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AQA CHEM5 A2 Chemistry - 19th June 2013

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Reply 1780

neelam123h

Can someone please help me with q8b in the June 2012 paper.

Thanks :smile:

Reply 1781

ayshapatel

can anybody explain why complex ions are coloured- as in visible light and stuff??

and when do u multiply by 2 or by 0.5 when doing born haber cycles and lattice calculations?

Reply 1782

marleyxd

I hate how science works questions fffffffs

Reply 1783

rommy123

has everyone done the jan 13 paper and how well did you do?

Reply 1784

16dan2life

Original post by Learner_108

Fe(h20)3(oh)3 , Cr(h20)3(oh)3 , Cu(h20)3(oh)3 ,
Can anyone tell me the colours for these?

Posted from TSR Mobile

Brown solid, Green precipitate, respectively. Not sure about Cu(h2o)3(oh)3

Reply 1785

OFoster

Why does Al2O3 has a lower melting point than MgO, even though it is smaller and more highly charged?!

Reply 1786

lala12

Original post by ayshapatel

can anybody explain why complex ions are coloured- as in visible light and stuff??

and when do u multiply by 2 or by 0.5 when doing born haber cycles and lattice calculations?

Complex ions are coloured because light is absorbed by electrons in the d sub level, exciting them. They absorb some wavelengths of light but not all, the ones reflected are the ones you see.

And for the 1/2, the example I can think of is the atomisation enthalpy is half of the bond dissociation of Cl2.

Reply 1787

marleyxd

Original post by ayshapatel

can anybody explain why complex ions are coloured- as in visible light and stuff??

and when do u multiply by 2 or by 0.5 when doing born haber cycles and lattice calculations?

-Complex ions absorbs light from spectrum
-Electrons move to higher energy d orbital
-Combination of colours not absorbed is reflected (the colour of the complex)

You multiply the atomisation of Cl by 2 when its bonded to a group 2 metal.

Reply 1788

16dan2life

Original post by dizzy17

on the jan 2011 paper for 7c, how do they get the balanced equation between the hydrogen peroxide and manganate ions? is there a quick way of working it out or is it just trial and error?

You're meant to know it.

Oxidation- H2O2 -> O2 + 2H+ + 2e-
Reducation- MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O

Overall:
2MnO4- + 6H+ + 5H2O2 -> 2Mn2+ + 8H20

Reply 1789

lala12

Original post by OFoster

Why does Al2O3 has a lower melting point than MgO, even though it is smaller and more highly charged?!

It might be because MgO has ionic bonding whereas Al2O3 has ionic bonding but with covalent character so it weakens it.

Reply 1790

MrMeep2580

Original post by popnit

Please could someone explain the equation for reaction 2 in this paper. Qs 8a http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JUN12.PDF

Reaction two occurs when its left in the prescence of oxygen, oxygen oxidises it to Co3+

Or you can use hydrogen perioxide to oxidise Co2+

(edited 10 years ago)

Reply 1791

dobbie180

do we need to be able to draw EDTA?? STRESSING

Reply 1792

marleyxd

Original post by OFoster

Why does Al2O3 has a lower melting point than MgO, even though it is smaller and more highly charged?!

Al203 has covalent character

Reply 1793

lala12

Original post by 16dan2life

You're meant to know it.

Oxidation- H2O2 -> O2 + 2H+ + 2e-
Reducation- MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O

Overall:
2MnO4- + 6H+ + 5H2O2 -> 2Mn2+ + 8H20

Can you say H2O2 -> 2OH- + 2e- ??

Reply 1794

Kaii_Smith

Original post by dobbie180

do we need to be able to draw EDTA?? STRESSING

nope

Reply 1795

MrMeep2580

Original post by lala12

Can you say H2O2 -> 2OH- + 2e- ??

yes

Reply 1796

16dan2life

Original post by lala12

Can you say H2O2 -> 2OH- + 2e- ??

The overall equation won't come out right, as the reduction reaction has no OH- ions

when H2O2 is oxidising it's
H202 + 2e- -> 2OH-

Reply 1797

rommy123

Original post by dobbie180

do we need to be able to draw EDTA?? STRESSING

Nope!

Reply 1798

elliewoodheadx

How does Cr(H2O)6^3+ react with NH2CH2CH2NH2? In terms of the equation? Thanks

Posted from TSR Mobile

Reply 1799

DELETED ACCOUNT

Original post by dobbie180

do we need to be able to draw EDTA?? STRESSING

Doubt it. You need to know however that it acts as a multidentate by donating its lone pairs of electrons, which are present on 2 nitrogen and 4 oxygen atoms.