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Chemistry - reactions

I am having some trouble with the type of question which asks what colour a solution turns after a reaction. Is there a set rule to remember this by?
Thanks:smile:
Reply 1
Avatar for L33t
L33t
10
Original post by Offmyrocker
I am having some trouble with the type of question which asks what colour a solution turns after a reaction. Is there a set rule to remember this by?
Thanks:smile:


You need to be more specific in terms of what kind of chemistry you're talking about. Transition metal complexes for example, their colours are based on the oxidation state of the ion involved, other reactions like those of the halides with silver nitrate are precipitation and produce white, cream and yellow precipitates.

If you can tell me the type of reaction in question, I can help you a lot more :smile:
(edited 7 years ago)
Reply 2
Avatar for Lockton
Lockton
2
Original post by Offmyrocker
I am having some trouble with the type of question which asks what colour a solution turns after a reaction. Is there a set rule to remember this by?
Thanks:smile:


Are you looking for coloured precipitates?
Reply 3
Avatar for Offmyrocker
Offmyrocker
OP
6
Hi, thanks for your replies. The kind of question I am struggling with are things such as the following:
"State the colour of bromine water after it has reacted with an alkene"?
This isn't my strong point and I wondered if there is any easy way to remember/rule to follow in these types of questions?
Reply 4
Avatar for L33t
L33t
10
Original post by Offmyrocker
Hi, thanks for your replies. The kind of question I am struggling with are things such as the following:
"State the colour of bromine water after it has reacted with an alkene"?
This isn't my strong point and I wondered if there is any easy way to remember/rule to follow in these types of questions?

Not to be pedantic but you need to say "The solution goes from orange to colourless" in order to gain the mark in an exam question. It is just something that you need to remember i'm afraid, it is a fairly standard chemical result.

C2H4+Br2(aq)==>C2H4Br2C_2H_4 + Br_2(aq)==> C_2H_4Br_2

Bromine water is orange (because it just is) and when it reacts with an alkene, the double bond breaks and the bromine (which is diatomic therefore 2) will join onto the molecule. This happens because as bromine approaches the double bond (an area of high electron density) a dipole is induced in bromine, the double bond breaks and the bromine is bonded leaving a positive charge on the intermediate which the left over bromine species bonds to.This forms 1,2-dibromoethane. :smile: Hope that helps.
(edited 7 years ago)
Original post by Offmyrocker
Hi, thanks for your replies. The kind of question I am struggling with are things such as the following:
"State the colour of bromine water after it has reacted with an alkene"?
This isn't my strong point and I wondered if there is any easy way to remember/rule to follow in these types of questions?


Hi. For that particular question, you need to know that alkenes decolourise bromine water (because bromine breaks the double bonds in the alkene to form new bonds with the alkene to form a halogenoalkane in an addition reaction. This uses up all the orange* Br2 (aq) to form the halogenoalkanes, which are colourless, so the solution turns from brown to colourless). You might also be asked to describe the test for an alkene: the answer to this is that alkenes decolourise bromine water.

More generally, there aren't really any fixed rules, though there are trends. For example,the colours of silver halide precipitates go darker as you go down the halides (white/cream/yellow for AgCl/AgBr/AgI respectively). You also need to memorise the colours of the transition metal ions which come up in your specification.

I have a page of all the colours that I need to know for my exams, and a few hours before a chemistry exam I tend to just cram the colours (as well as the industrial properties such as temperatures, pressures and catalysts for various industrial reactions).
(edited 7 years ago)
Reply 6
Avatar for alow
alow
19
Bromine water is transparent, it's just brown. Colourless is the word you're looking for.
Reply 7
Avatar for L33t
L33t
10
Original post by alow
Bromine water is transparent, it's just brown. Colourless is the word you're looking for.


That's what I said!
Reply 8
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alow
19
Original post by L33t
That's what I said!


Good for you!
Reply 9
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L33t
10
Original post by alow
Good for you!


SMH try reading posts in future before posting dupes- simple :smile:
Reply 10
Avatar for alow
alow
19
Original post by L33t
SMH try reading posts in future before posting dupes- simple :smile:


I quoted the person who was wrong. You did not. They may have never seen your post.
Thank you all for your replies.

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