chemistry help pls
hi, please could i have some help with question 1b reaction with h2o2. It is not a reaction we learn so how can I figure it out?
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
Reply 1
Original post by anonymous56754
hi, please could i have some help with question 1b reaction with h2o2. It is not a reaction we learn so how can I figure it out?
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
As it says, you should refer to the data booklet.
H2O2 is an oxidising agent and so will classically oxidise most species (forming water or hydroxide as a side product), though there are a handful of species it can reduce (forming O2 and H^+ ions as side products).
Reply 2
Original post by anonymous56754
hi, please could i have some help with question 1b reaction with h2o2. It is not a reaction we learn so how can I figure it out?
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
1) NaOH
The hydroxide ions (OH⁻) take away protons from the water molecules, creating a neutral solid called chromium(III) hydroxide.
2) H2O2
Hydrogen peroxide can change Cr^3+ into Cr(VI), creating a peroxo complex like [Cr(O2)4]^3- or [Cr(O2)(OH)4]^3- when it's in a basic environment.
3) NH3
Ammonia takes the place of the water ligands, creating the hexaamminechromium(III) ion.
My 2 cents!
Original post by anonymous56754
hi, please could i have some help with question 1b reaction with h2o2. It is not a reaction we learn so how can I figure it out?
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
After what I have got in this reaction the oxidation number for the chromium species increase from +3 to +5. Thus electrons are donated. It must be an oxidation as type of reaction then.
@TypicalNerd and @Pigster can certainly help you better than I. I am honest, I am not so sure.
Reply 4
It is very pH-dependent what you get.
I know that in excess alkali, H2O2 oxidises chromium(III) to chromate(VI) and I am fairly certain that in acidic conditions, chromium(III) will oxidise to dichromate(VI).
It’s not a great question to be honest.
I know that in excess alkali, H2O2 oxidises chromium(III) to chromate(VI) and I am fairly certain that in acidic conditions, chromium(III) will oxidise to dichromate(VI).
It’s not a great question to be honest.
Reply 5
Original post by Nitrotoluene
1) NaOH
The hydroxide ions (OH⁻) take away protons from the water molecules, creating a neutral solid called chromium(III) hydroxide.
2) H2O2
Hydrogen peroxide can change Cr^3+ into Cr(VI), creating a peroxo complex like [Cr(O2)4]^3- or [Cr(O2)(OH)4]^3- when it's in a basic environment.
3) NH3
Ammonia takes the place of the water ligands, creating the hexaamminechromium(III) ion.
My 2 cents!
this was the ms
Reply 6
Original post by anonymous56754
this was the ms
Whilst Nitrotoluene is correct that you can form a range of other chromium complexes ([Cr(O2)4]^3- being the only example that I can recall where the oxidation state of chromium is +5), it’s best to use the electrode potentials in the data book and predict them that way.
Original post by TypicalNerd
It is very pH-dependent what you get.
I know that in excess alkali, H2O2 oxidises chromium(III) to chromate(VI) and I am fairly certain that in acidic conditions, chromium(III) will oxidise to dichromate(VI).
It’s not a great question to be honest.
I know that in excess alkali, H2O2 oxidises chromium(III) to chromate(VI) and I am fairly certain that in acidic conditions, chromium(III) will oxidise to dichromate(VI).
It’s not a great question to be honest.
Wait for a second! according to ph-value this kind of reaction can even be a reduction? does this value has such a strong effect on the ions in the species?
Reply 8
Original post by Kallisto
Wait for a second! according to ph-value this kind of reaction can even be a reduction? does this value has such a strong effect on the ions in the species?
Well, it’s reduction in so far as the oxygens in H2O2 each gain a single electron to change oxidation state from -1 to -2, but the chromium is always oxidised.
Chromium(III) is pretty difficult to reduce to chromium(II) or even chromium metal (e.g Cr(0)). You would need to use infinitely better reducing agents (say magnesium, for example) than H2O2 for starters and you’d ideally want to use a Schlenk line to access these oxidation states.
Original post by anonymous56754
hi, please could i have some help with question 1b reaction with h2o2. It is not a reaction we learn so how can I figure it out?
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
qp: https://bestexamhelp.com/exam/cambridge-international-a-level/chemistry-9701/2021/9701_s21_qp_42.pdf
thanks!
My two pence:
Rather than worry about what reactions Cr does or doesn't do in acid/alkaline conditions, do you know what reactions Co does in the conditions stated in the Q? and then assume that Cr reacts in a similar way.
Basically, H2O2 is going to push the Cr into a highest oxidation state and you should know about dichromate(VI) as it is the standard oxidant for alcohols. The MS would also allow chromate(VI), which perhaps might be the more likely product, but agreed it would be pH dependant.
Reply 10
Original post by Pigster
My two pence:
Rather than worry about what reactions Cr does or doesn't do in acid/alkaline conditions, do you know what reactions Co does in the conditions stated in the Q? and then assume that Cr reacts in a similar way.
Basically, H2O2 is going to push the Cr into a highest oxidation state and you should know about dichromate(VI) as it is the standard oxidant for alcohols. The MS would also allow chromate(VI), which perhaps might be the more likely product, but agreed it would be pH dependant.
Rather than worry about what reactions Cr does or doesn't do in acid/alkaline conditions, do you know what reactions Co does in the conditions stated in the Q? and then assume that Cr reacts in a similar way.
Basically, H2O2 is going to push the Cr into a highest oxidation state and you should know about dichromate(VI) as it is the standard oxidant for alcohols. The MS would also allow chromate(VI), which perhaps might be the more likely product, but agreed it would be pH dependant.
the reactions we learn for Co in transition metals is only for cl-, nh3 and oh- which is how i figured out the other two. Cr has multiple higher oxidation states so how do we know which one? Also how would we know if it is Cr3+ or Cr2o72-?
Reply 11
Original post by anonymous56754
the reactions we learn for Co in transition metals is only for cl-, nh3 and oh- which is how i figured out the other two. Cr has multiple higher oxidation states so how do we know which one? Also how would we know if it is Cr3+ or Cr2o72-?
The hint is in the question - it tells you to use the data booklet (i.e the index of electrode potentials).
You are starting from Cr^3+, so it would make sense to find one electrode system that involves Cr^3+ in some way and any others that involve H2O2.
Reply 12
Original post by TypicalNerd
The hint is in the question - it tells you to use the data booklet (i.e the index of electrode potentials).
You are starting from Cr^3+, so it would make sense to find one electrode system that involves Cr^3+ in some way and any others that involve H2O2.
You are starting from Cr^3+, so it would make sense to find one electrode system that involves Cr^3+ in some way and any others that involve H2O2.
Yep, I got it now thanks!
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