* Chemistry * Predictions / discussion / aftermath * (1 thread please)

aranciata

jumpguy wrote: »

I was thinking about doing this but instead I decided to square it out and use the -b formula (wasn't feeling brave :P ). It was long and fairly tedious, but I'm sure they'll accept both methods.

Yes let's hope so! I think I got a figure of .5354 - not sure about that cause i literally did it in the last minute of the exam.

Raging that I used the wrong ratio in the bleach titration, could have lost 20 marks..

jumpguy

KoolAidRelic wrote: »

Yes we did... The equilibrium constant equation uses concentrations (i.e. mol dm^-3).

Five moles in a twelve litre flask = 5/12 mol dm^-3.

Are you sure? I assumed it was just 5 moles reacting away in a vessel of total capacity 12 litres. The capacity of the container wouldn't have any effect of the concentration of anything, and it asked for the concentration of hydrogen gas.

5 moles in a 12 litre flash is...5 moles, just in a 12 litre flask. 5 moles is the concentration. I'm not trying to be stand-offish here, I may be wrong.

partyatmygaff

I loved that paper. Much better than last years.

Q1: Couldn't be any nicer. I got 4.8% W/V like everyone else so hopefully that's right.
Q2: An odd style of question but nevertheless very handy.
Q3: Lovely question, the only thing mildly challenging was finding the melting point of the crystals but I had just studied that earlier this morning.
Q4: Bog standard short questions. No complaints.
Q5: Quite nice, I may have lost the odd three marks somewhere in the Mendeleev question.
Q6: A nice fuels question. Hydrolysis is a valid way of obtaining hydrogen from water, isn't it?
Q7: The first part of this question was a bit odd but the rest was pretty standard acids/base.
Q8: Loved this question. Especially because the ionic addition mechanism didn't come up.
Q9: B was fine but A was a bit odd. I got 0.4325 for the equilibrium calculation, did anyone else get that?
Q10: A and C were fine. B looked interesting but I was running out of time and didn't want to risk making an error in the calculation.
Q11: I did A and B in ten minutes as a final extra question, pretty bog standard bonding and water question.

All in all, it was one of the nicer papers I think.

partyatmygaff

aranciata wrote: »

anyone??? DID WE HAVE TO CONSIDER THE SIZE OF THE VESSEL IN THE EQUILIBRIUM CONSTANT QUESTION?

Even if you did take it into account it would have cancelled out anyway.

KoolAidRelic

jumpguy wrote: »

Are you sure? I assumed it was just 5 moles reacting away in a vessel of total capacity 12 litres. The capacity of the container wouldn't have any effect of the concentration of anything, and it asked for the concentration of hydrogen gas.

5 moles in a 12 litre flash is...5 moles, just in a 12 litre flask. 5 moles is the concentration. I'm not trying to be stand-offish here, I may be wrong.

Moles is not a concentration. Molarity is a concentration. They're not the same.
A concentration is an amount (moles, grams) per unit volume (lites, 100ml, etc).

Bogger_92

A very nice paper! over all Im very happy!

I think I may have made some basic numerical mistakes with my answers, i.e being two or three decimal places off? anyone know if Ill be docked many marks if so?

still a very nice paper, long questions were a delight!

partyatmygaff

KoolAidRelic wrote: »

Moles is not a concentration. Molarity is a concentration. They're not the same.
A concentration is an amount (moles, grams) per unit volume (lites, 100ml, etc).

It makes absolutely no difference to that question though. There are two molecules on either side so both the products and reactants will have a denominator of 144 which cancels out.

KoolAidRelic

partyatmygaff wrote: »

It makes absolutely no difference to that question though. There are two molecules on either side so both the products and reactants will have a denominator of 144 which cancels out.

I had:
Kc = (x^2)/(5/12 -2x)... which wouldn't cancel out.

Which gave x as being .04455...

oh! I realised that you had it:
Kc = (x/12)^2/((5-2x)/12)^2

That'd give x as .5346... but you'd have to then divide that by 12 because the concentration of Hydrogen is now x/12 and not just x.

YodaBoy

Got x=7 for the hydration in Q4
Q1: e(i) 0.322M
(ii) 6.44M

f(i) 479.78g/L
(ii) 4.79%

Q9...what was x??? I got 1.6 moles but think i made more than 1 mistake?

