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AQA CHEM5 A2 Chemistry - 19th June 2013

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depends on the compound. Let's say the question is to do with NaCl, we'd expect atomisation to be 1/2Cl2 -> Cl

If they gave us, Cl2 -> 2Cl, then this is bond dissociation, so we need to half this value, to work out the atomisation.

So if they gave us Cl2 -> Cl2 for a a compound like MgCl2, we would just use the bond enthalpy?

Reply 1821

flyylikejetz

Original post by lifeisgood2012

you blud... in the table it says lattice DISSOCIATION... YOU NEED LATTICE FORMATION SO YOU CHANGE THE + TO A NEGATIVE

Thanks man, didn't realise that! i definitely won't make that mistake in the exam tomorrow now!!! :biggrin:

Reply 1822

elliewoodheadx

Original post by 16dan2life

thats with NaOH, not diaminoethane

Oh okay thanks very much

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Reply 1823

MrMeep2580

Original post by problematic

Hey guys, quick question and sorry if this has already been answered.

I;ve attached a table from june11 paper q 5 and its asking to do a cell representation using the Zn and Ag2o equestions.

I just wanted to know why Pt boundaries are penalised, and what is the general rule when to put the Pt and when not to.

thank you

You have two solids on both sides therefore a platinum electrode isn't required.

Reply 1824

16dan2life

Original post by Atz23

hello, can someone explain to me question 3d on Jan 2013 please? it would be a big help....I do not get why you do 70-189.

question is saying that gaseous water has condensed to liquid water. In other words, H2O(g)-->H2O(l)

seeing as delta G is zero, and we're not given a value for delta H of liquid water, we're left with deltaH - T(deltaS). We need to work out delta H, so our equation is:

delta H = deltaS(T)
delta S = 70-189 = -119, divide it by 1000 to change the units of it into K J mol -1 = -0.119

so, -0.119 times 373 = -44.4 Kjmol-1

Reply 1825

marleyxd

Original post by lewiszorz

In the electrochemical cell, where it's like Zn and Cu, how do you know that Zn has more tendency to oxidise to Zn2+ than Cu?

If the emf of Zn is more negative than Cu it will be oxidised

Reply 1826

16dan2life

Original post by marleyxd

So if they gave us Cl2 -> Cl2 for a a compound like MgCl2, we would just use the bond enthalpy?

Cl2 -> 2Cl *

yes

Reply 1827

starfish232

Original post by hawraaj313

thanks, it was on the mark scheme thats why :s-smilie:

which markscheme was this?

Reply 1828

MrMeep2580

Original post by popnit

No i know that, how do you figure out the equation? More specifically the 2 moles of each reactant except H2O2

You know its yellow solution so you know its:

2[Co(NH3)6]^(2+) + H2O2 ----> 2[Co(NH3)]^(3+) + 2OH-

Reply 1829

marleyxd

Original post by 16dan2life

Cl2 -> 2Cl *

yes

yeah thats what i meant haha, cheers!

Reply 1830

Kaii_Smith

Original post by Atz23

hello, can someone explain to me question 3d on Jan 2013 please? it would be a big help....I do not get why you do 70-189.

Your trying the work out delta S so you do:
sum of delta S of products - sum of Delta S of reactants = delta S
so use the Detla S values of H2O (g) -> H2O (l)
So 70 (H2O (l)) - 189 (H2O (g))

Hope this helped and you can understand what I've written :smile:

Reply 1831

popnit

Could someone pls explain the Qs8b, the final bit that says Suggest a mechanism for the catalysed reaction....
http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN12.PDF
Thanks

Reply 1832

mcollin1

do we need to know the CrO4 2- -> Cr2O7 2- equation?

Reply 1833

popnit

Original post by MrMeep2580

You know its yellow solution so you know its:

2[Co(NH3)6]^(2+) + H2O2 ----> 2[Co(NH3)]^(3+) + 2OH-

I know but I can't figure out why you put 2 moles!

Reply 1834

edward28

Original post by Melissa.S.

So confused, for lattice enthalpies , when the experimental values in born haber cycles is bigger than the value from theory, it means the bonding is stronger because the ions are polarised, so it contains partial covalent bonding. But then in the periodicity unit Al203 has a lower melting point because it contains covalent bonding??

So does covalent character in ionic bonding make bonds stronger or weaker??

I was also really confused about that!!! But i think the polarisation makes it stronger than the theoretical. I am not too sure but I have made myself just accept it and live with it.

Reply 1835

16dan2life

Original post by mcollin1

do we need to know the CrO4 2- -> Cr2O7 2- equation?

Yes. It was asked in Jan 2013.

Reagent- H2SO4

2CrO42- + 2H+ --> [Cr2O7]2- + H2O

CrO42- = yellow solution
Cr2O7 2- = Orange solution

Reply 1836

PeanutButter2000

I know it's been posted before but i can't find the link to the January paper, could someone put up the link or quote the post it was in please! thanks :smile:

edit: found it!

(edited 10 years ago)

Reply 1837

peebs123

Can anyone explain how you calculate the temperature at which the reaction is NOT feasible?