Hudl
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I was reading this book and didnt quite understand something under the title, Amphoteric hydroides,

Al(H2O)3(OH)3 + OH- -----> [Al(OH)4]- + 3H2O

for when the complex acts as an acid

I would expect the equation to be:

Al(H2O)3(OH)3 + 3OH- -----> [Al(OH)6]3- + 3H2O


As looking at a similar amphoteric question, the reaction equation is:

Cr(H2O)3(OH)3 + 3OH- ----> [Cr(OH)6]3- + 3H2O





Also I think the AQA Nelson thornes book has made a mistake as it says that chromium ion exist as CrO4- and Cr2O7 2-
It then latter on in the book writes the equation as

2CrO4 2- + 2H+ ----> Cr2O7 2- + H20

So which is correct CrO4 - or CrO4 2-
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GDN
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(Original post by Hudl)
I was reading this book and didnt quite understand something under the title, Amphoteric hydroides,

Al(H2O)3(OH)3 + OH- -----> [Al(OH)4]- + 3H2O

for when the complex acts as an acid

I would expect the equation to be:

Al(H2O)3(OH)3 + 3OH- -----> [Al(OH)6]3- + 3H2O


As looking at a similar amphoteric question, the reaction equation is:

Cr(H2O)3(OH)3 + 3OH- ----> [Cr(OH)6]3- + 3H2O





Also I think the AQA Nelson thornes book has made a mistake as it says that chromium ion exist as CrO4- and Cr2O7 2-
It then latter on in the book writes the equation as

2CrO4 2- + 2H+ ----> Cr2O7 2- + H20

So which is correct CrO4 - or CrO4 2-
As for the chromate(VI) ion its definitely CrO4 2-.

Most textbooks do give the formula of the aluminate ion as [Al(OH)4]- but AQA mark schemes in the past have accepted equations for Al species the same as the chromium species
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Hudl
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(Original post by GDN)
As for the chromate(VI) ion its definitely CrO4 2-.

Most textbooks do give the formula of the aluminate ion as [Al(OH)4]- but AQA mark schemes in the past have accepted equations for Al species the same as the chromium species
Oh thank you, why is there this discrepancy surely its one or the other?


Also just reading the book further now and it says that for the reaction of [Fe(H2O)6]2+ with a little OH-

A green gelatinous ppt of Fe(H20)4(OH)2 is formed and the same thing is formed when OH- is added in excess.
But surely in excess OH- a full substitution reaction will occur and not just hydrolysis, meaning [Fe(OH)6]4- will form?
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GDN
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(Original post by Hudl)
Oh thank you, why is there this discrepancy surely its one or the other?


Also just reading the book further now and it says that for the reaction of [Fe(H2O)6]2+ with a little OH-

A green gelatinous ppt of Fe(H20)4(OH)2 is formed and the same thing is formed when OH- is added in excess.
But surely in excess OH- a full substitution reaction will occur and not just hydrolysis, meaning [Fe(OH)6]4- will form?
the precise nature of hydroxide precipitates and hydroxyl complexes is uncertain - the info given in textbooks is just an approximation to the truth. Unfortunately there is not simple pattern to the reactions of metal aqua ions with NaOH - you just have to learn the specifics for each of the meatal aqua ions studied
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Hudl
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(Original post by GDN)
the precise nature of hydroxide precipitates and hydroxyl complexes is uncertain - the info given in textbooks is just an approximation to the truth. Unfortunately there is not simple pattern to the reactions of metal aqua ions with NaOH - you just have to learn the specifics for each of the meatal aqua ions studied
Thanks a lot, +Ve

To finish off, my book says that [Fe(H2O)6]3+ is pruple/yellow/brown wtf lol. Which one of these colours is it or in an exam do you mention all 3?
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rakib567
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(Original post by Hudl)
Thanks a lot, +Ve

To finish off, my book says that [Fe(H2O)6]3+ is pruple/yellow/brown wtf lol. Which one of these colours is it or in an exam do you mention all 3?
Iron(III):: yellow / purple / brown / lilac / violet (solution) gives a brown /
rusty precipitate

from mark scheme
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Hudl
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(Original post by rakib567)
Iron(III):: yellow / purple / brown / lilac / violet (solution) gives a brown /
rusty precipitate

from mark scheme
Oh ok thanks, they will give you a mark for either of those colours....thanks

Ive been learning it as brown gelatinous ppt
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rakib567
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(Original post by Hudl)
Oh ok thanks, they will give you a mark for either of those colours....thanks

Ive been learning it as brown gelatinous ppt
fancy!
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