River Song

I got 4.5% or there abouts for the (w/v) question. Not happy with the paper tbh, all the wrong stuff for me came up ><

M_ark

Ah I thought that was lovely yeah! Was a bit confused by the periodic table question in 5(d)(iii) though, wasn't sure whether they were asking for transition metals or just a d sublevel...

._.

Pretty sure I got a B2.. Pretty disappointed tbh, went down from 83% in my mocks... Wanted an A, etc etc..
A B2 will probably be fine for me for points, but still..

YodaBoy

M_ark wrote: »

Ah I thought that was lovely yeah! Was a bit confused by the periodic table question in 5(d)(iii) though, wasn't sure whether they were asking for transition metals or just a d sublevel...

yeah i was just tryin that bit when we were told to stop
one has a full 3d10 and the other has 3d1, no?? doesnt look too similar!

YodaBoy

has any1 got the figures for all the calculations please!!!!!!?

M_ark

YodaBoy wrote: »

yeah i was just tryin that bit when we were told to stop
one has a full 3d10 and the other has 3d1, no?? doesnt look too similar!

Was it not asking for the series from one to the other? I just wasn't sure whether they were taking Zinc to Scandium inclusive or exclusive... ARGH!

aranciata

M_ark wrote: »

Was it not asking for the series from one to the other? I just wasn't sure whether they were taking Zinc to Scandium inclusive or exclusive... ARGH!

I think the answer is that because they're transitional metals, they all form ions with partially filled d-sublevels? At least that's what I said..

Pre Judas

aranciata wrote: »

I think the answer is that because they're transitional metals, they all form ions with partially filled d-sublevels? At least that's what I said..

Wasn't it that neither of them obey the Aufbau Princepal because they don't occupy the lowest available energy level or something?

stealinhorses

I thought the exam wasn't too hard. I might have gotten an A, though I made a couple of mistakes. Delighted to finish my exams on a high though .

mrboswell

It will probably depend on what they designed the question to be and what students perceived it to be.

They are all d-block/transition metals - might be some marks for that alone.

Zinc does obey aufbau because electrons have filed 3d so it is at the lowest energy, as does scandium.
Scandium is 3d1 so if anything they might be looking for looking for Hunds rule to show the orbitals are filled singly starting with Scandium until zinc fill them up. They may have expected chromium and copper to show the exceptions.

Haven't seen the question/paper though so I'm 100% sure.

Pre Judas

mrboswell wrote: »

It will probably depend on what they designed the question to be and what students perceived it to be.

They are all d-block/transition metals - might be some marks for that alone.

Zinc does obey aufbau because electrons have filed 3d so it is at the lowest energy, as does scandium.
Scandium is 3d1 so if anything they might be looking for looking for Hunds rule to show the orbitals are filled singly starting with Scandium until zinc fill them up. They may have expected chromium and copper to show the exceptions.

Haven't seen the question/paper though so I'm 100% sure.

I know they both partially fill the 4s sublevel, but not fully because they either half-fill or fully fill the d level instead of fully filling the 4s, which means they haven't occupied the lowest available energy level.

KoolAidRelic

They both fully fill the 4s sub-level - only Copper and Chromium (out of the ten elements in that series) don't obey the Aufbau principle.

The 4s is of a lower energy than the 3d, and consequently fills up completely and before the 3d does.
(Though actually, Copper and Chromium probably do obey it, having five in 3d and one in 4s is probably of lower energy than four in 3d and one in 4s [10 for Copper] due to half-full sub-levels being more stable)

macskanadrag

I thought the question was asking from one element to the other and not just asking about those 2. So i wrote they all have an at least partially filled d-sublevel. Zn has a full one though and is not a transition metal afaik. Too unbothered to look up if this is true

mrboswell

Pre Judas wrote: »

I know they both partially fill the 4s sublevel, but not fully because they either half-fill or fully fill the d level instead of fully filling the 4s, which means they haven't occupied the lowest available energy level.

Zinc and Scandium have fully filled 4s, so they have occupied the lowest energy level.

