Edexcel Chemistry A2 Unit 5 ~ Wednesday 19th June 2013 (Now Closed) Watch

Poll: How pumped up are you for this exam?-(warning)-(bad jokes arene this poll!)
"Titanium-I'm not going to corrode (even at high temperatures)" (A*) (22)
16.67%
"Benzene's my middle name, give me the paper in a week and I'll ace it!" (A) (27)
20.45%
"Yeah, I'm fairly electrophillic (positively charged) about the exam" (B) (27)
20.45%
"I'm in the middle of the salt bridge, but I will pass-eventually" (C) (21)
15.91%
"I'm feeling rather electroNegative about this exam" (D) (18)
13.64%
"Benzene, what's that?" (E) (6)
4.55%
"Chemistry, what's that?" (F) (11)
8.33%
This discussion is closed.
posthumus
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(Original post by LeaX)
if we were asked to show how zinc hydroxide is amphoteric by equations are these correct:

(Zn(OH)2(H2O)4] + H+ --> [Zn(OH)(H2O)5]+1
[Zn(OH)2(H2O)4] + OH- --> [Zn(OH)3(H2O)3]-1 +H2O
I don't see why not
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MustaphaMond
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#1562
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Run out of past papers, any unofficial ones or extra questions anywhere?
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freakynerdlol
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Why is the answer B? Isnt 1moldm-3 the requirement for a standard reaction?

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AtomicMan
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(Original post by freakynerdlol)
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Why is the answer B? Isnt 1moldm-3 the requirement for a standard reaction?

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You have 2 cromium atoms in each chromium sulphate molecule, so you need 0.5moldm-3 of chromium sulphate.
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LeaX
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(Original post by posthumus)
I don't see why not
thank you
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LeaX
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can someone please explain combustion analysis calculations? i've always struggled with knowing with diatomic molecules and their Mr value.

i know for working out masses of example C you do (12/44) X mass of CO2. but if we are given a mass of oxygen do we divide it by 16 or 32? also if we're given moles do we times by 16 or 32 to get mass?
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GeorgeL3
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(Original post by LeaX)
can someone please explain combustion analysis calculations? i've always struggled with knowing with diatomic molecules and their Mr value.

i know for working out masses of example C you do (12/44) X mass of CO2. but if we are given a mass of oxygen do we divide it by 16 or 32? also if we're given moles do we times by 16 or 32 to get mass?
You will be given the mass of the sample initially, mass of CO2 produced and mass of H2O produced:
1 – Calculate the mass of C and H produced.
C is 12/44 x mass of CO2
H is 2/18 x mass of H20
2 – Subtract the mass of C & H from initial mass to find the mass of Oxygen produced.
3 – Calculate moles of C, H & O produced.
Mass of C / 12
Mass of H / 1
Mass of O / 16
4 – Find the empirical formula by putting moles in a ratio and dividing through by the smallest value.

There was also 1 question where the compound had N in it too, you had to work this out in a different way and then take it away from the initial mass along with C & H to find the mass of O.

If they gave you the mass of Oxygen you'd just divide by 16 although this is unlikely, it's more likely if they gave you anything that it'd be volume and you could use moles = volume / 24
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Hunarench95
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Hi All, I'm struggling a little bit on a 'Part C' question.
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I've worked it through, and have found the moles of thiosulfate, and then the moles of iodine - but I'm unsure as to why the markscheme then suggests dividing the moles of iodine by three to get the moles of dichromate unreacted?

EDIT: This is June 2011
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Hunarench95
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^ I think I may have answered my own question! I get it.

Thanks anyway!
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LeaX
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(Original post by GeorgeL3)
You will be given the mass of the sample initially, mass of CO2 produced and mass of H2O produced:
1 – Calculate the mass of C and H produced.
C is 12/44 x mass of CO2
H is 2/18 x mass of H20
2 – Subtract the mass of C & H from initial mass to find the mass of Oxygen produced.
3 – Calculate moles of C, H & O produced.
Mass of C / 12
Mass of H / 1
Mass of O / 16
4 – Find the empirical formula by putting moles in a ratio and dividing through by the smallest value.

