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Edexcel AS Chemistry: 4th June 08

Hi ,

the last question on unit 1 (not the calculations./... before it...) it said something about:

H-C-H bonds and their angle... wot did u write? I had no idea!

Thanks!

Reply 181

arcoxia

H-C-H bond angles greater because of lone pair repulsion in the other molecule

Reply 182

abcd1990

mrpatel

Hi ,

the last question on unit 1 (not the calculations./... before it...) it said something about:

H-C-H bonds and their angle... wot did u write? I had no idea!

Thanks!

pcl3 had a lone pair of electrons, so the angle was 107 not 109 degrees. thats because lone pair of electrons have greater repulsion than bond pairs.

has anyone made a mark scheme for unit 1? i would love to see it :smile:

Reply 183

mrpatel

can you possibly remind me of the question?

Reply 184

redsox20

I think some of you totally misread the question on 3B, about the graph.

The graph showed the temperature change when varying amounts of zinc were added to DIFFERENT samples of copper sulphate, so the graph curves because, to my knowledge, at first there is not enough zinc to react fully but then as it flattens there is not enough CuSO4 to react with the extra zinc so after a certain point the temp change stays constant.

Also, for why the metal is not used in the calculation, I think it is because you dont know the exact mass that reacts with 50cm3 of copper sulphate.

Sorry if I have misread the question, or your posts, but that is my take.

Thanks

Reply 185

RMIM

9

callum9999

I said 50 to 52 degrees (+/- 1)

yes but is that right? i cant find it - but i have come across a past paper with the same question. now i think the mark scheme said if you have a higher temp than the boiling u get no marks. not sure. none of my tx books etc seem to explain what to do.

Reply 186

mrpatel

Sorry about this.... but can anyone help me with this question?

Explain why molten sodium iodide conducts electricity but solid sodium iodide does
not.

Thanks

Reply 187

redsox20

I think you can go a couple of degrees either side of the boiling point.

Thanks

Reply 188

tweediness

4

Going through this thread, it seems as though majority found paper 3B dodgey? So hopefully, this can be representitive of the chem students, SO HOPEFULLY...grade boundaries could go lower?????? *hopes*

Damn that paper

Unit 1 and 2 were amazing in comparison

Reply 189

charmainelaw

1

mrpatel

Sorry about this.... but can anyone help me with this question?

Explain why molten sodium iodide conducts electricity but solid sodium iodide does
not.

Thanks

Ions in solid sodium iodide are fixed in position by the strong ionic bond whereas in molten sodium iodide the ionic lattice is broken down so ions can move (when a potential difference is applied)

How about "Suggest why sodium iodide has a lower melting temperature than that of sodium chloride." Does anyone know the answer to this? Never have come up in an the exam before.
I said iodide and chloride ion have the same charge but iodide ions are bigger that chloride ion. This means that they cannot pack around the sodium ion as closely as the chloride ions would be able to. Hence attraction is less.

Reply 190

callum9999

16

RMIM

yes but is that right? i cant find it - but i have come across a past paper with the same question. now i think the mark scheme said if you have a higher temp than the boiling u get no marks. not sure. none of my tx books etc seem to explain what to do.

I think its right. If I remember correctly, the only other substance in the solution had a boiling temperature of at least 60 degrees, therefore going over by 1 degree isn't going to cause any contamination. I think I also remember seeing an answer like this in a markscheme. Sorry I can't be certain but does it really matter now? Its too late to do anything about it.

Reply 191

RMIM

9

E.M.C.

they asked the same question in the specimen paper. it said- initially there were high concentration of CuSO4 and large surface area of zinc, so reaction rate was increasing. then these factors become limiting, so rate decreases.

i dont think the cases are the same, so that answer is not correct for this situation. in this case we varied the mass, and it was powered anyway.

Reply 192

RMIM

9

callum9999

I think its right. If I remember correctly, the only other substance in the solution had a boiling temperature of at least 60 degrees, therefore going over by 1 degree isn't going to cause any contamination. I think I also remember seeing an answer like this in a markscheme. Sorry I can't be certain but does it really matter now? Its too late to do anything about it.

well it's about learning how to work it out properly if it comes up again in the other papers. such a waste of 2% for something so simple. throw away enough 2% and your left with a shameful grade. 2% is a lot, 5 of them make 10% - there were so many 1mark questions like that.

Reply 193

gm88

5

i found unit 3b alright actually. unit 2 was a ****ing joke - ridiculously hard gonna have to resit it. for me unit was a resit but i was so tired that i didnt bother about it and just made uo most of my answers - it was easier than the jan 08 paper though. my head was actually physically aching after walking out of the exam - felt like i had a hangover - 3 hrs of chem was waaaay too much

Reply 194

callum9999

16

RMIM

well it's about learning how to work it out properly if it comes up again in the other papers. such a waste of 2% for something so simple. throw away enough 2% and your left with a shameful grade. 2% is a lot, 5 of them make 10% - there were so many 1mark questions like that.

Yeah. If no-one can say for certain just ask a chemistry teacher next time your in school.

I was thinking you were just obsessing over what grade you might have (like I was!)

Reply 195

RMIM

9

callum9999

Yeah. If no-one can say for certain just ask a chemistry teacher next time your in school.

I was thinking you were just obsessing over what grade you might have (like I was!)

i found the mark scheme. you were right. i miss read it last time!!! thought it was weird to do it that way. like you said just +/-1 or 2 or 3. what a waste of 2%

Reply 196

callum9999

16

charmainelaw

Ions in solid sodium iodide are fixed in position by the strong ionic bond whereas in molten sodium iodide the ionic lattice is broken down so ions can move (when a potential difference is applied)

How about "Suggest why sodium iodide has a lower melting temperature than that of sodium chloride." Does anyone know the answer to this? Never have come up in an the exam before.
I said iodide and chloride ion have the same charge but iodide ions are bigger that chloride ion. This means that they cannot pack around the sodium ion as closely as the chloride ions would be able to. Hence attraction is less.

Thats what I put and I've never seen it before either. I also mentioned something about charge density to try and "pad out" my answer but I don't know if that matters?

Has anyone here sat the January 08 papers (or did them during revision)? Did you find these exams easier, harder or about the same?

Reply 197

moves-my-world

8

(v) Calculate the percentage of the ethanol that has oxidised, given that one mole of
ethanol forms one mole of ethanoic acid.

(b) Suggest a physical test to confirm that the sample was hex-1-ene, rather than one of
its isomers.

Anyone?

Reply 198

RMIM

9

no idea about the % one. but the other one I think was measure its boiling point (that is a physical test) - bet you're kicking yourself - 2%