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Edexcel Chemistry A2 Unit 5 ~ Wednesday 19th June 2013 (Now Closed)

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Original post by Jammie Dodgers
I'm pretty sure we don't need to know about the reactions of Cobalt as a transition metal/ion, right? (I can't find it on the specification)


Nope. Copper and Chromium are the main ones. But of course you will have to know about deprotonation of the hydrated copper ion, the colours etc
Reply 821
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GeorgeL3
5
Does anybody have a list of/know exactly which transition metal colours we need to learn?
Do we need to remember colours in aqueous solution, with small NH3/NaOH & with excess NH3/NaOH for the different metals (Cr,Mn,Fe,Ni,Cu)?
Do we also need to remember colours of different oxidation states and with other ligands?
I had a look at the specification but I couldn't see it mention learning specific colours anywhere.
Thanks a lot if you can help.
Original post by bubblegummer
Can anyone explain what is the difference between E, Edelta and Ecell ?


E on it's own is the electrode potential of that particular half cell ... or "half equation" :smile:

Ecell is 2 half cells combined together.... or the sum of two E (electrode potentials) .... or the sum of two half equations :biggrin: However you want to look at it :tongue:

I've never come across Edelta... but I would assume the change in E would be the E cell....
I think it may have been linked before but i can't find it with a search (or I'm dreaming that it ever existed...) but does anyone have the old paper questions available? Thanks :smile:
Original post by Weaselmoose
I think it may have been linked before but i can't find it with a search (or I'm dreaming that it ever existed...) but does anyone have the old paper questions available? Thanks :smile:



http://www.thestudentroom.co.uk/showthread.php?t=2230370&page=4&p=41205680&highlight=happy%20to#post41205680


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Guys, this always gets me...

We know that a negative rod is called the cathode (because they attract cations)
& positive rods anodes....but in the case of redox electron transfer:

"Oxidation takes place at the zinc rod, which is called the anode"
"Reduction takes place at the copper rod, which is called the cathode"

Is there a reason for this?? :confused: I initially thought this was wrong because the overall net charge suggests the opposite.

Thanks in advance :smile:
Original post by posthumus
Guys, this always gets me...

We know that a negative rod is called the cathode (because they attract cations)
& positive rods anodes....but in the case of redox electron transfer:

"Oxidation takes place at the zinc rod, which is called the anode"
"Reduction takes place at the copper rod, which is called the cathode"

Is there a reason for this?? :confused: I initially thought this was wrong because the overall net charge suggests the opposite.

Thanks in advance :smile:


Does OIL RIG, ring any bells?

Oxidation is Loss of Electrons.

Reduction is Gain of Electrons.
Original post by James A
Does OIL RIG, ring any bells?

Oxidation is Loss of Electrons.

Reduction is Gain of Electrons.


But the rod itself is negative though ? :frown: Could you explain further please...

It also states that negative electrolytes move towards to anode... which I'm so used to being positive.

Oxidation at Zinc rod would mean that Zn^2+ is produced, is that what makes it an anode ? but that's in the solution, the rod is still negative :confused:

EDIT: Oh no... it says Anions go to the anode... therefore to anions such as Cl- go to the negative zinc rod ? Since that's what was suggested earlier

EDIT no.2: Well I read further on :tongue: Anions go towards the anode because the solution is positively charged .... so which one is that ? with the most positive charge.

Is it the Zinc side because equilibrium is more to the right Zn <------> Zn2+ + e-

Severely confused now :frown:
(edited 10 years ago)
Reply 828
Avatar for GeorgeL3
GeorgeL3
5
Original post by posthumus
Guys, this always gets me...

We know that a negative rod is called the cathode (because they attract cations)
& positive rods anodes....but in the case of redox electron transfer:

"Oxidation takes place at the zinc rod, which is called the anode"
"Reduction takes place at the copper rod, which is called the cathode"

Is there a reason for this?? :confused: I initially thought this was wrong because the overall net charge suggests the opposite.

Thanks in advance :smile:


Cathodes attracts cations.
Cations are positively charged so the cathode is negative.
The positive cations are drawn into the negative cathode and when they get there, electrons are gained by the cation from the cathode. Reduction is gain of electrons so it happens here.

Anodes attract anions.
Anions are negatively charged so the anode is positive.
The negative anions are drawn towards the positive anode and lose their extra electrons to the anode. Oxidation is the loss of electrons so it happens here.

Remember it is the ions in solution which are oxidised/reduced, not the electrode itself.
Original post by GeorgeL3
Cathodes attracts cations.
Cations are positively charged so the cathode is negative.
The positive cations are drawn into the negative cathode and when they get there, electrons are gained by the cation from the cathode. Reduction is gain of electrons so it happens here.

Anodes attract anions.
Anions are negatively charged so the anode is positive.
The negative anions are drawn towards the positive anode and lose their extra electrons to the anode. Oxidation is the loss of electrons so it happens here.

Remember it is the ions in solution which are oxidised/reduced, not the electrode itself.


