The big giant AQA CHEM5 thread!!! (May/June 2013)

Boom boom boom!! Know all the colours off by heart now


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What method did you use to memorise them?

What method did you use to memorise them?

I just wrote the different complexes that the different metals form because a few of them have the same ones repeated then just write them out several times.

Now I just gotta memories what you've gotta add to get that complex


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Just some questions. I would be really grateful if someone could have a go at explaining these to me! thanks in advance for all the help offered!

1) What is the difference when excess OH- or excess NH3 is used?

When excess OH- is used only Al and Cr react. Al reacts with 1 more OH to form [Al(OH)4]- and Cr reacts with 3 more to form [Cr(OH)6]3-. Because both of these are charged, they dissolve so form a colourless solution (Al) and a dark green solution (Cr).

With excess ammonia I remember all the metals beginning with 'C' react in excess. So Cobalt reacts with 6 more NH3 to form [Co(NH3)6]2+ which is a straw-coloured solution. Copper reacts with 4 NH3 to form [Cu(NH3)4(H2O)2]2+ which is deep blue. And Chromium reacts with 6 NH3 to form [Cr(NH3)6]3+ which is supposed to be purple.

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When excess OH- is used only Al and Cr react. Al reacts with 1 more OH to form [Al(OH)4]- and Cr reacts with 3 more to form [Cr(OH)6]3-. Because both of these are charged, they dissolve so form a colourless solution (Al) and a dark green solution (Cr).

With excess ammonia I remember all the metals beginning with 'C' react in excess. So Cobalt reacts with 6 more NH3 to form [Co(NH3)6]2+ which is a straw-coloured solution. Copper reacts with 4 NH3 to form [Cu(NH3)4(H2O)2]2+ which is deep blue. And Chromium reacts with 6 NH3 to form [Cr(NH3)6]3+ which is supposed to be purple.

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May help people learn the colours:
http://www.youtube.com/watch?v=_yVB85ugnig&list=UU9F-vfa-lqPgYUzE2mQoAWg&index=5
http://www.youtube.com/watch?v=7FFP2PFLPcU

Hello there,
When the metal aqua ions react to form a ppt , which one of the following is the equation for the reaction:

1) the one where the metal aqua ion reacts with H2O and then on the RHS there is H3O+ (along with the ppt)

or

2) the one where the metal aqua ion reacts with OH- ions and on the RHS you have got H2O as one of the products along with the ppt.


I looked in two different books and they give these two diff versions! which is right?

Probably being stupid but I don't really get what you're asking, sorry

If you're asking when it's a precipitate its when the product has not charge, eg [M(H2O)3(OH)3] is a precipitate.

Hello there,
When the metal aqua ions react to form a ppt , which one of the following is the equation for the reaction:

1) the one where the metal aqua ion reacts with H2O and then on the RHS there is H3O+ (along with the ppt)

or

2) the one where the metal aqua ion reacts with OH- ions and on the RHS you have got H2O as one of the products along with the ppt.


I looked in two different books and they give these two diff versions! which is right?

If you think about it, the reagent being added is sodium hydroxide. Sodium hydroxide dissociates into Na⁺ and OH^- ions..so you're effectively adding OH^- ions. The reaction is of hydroxide ions with the metal aqua ion, so the equation to write is that containing the hydroxide ion.

Remember for M²⁺ ions:

[M(H₂O)₆]²⁺(aq) + 2OH^-(aq) -> [M(H₂O)₄(OH)₂](s) + 2H₂O(l)

Each hydroxide ion acts as a base removing a proton from one water ligand, leaving a negatively charged OH- ligand.

If two of these OH^- ligands are formed, they result is an overall neutral charge on the complex. Because there is no charge, there is no reason for it to stay in solution (I've read that it is now considered "covalent", don't know if that's correct though), so it falls out of the solution as a solid precipitate.