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

Poll: How pumped up are you for this exam?-(warning)-(bad jokes arene this poll!)
"Titanium-I'm not going to corrode (even at high temperatures)" (A*) (22)
16.67%
"Benzene's my middle name, give me the paper in a week and I'll ace it!" (A) (27)
20.45%
"Yeah, I'm fairly electrophillic (positively charged) about the exam" (B) (27)
20.45%
"I'm in the middle of the salt bridge, but I will pass-eventually" (C) (21)
15.91%
"I'm feeling rather electroNegative about this exam" (D) (18)
13.64%
"Benzene, what's that?" (E) (6)
4.55%
"Chemistry, what's that?" (F) (11)
8.33%
This discussion is closed.
LeaX
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#781
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How do you guys revise for the organic synthesis stuff? I just attempted to do a flow chart of all the AS and A2 reactions and describing it as chaotic and messy is an understatement.
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phaseshift
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(Original post by LeaX)
How do you guys revise for the organic synthesis stuff? I just attempted to do a flow chart of all the AS and A2 reactions and describing it as chaotic and messy is an understatement.
I am yet to find a better way of revising the reaction mechanisms. I agree that its messy though.
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posthumus
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#783
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Guys, how much priority should content from other units be given if we are focusing just on the unit 5 paper ?

...like is it just as important.

Thanks in advance
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posthumus
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Has anyone done any redox equilibria calculations from the Jim Clark book, it is so confusing

They give you things like:

Ag(s) | Al3+ (aq) || Cd2+ (aq) | Cd (s)

They state:

Ecell= E right hand - E left hand

I haven't seen anything like this before ! But here you do not need to know which is cathode and anode... you just follow this equation rule ?

What does it mean if it is that order? What if I switch the volt meter around it would give me a value with opposite sign.... How do I know which is the cathode & which is the anode?

Is the alternative method, finding out what is happening at each (oxidation or reduction) then adding the two equlibrias up... where the electrons cancel on either side and you are left with Ecell

^ if someone knows method would also work, then could you please say so... it is what I'm used to
This right hand - left hand stuff is confusing me Because you don't switch any of the two reduction potentials around...

Hope that all made sense !

Thanks in advance to any responses
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LeaX
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(Original post by posthumus)
Has anyone done any redox equilibria calculations from the Jim Clark book, it is so confusing

They give you things like:

Ag(s) | Al3+ (aq) || Cd2+ (aq) | Cd (s)

They state:

Ecell= E right hand - E left hand

I haven't seen anything like this before ! But here you do not need to know which is cathode and anode... you just follow this equation rule ?

What does it mean if it is that order? What if I switch the volt meter around it would give me a value with opposite sign.... How do I know which is the cathode & which is the anode?

Is the alternative method, finding out what is happening at each (oxidation or reduction) then adding the two equlibrias up... where the electrons cancel on either side and you are left with Ecell

^ if someone knows method would also work, then could you please say so... it is what I'm used to
This right hand - left hand stuff is confusing me Because you don't switch any of the two reduction potentials around...

Hope that all made sense !

Thanks in advance to any responses
the right-left rule is when you make the electrode with the more negative electrode potential the left hand cell (which would make it the anode). however, my teacher said not to use this method and instead reverse the sign of the smallest electrode potential value for oxidation and then add them (like you said).
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posthumus
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(Original post by LeaX)
the right-left rule is when you make the electrode with the more negative electrode potential the left hand cell (which would make it the anode). however, my teacher said not to use this method and instead reverse the sign of the smallest electrode potential value for oxidation and then add them (like you said).
Thank you ! So when do I not use that, when they have given which direction of the reaction they want you to calculate Ecell?

What is the significance of doing that is it to find the standard electrode potential of the overall cell.

And wouldn't the one on the left be the cathode (negatively charged rod) because it is more negative than the other rod ? (making the one on the right an anode)
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LeaX
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(Original post by posthumus)
Thank you ! So when do I not use that, when they have given which direction of the reaction they want you to calculate Ecell?

What is the significance of doing that is it to find the standard electrode potential of the overall cell.

And wouldn't the one on the left be the cathode (negatively charged rod) because it is more negative than the other rod ? (making the one on the right an anode)
you can use either, whichever works best for you. remember in the data booklet the electrode potential values are always for reduction so the greatest value will be the half cell which undergoes reduction. both right-left and adding them up (when you reverse the sign for oxidation) give the electrode potential for the overall cell. both methods get the same result. the value can also be used to see if the reaction is feasible (a positive cell potential indicates it's feasible).

and yeah i meant to put that not sure why i put anode haha. it was late :P
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posthumus
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(Original post by LeaX)
you can use either, whichever works best for you. remember in the data booklet the electrode potential values are always for reduction so the greatest value will be the half cell which undergoes reduction. both right-left and adding them up (when you reverse the sign for oxidation) give the electrode potential for the overall cell. both methods get the same result. the value can also be used to see if the reaction is feasible (a positive cell potential indicates it's feasible).