So the zinc rod even though it is negative itself - I shalll called it the anode? :frown: I would understand if the solution it was dipped in was called the anode (since it's positive) & I think it attracts anions too since the equilibrium is more to the right:

Zn <------> Zn2+ + e-

Is the rod called anode because of this ? If so, then I'm confused why George Facer says the zinc rod itself is negative relative to the solution :confused:
Reply 830
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GeorgeL3
5
Original post by posthumus
So the zinc rod even though it is negative itself - I shalll called it the anode? :frown: I would understand if the solution it was dipped in was called the anode (since it's positive) & I think it attracts anions too since the equilibrium is more to the right:

Zn <------> Zn2+ + e-

Is the rod called anode because of this ? If so, then I'm confused why George Facer says the zinc rod itself is negative relative to the solution :confused:

Zn(2+) + 2e- → Zn E = -0.76V
Cu(2+) + 2e- → Cu E = +0.34V

Zn + Cu(2+) → Zn(2+) + Cu E = +1.10V

Zn(s) | Zn(2+) || Cu(2+) | Cu(s)

The RH electrode is positive with respect to the LH electrode because electrons flow from Zinc to Copper.
I don't know if you do physics or remember from GCSE but the negative terminal (cathode) in a cell is 'where the electrons come from' and the positive terminal (anode) is 'where the electrons flow to'.

I wouldn't worry about it too much though, as long as you can write the half equations and work out Ecell values you'll probably be fine.
How part iii??????? For 3 Marks, Can someone fully explain then I will tell you mark scheme answer :colondollar:
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Reply 832
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jojo1995
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Original post by Knoyle quiah
How part iii??????? For 3 Marks, Can someone fully explain then I will tell you mark scheme answer :colondollar:



steel is made up of iron.
a more negative e value means that the element is oxidized more easily.
zinc is therefore oxidised more eaily than iron.
so in the presence of oxygen in air, zinc will protect iron from being oxidsed by being oxidised itself.
whilst in the case of sn, sn is harder to oxidise than fe so sn will not protect steel in the presence of air (oxygen).

have i explained it okay?
Original post by Knoyle quiah
How part iii??????? For 3 Marks, Can someone fully explain then I will tell you mark scheme answer :colondollar:


Ah damn I would have thought Tin would be preferred more :frown:

If it's not a problem... could you post the previous parts to the question as well please?
Original post by jojo1995
steel is made up of iron.
a more negative e value means that the element is oxidized more easily.
zinc is therefore oxidised more eaily than iron.
so in the presence of oxygen in air, zinc will protect iron from being oxidsed by being oxidised itself.
whilst in the case of sn, sn is harder to oxidise than fe so sn will not protect steel in the presence of air (oxygen).

have i explained it okay?


I thought you would cover the iron and make sure the material around it doesn't get oxidized easily/ corrode .... so it doesn't reach the iron :frown:

But what you said is probably right :redface:
Reply 835
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jojo1995
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Original post by posthumus
I thought you would cover the iron and make sure the material around it doesn't get oxidized easily/ corrode .... so it doesn't reach the iron :frown:

But what you said is probably right :redface:


yeah that is what you do - you are right ... did i say something different ? :redface:
Original post by jojo1995
steel is made up of iron.
a more negative e value means that the element is oxidized more easily.
zinc is therefore oxidised more eaily than iron.
so in the presence of oxygen in air, zinc will protect iron from being oxidsed by being oxidised itself.
whilst in the case of sn, sn is harder to oxidise than fe so sn will not protect steel in the presence of air (oxygen).

have i explained it okay?


Yes thanks this had made it clearer :smile: but why does a more negative e value mean its oxidised more easily??? and what side of the equation is oxidised more easily if more negative, cant be both sides???????????
Reply 837
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jojo1995
2
Original post by Knoyle quiah
Yes thanks this had made it clearer :smile: but why does a more negative e value mean its oxidised more easily??? and what side of the equation is oxidised more easily if more negative, cant be both sides???????????


you are welcome :- )

the side without the electron on it is oxidised more easily to release the electron (which is on the other side)

the e value represents reduction potential of a species. so if the value is negative it means its harder to reduce but easier to oxidise
Original post by jojo1995
yeah that is what you do - you are right ... did i say something different ? :redface:


Hehe well I thought Tin... since tin has a more positive electrode potential than zinc therefore its a good oxidizing agent and is easily reduced.

Zinc is more easily oxidized (meaning it corrodes more easily?), and that's what you said is needed :frown:
Reply 839
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jojo1995
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Original post by posthumus
Hehe well I thought Tin... since tin has a more positive electrode potential than zinc therefore its a good oxidizing agent and is easily reduced.

Zinc is more easily oxidized (meaning it corrodes more easily?), and that's what you said is needed :frown:


a more negative e value means it is oxidised more easily. and if a species is oxisided more easily, it will react with oxygen and thus prevent fe reacting with it as it is oxidised more eaily than fe and therefore more reactive than fe when it comes to oxidation.

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