Edexcel A2 Chemistry Exams -6CH04 (14th June) and 6CH05 (22nd June) Discussion Thread

Original post by dpoojaraa

Which multiple choice questions in particular did you not expect? I assume those are the same ones which you had to guess? No I haven't looked for an unofficial mark scheme. Are you retaking unit 2?

I only remember two now. One was which molecule has the same empirical and molecular formula, and the other one asked for the volume of gas produced in the reaction they gave but they only gave the volume of the reactants. Yeah, I did have to guess. I can't find one anywhere. Nope, I'm not retaking unit 2.

Reply 163

Original post by ayvaak

Thank you so much :smile:

Np.

Reply 164

Hi. I have two questions from Unit 4. It would be helpful if you can answer them. :smile:

1-In a Reaction Profile Diagram for Multi Step reactions, it is said that the intermediate formed has a greater activation energy than the products. What makes it greater actually? Is it something to do with stability of the intermediate?
2-In a Pseudo Zero Partial Order reaction, what's the main concept behind it? Is it assumed to be a reaction which has a reaction order of zero?

Reply 165

OP

Original post by Funky_Giraffe

Hey guys, I've got a couple of multiple choice queries -

this first one, I guessed right as D but I have no idea why it couldn't be some of the others... what are you supposed to look at? number of shared pairs or something...?? tbh I have no idea

then for the second question, is there a rule about how to determine how fast a pair of compounds will react to form the polymer? (ans is B)

thanks guys!

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For the first one, you need to look at what the ox state of titanium would be in the compound - the structure of the titanium suggests that it is unlikely for it to form anything further than +4, as there are 4 electrons in the 4s and 3d. For part 2, we can either react a carboxylic and an amine or an acyl chloride and an amine. From unit 4 we know that acyl chlorides tend to be much more reactive than carboxylic acids so hence the polymer forms faster

Reply 166

OP

Original post by sabahshahed294

Hi. I have two questions from Unit 4. It would be helpful if you can answer them. :smile:

1-In a Reaction Profile Diagram for Multi Step reactions, it is said that the intermediate formed has a greater activation energy than the products. What makes it greater actually? Is it something to do with stability of the intermediate?
2-In a Pseudo Zero Partial Order reaction, what's the main concept behind it? Is it assumed to be a reaction which has a reaction order of zero?

Question 2 isn't something you need to worry about for the spec - from what I know I believe it typically happens for something like water, e.g. if the reaction takes place in aqueous solution and is first order wrt water, the water is going to be in such excess that the reaction takes place at the same rate the whole time anyway. The first one again isn't really on the spec, but is normally to do with the mechnism of the reaction - to form the intermediate is normally where a lot of bonds are broken whereas the second hump tends to be where the bonds start to be reformed

Reply 167

OP

Original post by Funky_Giraffe

Thank you! By the way, an unrelated question but why can potassium manganate oxidise alkenes to diols, yet potassium dichromate, which is also an oxidising agent, cannot?

I suspect it is because manganate is a much stronger oxidising agent than dichromate is

Reply 168

n2697

11

could someone please explain what they mean in the first purple box - I don't understand why the total Ecell value would be lowered by an increase in Zn2+

Reply 169

OP

Original post by n2697

could someone please explain what they mean in the first purple box - I don't understand why the total Ecell value would be lowered by an increase in Zn2+

Ecell is a measure of the equilibrium they show - the more negative the ecelll the more that species tends to form ions and release electrons. If we add zn2+ we shift the equilibrium back towards the Zn, meaning there are less electrons so since in a copper zinc cell the zinc electrode is the negative one if there are less electrons that electrode is less negative so the potential difference is smaller

Reply 170

n2697

11

But i thought that since the equilibrium shifts to the right when more Zn2+ is added, electron gain becomes easier(as more Zn2+ is combining with 2e-) so as electron gain becomes easier the value is more negative as it's getting reduced and so ecell value is more positive

Reply 171

Ayman!

15

Does anyone know how strict they are on awarding the QWC mark? Idk, the mark scheme mostly has two or three line answers for these

Reply 172

Original post by samb1234

Question 2 isn't something you need to worry about for the spec - from what I know I believe it typically happens for something like water, e.g. if the reaction takes place in aqueous solution and is first order wrt water, the water is going to be in such excess that the reaction takes place at the same rate the whole time anyway. The first one again isn't really on the spec, but is normally to do with the mechnism of the reaction - to form the intermediate is normally where a lot of bonds are broken whereas the second hump tends to be where the bonds start to be reformed

Oh okay! Thank you so much! :smile:

Reply 173

gabby07

11

Original post by Ayman!

Does anyone know how strict they are on awarding the QWC mark? Idk, the mark scheme mostly has two or three line answers for these

I think it's literally just checking that you are using terms in the correct way and spelling them correctly, with clear and correct grammar and punctuation throughout. To be honest Edexcel don't take it as seriously as some of the other boards (where a separate mark is awarded for QWC). There is no standalone mark for QWC in Edexcel papers.

Reply 174

OP

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Original post by Ayman!

Does anyone know how strict they are on awarding the QWC mark? Idk, the mark scheme mostly has two or three line answers for these

Probably not very. As long as you are using proper terms and aren't writing stuff like 'alcohols have a higher gasing point because they have hydrogen thingies' or similar than they probably aren't going to dock you any marks - I believe it is more about using proper scientific vocab rather than whether you have used 4 semi colons and an exclamation mark in your answer

Reply 175

tgygt

2

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Why is the cell potential the same for reactions with same overall equation? :frown:

Reply 176

Original post by tgygt

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Why is the cell potential the same for reactions with same overall equation? :frown:

I have the same question as you. Found out while studying..but I guess the type of reaction is same and maybe that's why Ecell is the same!

Reply 177

Hey! need some help in these MCQs please :smile:

http://qualifications.pearson.com/content/dam/pdf/A-Level/Chemistry/2013/Exam-materials/6CH04_01_que_20150610.pdf

6(a)- How come the answer is D instead of B?
6(b)-Not getting correct answer so just want to know my mistake here.
12(c)-Why is the answer C? Shouldn't it be D?

Reply 178

ayvaak

2

Just out of curiosity how many chemistry past papers can people get through in a day? I would love to have done them all before the exam but I'm not sure theres enough time. Instead I might just tackle the tough ones. Any recommendations for difficult both Unit 5 and 4 past papers?

Reply 179

OP