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Equilibria, Energetics and Elements (F325) - June 2011 Exam.

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Guys lets make the Unofficial Mark Scheme Mk1
Please say if you have things to add, modify, or correct (theres lots!) :smile:

?)definition of lattice enthalpy: Energy given out when one mole of an ionic compound is formed from its constituent gaseous ions.[2]
?)Why is hydration enthalpy for Cl- exothermic? Bonds formed with deltaH+ on water molecules, heat given out.
?)Born Haber diagram of Enthalpy of solution, hydration, lattice enthalpy. 4 stages, incomplete answers: Mg2+(g) + 2Cl-(aq), Mg2+(aq) + 2Cl-(aq) [2]
?)
?)
graph question. Marks for calculating 3 half lives, ~200s, stating that constant half life means its 1st order. State that methanoic acid in excess, so [methanoic] is constant and can be disregarded in the rate equation. calculate k, answer and units= ?? s^-1
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?)
?)
?)
?)Magic tang question: use lactic acid because its pKa is close to our desired pH (within one unit), so it buffers better. I used henderson hasselbach equation, pKa = pH + log [A-]/[HA] or you can use the ka expression to work out [A-]/[HA], which (I got) ~0.49
I think it will be tangy because the buffering range (+/- 1 pH unit of pKa) certainly ecompasses the tangy pH of 3.55
?)
?)
?)
?)
?)
?)monomer unit for the complex ion polymer

?)

?)

?)

?)

?)

?) % of Cu in Brass question
Equations with state symbols:
Cu (s) + 4HNO3 (l) --> Cu(NO3)2 (aq) + 2NO3- (aq) + 2NO + 2H2O
Cu2+ + 2I- (aq) --> CuI (s) + 0.5 I2 (aq)
(one more which i can't rememebr)
n(Cu) = 0.5n(I2) = n(S2O3)
titre of 24.98 x 10^-3 cm^3 or summin, 0.1moldm^-3, n (S2O3)=that, so n(Cu) = 24.98 x 10^-3 cm^3
so m(Cu) = nM = 24.98 x 10^-3 * 63.5 = 1.59 g of Cu in 2.8g or summin brass.
% by mass= 67 point something percent, it asked to 1dp.

(edited 12 years ago)

Reply 2101

786girl

58/100 = A

plus a tangy sweet as compensation for mental torture.

Reply 2102

Student21

Original post by dan_dan_dan

I'll be happy with half marks. Drew a wizard on the magic tang question, only relevant thing I could think of.

I left an note in the paper towards the examiner pleading with him/her to slip in a couple of marks my way. Not going to happen though. :frown:

Reply 2103

Kalamari Dave

Original post by pavan pong

is anybody going to put up a mark scheme or something

workin on it, help out! :wink:

Reply 2104

wbataylor

Original post by pavan pong

it didnt say it was endothermic did it? it said it took place at zero so must it be exo as it requires no energy

Exactly what i thought

Reply 2105

Flint_09

Original post by abloomfield

i got 33.8% hmm.. The ionic equation for stage 4 was just next

yes!!!!!!!!!!!!!!!!!!!!!!!!! Thats what i got

Reply 2106

Rickesh

I thought the Br was a second order? :| !!!! Is that wrong!?!? It didnt have a constant half life did it?

Reply 2107

bexx-ry

Original post by Thuzz

sooo...
i failed that nicely

f324 retake next week better be merciful

This & my physics re-sit is my last hope...

Reply 2108

Kalamari Dave

Original post by Rickesh

I thought the Br was a second order? :| !!!! Is that wrong!?!? It didnt have a constant half life did it?

It did I'm afraid, ~200s :frown:

Reply 2109

tashahobo

Original post by susan23

Same ...actually believe it or not...i'm crying here just staring at the computer....93 is pretty low! You'll make it :smile:

i thought i was the only one, cant believe an exam actually brought me to tears :frown:

Reply 2110

Rickesh

Original post by Flint_09

yes!!!!!!!!!!!!!!!!!!!!!!!!! Thats what i got

I got that too !!

Reply 2111

RodrigoB

Original post by Student21

I left an note in the paper towards the examiner pleading with him/her to slip in a couple of marks my way. Not going to happen though. :frown:

PROPER LOL'ed now

Reply 2112

Vidja

From wikipedia: There are many different bronze alloys but modern bronze is typically 88% copper and 12% tin

Then I suppose my answer of 3.38% isn't correct.

Reply 2113

student777

Original post by Kalamari Dave

Guys lets make the Unofficial Mark Scheme Mk1
Please say if you have things to add, modify, or correct (theres lots!) :smile:

For the repeat unit I think you need:
Complex ion with 4 waters and the weird ligand thing
All inside square brackets with the charge
All inside curly brackets (like in the question) with an n for the repeak unit

Reply 2114

pavan pong

the exam was really hard and on the last q if i read it propely could have got a few marks. i forgot to divide the moles for sodium thiosulfate by 2 as cu was only one mole

Reply 2115

Rickesh

Original post by Kalamari Dave

It did I'm afraid, ~200s :frown:

I didnt get that, nor did a few people I asked, they said its not a first order, as it didnt have a constant half life. when i worked it out i didnt get a constant half life?

Reply 2116

RodrigoB

Original post by Flint_09

yes!!!!!!!!!!!!!!!!!!!!!!!!! Thats what i got

so did I !!! but I heard in real life brass has approx 60% of cupper lol

Reply 2117

missred93

Original post by Rickesh

I thought the Br was a second order? :| !!!! Is that wrong!?!? It didnt have a constant half life did it?

I got 2nd order aswell, because it didnt have a constant half life --the time went from around 190 to 400 which means it hasnt doubled = no constant half life?

I may be wrong though?

Reply 2118

Tphys

Well, I think I got about 55ish mark in this paper. Hopefully that'll equate to at least a high C. My mind just went blank and I swear a lot of the stuff I've never seen before. GG OCR.

Reply 2119

goldlock

Guys, if you write down a load of correct working and the correct answer but cross out the right answer and some of the working and put down a wrong answer instead, do you think you get to keep a few of those marks? I shot myself in the foot like that on the final question, so annoyed.. :/