam overall, I found it really damn hard so I'll be re sitting this and hopefully scoring high in my favourite CH2.
Original post by ChemSinceDaay...
Well if in the question it states to specific sf then u must give it in that form. However if it's not stated then u don't have to because examiners will consider ecf (error carried forward) so they should give u the mark if reasonable.

Anyways how did u find the exam overall, I found it really damn hard so I'll be re sitting this and hopefully scoring high in my favourite CH2.


so will i get marks deducted because i check some past paper mark scheme and they said cao [3] which means only all 3 marks forcorrect answer only, so will i get all the marks or lose one mark. this exam had so xxxxxxx many calculations, if i get one mark deducted from every single calculation, i would pretty sad.
lol should i send an email to wjec?

i find some calculations are really complex and quite new to us but i should be alright if no mark deducted for incorrect s.f. because in my memory there are no question stated need specific s.f.
(edited 7 years ago)
Original post by RasputinReborn
As mentioned above, you will only be deducted one mark if it's EXPLICITLY stated. Didn't the relative atomic mass of Si require certain significant figures? I can't quite remember.

i dont think they did mention
Original post by matthewyakgw
i dont think they did mention

Yes that questions did require 3sf I remember

It was 28/29/30 Si
28=91.6 29=5.6%. 30=2.8%
(28x91.6)+(29x5.6)+(30x2.8)
Divide by 100
=28.112
3sf=28.1% that's what I wrote
Original post by ChemSinceDaay...
Yes that questions did require 3sf I remember

It was 28/29/30 Si
28=91.6 29=5.6%. 30=2.8%
(28x91.6)+(29x5.6)+(30x2.8)
Divide by 100
=28.112
3sf=28.1% that's what I wrote


That's what I had, some people had like 21% and I was confused because I thought it had to be 28-30 and considering 28 had the biggest percentage it had to be closer to that

Quick Reply