Aqa chem 4/ chem 5 june 2016 thread

Can someone help me with bidentate ligands and M3+ ions? I assumed they would react in the same way as M2+ ions but according to http://filestore.aqa.org.uk/subjects/AQA-CHEM5-QP-JAN12.PDF Q8ei they don't. Why is this?

Reply 1881

thehollowcrown

12

Original post by shiney101

When they ask why a coloured ion is formed do you say the d-electrons absorb visible light or white light? I've always said it's visible light but then in one of the 2010 papers it said white light so now i'm not sure.

I always visible light

Reply 1882

GO97

8

Why are enthalpy changes approximate?
is it because averages are used?

Reply 1883

GO97

8

Original post by shiney101

When they ask why a coloured ion is formed do you say the d-electrons absorb visible light or white light? I've always said it's visible light but then in one of the 2010 papers it said white light so now i'm not sure.

i usually just talk about the visible light spectrum generally unless it tells u (or implies) in the question what colour it shows

Reply 1884

SirRaza97

7

Original post by anonymous-97

hi guys does anyone know how to get round thermodynamics im really struggling with it especially with the rearranging the born haber cycle ?

thanks in advance

I explained it here before. It might help

http://www.thestudentroom.co.uk/showthread.php?p=65043249#post65043249

Reply 1885

thenextchemist

21

Do we have to know how to name complexes?
Lie tetrachlorocobalate (II)

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Reply 1886

lahigueraxxx

2

Can someone please explain to me why Al2O3 has a higher melting point than SiO2?? Surely SiO2 should have the higher melting point as it's giant covalent?? Thanks in advance x

Reply 1887

Rubato

14

Original post by Crankyla

I think it is supposed to be [Al(OH)4(H2O)2]-. At least, that's how I've always written it.

Also, I'm unclear on the 'why' of substitution reactions. For example, [Cu(H2O)6]2+ reacts with an excess of ammonia, it forms [Cu(H2O)2(NH3)4]2+. When my class asked our Chemistry teacher why that happens and why aren't all the H2O ligands replaced, all she said was "It just does". Can someone explain it, please??

I asked my teacher this as well and it basically doesn't matter. I can't remember what they said either - something about NH3 being a better ligand than water although that doesn't explain why some water remains. Here's a link but as you can see, its super complicated and way beyond the spec.
http://chemistry.stackexchange.com/questions/29126/why-is-ligand-substitution-only-partial-with-copperii-ions-and-ammonia

Original post by shiney101

When they ask why a coloured ion is formed do you say the d-electrons absorb visible light or white light? I've always said it's visible light but then in one of the 2010 papers it said white light so now i'm not sure.

It absorbs a certain frequency of white light does it not? I'm pretty sure white light and visible light are the same thing but my physics isn't very strong. I wouldn't just say it absorbs white light though because then there will be no light remaining/ transmitted right?

Original post by GO97

Why are enthalpy changes approximate?
is it because averages are used?

What paper is this? If you're talking about enthalpy change calculated using mean bond enthalpy, then its because the values are averaged over a range of compounds.

Reply 1888

26december

2

Original post by lahigueraxxx

Can someone please explain to me why Al2O3 has a higher melting point than SiO2?? Surely SiO2 should have the higher melting point as it's giant covalent?? Thanks in advance x

Al2O3 is ionic and forms a giant ionic lattice.
SiO2 has macro molecular structure but only has covalent bonds holding structure

Basically it's because Al2O3 is ionic and I've always been told that ionic bonding is much stronger and harder to break apart than covalent bonding

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Reply 1889

Lilly1234567890

9

Original post by nutcase13

aluminium oxide

why?

Reply 1890

nutcase13

2

Original post by Lilly1234567890

why?

idk, im just quoting the book, i think it may have something to do with the electronegativity difference between Al and Si, although that could be wrong

Reply 1891

lahigueraxxx

2

Original post by 26december

Al2O3 is ionic and forms a giant ionic lattice.
SiO2 has macro molecular structure but only has covalent bonds holding structure

Basically it's because Al2O3 is ionic and I've always been told that ionic bonding is much stronger and harder to break apart than covalent bonding

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Oh I always thought that covalent bonding was stronger, especially if the structure is macromolecular :frown:

Thanks!

Reply 1892

12284

8

When AS (entropy) is negative for a reaction, is there more order or more disorder?

Reply 1893

shiney101

10

Original post by 26december

Al2O3 is ionic and forms a giant ionic lattice.
SiO2 has macro molecular structure but only has covalent bonds holding structure

Basically it's because Al2O3 is ionic and I've always been told that ionic bonding is much stronger and harder to break apart than covalent bonding

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Why is the melting point for Na2O less than SiO2?

Reply 1894

shiney101

10

Original post by 12284

When AS (entropy) is negative for a reaction, is there more order or more disorder?

More order, so basically less disorder if entropy is negative.

Reply 1895

thenextchemist

21

What are all the types of reactions?

There's ligand substitution
There's acid base

Anymore?

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Reply 1896

12284

8

Original post by shiney101

More order, so basically less disorder if entropy is negative.

Wait I'm confused now, I thought a reaction was more likely to occur if AS is negative?

Reply 1897

Crankyla

11

Original post by Manexopi

I asked my teacher this as well and it basically doesn't matter. I can't remember what they said either - something about NH3 being a better ligand than water although that doesn't explain why some water remains. Here's a link but as you can see, its super complicated and way beyond the spec.
http://chemistry.stackexchange.com/questions/29126/why-is-ligand-substitution-only-partial-with-copperii-ions-and-ammonia

Okay, I think I understand it... kind of. It makes sense that the water ligands would be competing for the binding sites with ammonia, especially if it's in high concentration. Thanks so much :smile:

Reply 1898

12284

8

Original post by shiney101

Why is the melting point for Na2O less than SiO2?