and yeah i meant to put that not sure why i put anode haha. it was late :P
Ah I see now
The more negative (less positive) reduction potential means a great reducing agent & therefore sends a lot of electrons to go around flowing

And the greater the reduction potential (more positive), the better it is at oxidising! Therefore increase electron flow even more ? (by attracting electrons)

The greater the difference between the too... the greater the flow of electrons and therefore greater current/EMF

Thanks this is actually starting to make a lot of sense
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Mollymod
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(Original post by LeaX)
How do you guys revise for the organic synthesis stuff? I just attempted to do a flow chart of all the AS and A2 reactions and describing it as chaotic and messy is an understatement.
It's hard, but you have to try and think systematically.
You have to think my start product and my end product.
What can I make my end product out of, and how can I make that compound from my start product. Often, making the end product is just 2 reactants, like an acyl chloride with an alcohol to make an ester. And if your starting compound is an alkene for example, you'd think "How can I make a primary alcohol from this alkene?" And having learnt all your reactions, you'd be able to tell: Oh, I'd react it with steam and a Phosphoric Acid Catalyst.

I'll give you an example.
Making Butyl Ethanoate from Ethene.

Jot down all the possible things you make an ester from with your starting product as an alkene.

Alkene ----> Alcohol ----> Carboxylic Acid ----> Acyl Chloride ----> Ester
(And it's the ester that we want)

My first step is to think of all 8 reactions of alkenes.
To make an alcohol from an alkene, Phosphoric Acid catalyst 300 degrees with steam. This has made Ethanol in an electrophilic addition reaction.

Right, then how do we make the next thing? We heat the Ethanol under reflux with an oxidising agent like acidified Dichromate (Orange to green solution change) forming Ethanoic Acid.

How do we make an acyl chloride from a carboxylic acid? Add PCl5 to the mixture forming ethanoyl chloride. And finally, we react that with butan-1-ol under reflux with a sulphuric acid catalyst to make our ester.

Remember, they assume you have simple compounds on hand.
So you wouldn't have to then make things like butan-1-ol or whatever to aid your synthesis, they just want the main thing they asked for in the question.

Put conditions in any case, just in case, and test yourself randomly on reactions. Write down the 6 reactions of alcohols randomly without having revised it, or alkenes, or carbonyls. Just keep yourself on your feet with unprepared questions, so you can see if you can recall that information off the top of your head. This is how I get them into my memory. Try not to panic and think stage by stage, like a bit of a map, what have I learnt that would help me get from here to there, from Point A to Point B.
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LeaX
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(Original post by Mollymod)
x
Ooh, thank you so much! This was so helpful.
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Mollymod
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In other news, I upped my practicals a few UMS this year
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F1's Finest
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(Original post by Mollymod)
In other news, I upped my practicals a few UMS this year
Good stuff, every ums counts!
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Mollymod
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(Original post by James A)
Good stuff, every ums counts!
I saw that last year! I wish I had done better in Unit 3 Assessed practicals now!

I got a high C which I could easily have bettered if I could have been bothered. This year I've got 39/40, so that hopefully translates into about 56/60 unless the boundary moves.

It makes it marginally, but easier for me to get an A overall which, considering where I came from with my low D at AS, i'd be over the moon with
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Gnome :)
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(Original post by Mollymod)
I saw that last year! I wish I had done better in Unit 3 Assessed practicals now!

I got a high C which I could easily have bettered if I could have been bothered. This year I've got 39/40, so that hopefully translates into about 56/60 unless the boundary moves.

It makes it marginally, but easier for me to get an A overall which, considering where I came from with my low D at AS, i'd be over the moon with
Well done
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LeaX
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I don't suppose any of you have found any good resources on the organic synthesis stuff? I went through the AS and A2 textbook of mine and wrote down every single organic reaction but I still don't have some that are mentioned in mark schemes.
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Jammie Dodgers
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Has anyone done any mocks at school yet?
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posthumus
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(Original post by Jammie Dodgers)
Has anyone done any mocks at school yet?
Our teacher only gives us those 6 marker calculation questions to do...

apparently they are very difficult -_- But I found calculations in the unit 2 Jan paper harder
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sounique
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(Original post by Jammie Dodgers)
Has anyone done any mocks at school yet?
I've done both mocks at school. Unit 5 was awful.
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swahmad
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Does anyone have any advice for what to revise for the aspirin and groups 2 and 7 practical?

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GeorgeL3
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(Original post by swahmad)
Does anyone have any advice for what to revise for the aspirin and groups 2 and 7 practical?

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For the Aspirin practical make sure you look up it's melting point first.
The answer you give should be a few degrees lower due to impurities (but not 60°C lower like mine turned out :-S ).

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