The exceptions:
4s and 3d are very close in energy. Chromium is 4s1 3d5 because that configuration has a lower energy than 4s2 3d4. This is because there would be more repulsion between two electrons in an orbital than the six unpaired electrons with a 4s1 3d5 configuration.
Similarly with copper 4s1 3d10 is of lower energy than 4s2 3d9 would be because a full set of 3d orbitals combined with the 4s1 would be of a lower energy because it would be more stable with the full set of d orbitals.

To be honest I think you are over thinking it. Just forget it any enjoy your holidays

mrboswell

macskanadrag wrote: »

I thought the question was asking from one element to the other and not just asking about those 2. So i wrote they all have an at least partially filled d-sublevel. Zn has a full one though and is not a transition metal afaik. Too unbothered to look up if this is true

Well you are right, except for Zn. It is effectively a weaker metal but a metal and d-block element none the less.

KoolAidRelic

macskanadrag wrote: »

I thought the question was asking from one element to the other and not just asking about those 2.

It was. They'll probably give marks for any correct statement about them!

4s and 3d are very close in energy. Chromium is 4s1 3d5 because that configuration has a lower energy than 4s2 3d4. This is because there would be more repulsion between two electrons in an orbital than the six unpaired electrons with a 4s1 3d5 configuration.
Similarly with copper 4s1 3d10 is of lower energy than 4s2 3d9 would be because a full set of 3d orbitals combined with the 4s1 would be of a lower energy because it would be more stable with the full set of d orbitals.

I knew it'd be because of that! We were just told "they're exceptions, learn them and deal with it!".

mrboswell

KoolAidRelic wrote: »

I knew it'd be because of that! We were just told "they're exceptions, learn them and deal with it!".

At the end of the day my explanation wouldn't get you any extra marks!!

Enjoy the summer folks!

rainbowtrout

Haven't looked back on any previous marking schemes, but when doing out an equilibrium question you normally do out the initial line, then the equilibrium line and then the concentration at equilibrium. Now because you're not given equilibrium most of the time, a lot of people tend to skip this line as they see it as a waste of time cancelling out Vs. The volume would have mattered if the solution didn't fit so neatly as it could be square rooted. So personally I think the line should have been in there to show you understand how it works. Time will tell however.

aranciata wrote: »

Yes let's hope so! I think I got a figure of .5354 - not sure about that cause i literally did it in the last minute of the exam.

Raging that I used the wrong ratio in the bleach titration, could have lost 20 marks..

I got the same answer just now.

jumpguy

rainbowtrout wrote: »

Haven't looked back on any previous marking schemes, but when doing out an equilibrium question you normally do out the initial line, then the equilibrium line and then the concentration at equilibrium. Now because you're not given equilibrium most of the time, a lot of people tend to skip this line as they see it as a waste of time cancelling out Vs. The volume would have mattered if the solution didn't fit so neatly as it could be square rooted. So personally I think the line should have been in there to show you understand how it works. Time will tell however.

Rainbowtrout, could you tell me what's the story for making obvious mathematically slips/blunders in chemistry? Do you lose like 3 marks and they carry your answer or does it carry on and you continue to lose marks for subsequent parts? Just wondering because I got that hydrogen concentration in the Kc wrong and losing 12 or 9 marks wouldn't be welcome!


EDIT: NVM...just checked the marking scheme, -1, hopefully!

nommm

I got .5354 too!

rainbowtrout

jumpguy wrote: »

Rainbowtrout, could you tell me what's the story for making obvious mathematically slips/blunders in chemistry? Do you lose like 3 marks and they carry your answer or does it carry on and you continue to lose marks for subsequent parts? Just wondering because I got that hydrogen concentration in the Kc wrong and losing 12 or 9 marks wouldn't be welcome!

Not a clue to be honest. It's a few years since I had a LC chemistry class. I have a feeling ( don't quote me on this) that it's similar to maths, ie if you get the wrong answer somewhere along the line, you'll lose marks at that point but if you continue to do the question correctly using your answer through the rest of the question you should get the other marks.

I notice one or two people said they got 3.9 for the concentration of hydrogen iodide, but the question asked for the concentration of hydrogen. However I'm taking that to mean that if they were able to work out 2x correctly that they already got x

EDIT: From the 2010 Marking Scheme :

There is a deduction of one mark for each arithmetical slip made by a candidate in a calculation.

So it's not the end of the world.