There was also 1 question where the compound had N in it too, you had to work this out in a different way and then take it away from the initial mass along with C & H to find the mass of O.

If they gave you the mass of Oxygen you'd just divide by 16 although this is unlikely, it's more likely if they gave you anything that it'd be volume and you could use moles = volume / 24
thank you
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imasha.sj
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Hey guys!

I really need some help with the January 2012 paper question 21(b)(ii)...how is it that you select a suitable metal for the reaction by considering the standard electrode potential? Can someone please explain? Thanks in advance
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clydeshen411
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(Original post by Hunarench95)
Hi All, I'm struggling a little bit on a 'Part C' question.
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Name:  Screen Shot 2013-06-16 at 20.18.29.png
Views: 100
Size:  18.9 KB

I've worked it through, and have found the moles of thiosulfate, and then the moles of iodine - but I'm unsure as to why the markscheme then suggests dividing the moles of iodine by three to get the moles of dichromate unreacted?

EDIT: This is June 2011
moles of I2 in the reacted with thiosulphate ions is equal to I2 produced from dichromate ions. So moles of I2 in the second equation = 3I2 in the first equation, thus you divide it by 3 to get amounts of cr2o7.

Hope it helps
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clydeshen411
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(Original post by imasha.sj)
Hey guys!

I really need some help with the January 2012 paper question 21(b)(ii)...how is it that you select a suitable metal for the reaction by considering the standard electrode potential? Can someone please explain? Thanks in advance
CR3+ turns into cr2+, the E cell = -0.41, and its on right hand side since it gets reduced, so the left hand cell must have a E value more negative than -0.41 so the overall E is positive.
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imasha.sj
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(Original post by clydeshen411)
CR3+ turns into cr2+, the E cell = -0.41, and its on right hand side since it gets reduced, so the left hand cell must have a E value more negative than -0.41 so the overall E is positive.
Thank you but why can't we use any metal other than the ones mentioned in the markscheme?
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clydeshen411
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(Original post by imasha.sj)
Thank you but why can't we use any metal other than the ones mentioned in the markscheme?
umm, i guess it's because metals in items 1-5 are too reactive? So it's difficult to get a solid electrode of pure Na for example.
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clydeshen411
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(Original post by GeorgeL3)
You will be given the mass of the sample initially, mass of CO2 produced and mass of H2O produced:
1 – Calculate the mass of C and H produced.
C is 12/44 x mass of CO2
H is 2/18 x mass of H20
2 – Subtract the mass of C & H from initial mass to find the mass of Oxygen produced.
3 – Calculate moles of C, H & O produced.
Mass of C / 12
Mass of H / 1
Mass of O / 16
4 – Find the empirical formula by putting moles in a ratio and dividing through by the smallest value.

There was also 1 question where the compound had N in it too, you had to work this out in a different way and then take it away from the initial mass along with C & H to find the mass of O.

If they gave you the mass of Oxygen you'd just divide by 16 although this is unlikely, it's more likely if they gave you anything that it'd be volume and you could use moles = volume / 24
Hi George, the notes u shared are amazing! but i do not understand how reducing a nitrile then add a weak acid can produce amines? i thought reduce nitrile will give aldehyde.
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imasha.sj
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(Original post by clydeshen411)
umm, i guess it's because metals in items 1-5 are too reactive? So it's difficult to get a solid electrode of pure Na for example.
Awesome Thank you
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GeorgeL3
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(Original post by clydeshen411)
Hi George, the notes u shared are amazing! but i do not understand how reducing a nitrile then add a weak acid can produce amines? i thought reduce nitrile will give aldehyde.
Remember a nitrile group is -C≡N
When you add LiAlH4 and a dilute acid, it reduces the nitrile group down to a primary amine.
As always, chemguide can probably explain it better than I can.
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clydeshen411
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(Original post by GeorgeL3)
Remember a nitrile group is -C≡N
When you add LiAlH4 and a dilute acid, it reduces the nitrile group down to a primary amine.
As always, chemguide can probably explain it better than I can.
Thanks!
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avipsita
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What do you guys think will come up in this paper?
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