[EffKayy]

Thought I'd make a thread for this..

How's everyone doing in terms of revision?
We've got all of organic synthesis left to do at school which seems OK ish. The content is fair, but having looked at the pastpapers, some of the questions are just dreadful!

[This Honest]

Hey dude, I'm also sitting this. Atm we've just finished transition metals/cupper n chromium properties and we are in the middle of organic.

Yeah, I checked some of the pp and some of the Q were just but that was 3 weeks ago, hopefully it'll be ok now

[NutterFrutter]

Going well so far...

[EffKayy]

Can you guys please help me?
Im on my phone so I can't attach the paper here but the jan12 paper Which is available on student forum.biz is confusing. The multiple choice question about iron hydroxide separation - why isit ammonia? Where is this in the spec? Also with that disproportionation half cell thing- my equation was balanced etc but it was one of the 'wrong equations' Infact it was the second one on the markscheme. I flipped the second half equation as it was more negative ?? Confusing. Then I calculated the potential and it was positive so it should be feasible but it says not feasible with a positive value...? What?


Thanks guys

[raheem94]

(Original post by EffKayy)
Thought I'd make a thread for this..

How's everyone doing in terms of revision?
We've got all of organic synthesis left to do at school which seems OK ish. The content is fair, but having looked at the pastpapers, some of the questions are just dreadful!

I don't think there are any 'dreadful' questions in chem5, the dreadful things can be the grade boundaries, like the one of chem4 last year, a paper which required 88/90 to score full UMS, that was really high.

Unit 5 has a lot less content compared to unit 4, but unit 4 is easier in my opinion if the grade boundaries are not like june 2011.

The best way is to try to score full in all the calculation questions, because in the other questions there is a chance that a person may miss writing a point so he may lose some marks but calculations provide a easy way to score well.

By the way, i am not doing any of chem exams, i have already got an A* in edexcel a-level chem in june 2011.

Good luck

[clownfish]

(Original post by EffKayy)
Can you guys please help me?
Im on my phone so I can't attach the paper here but the jan12 paper Which is available on student forum.biz is confusing. The multiple choice question about iron hydroxide separation - why isit ammonia? Where is this in the spec? Also with that disproportionation half cell thing- my equation was balanced etc but it was one of the 'wrong equations' Infact it was the second one on the markscheme. I flipped the second half equation as it was more negative ?? Confusing. Then I calculated the potential and it was positive so it should be feasible but it says not feasible with a positive value...? What?


Thanks guys

The Cu(OH)₂ precipitate will react with the ammonia and dissolve, whereas the Fe(OH)₃ precipitate will remain insoluble. Hence they can be seperated this way. It's in the syllabus for unit 5, section 2.J

----

In order to have Cr³⁺ as the starting material you need to flip the dichromate equation. What you did was that would actually happen, by looking at the SEP values, but they were asking you to make Cr³⁺ react with itself. You then combine to get:

8Cr³⁺ + 7H₂O --> 6Cr²⁺ + Cr₂O₇^2- +14H⁺

Then work out the E_cell = the equation that goes forwards - the equation that goes backwards = -0.41 - 1.33 = -1.74V As this is negative the reaction is not feasible.

For half equations, as a general rule the more positive one goes forwards and the less positive one goes backwards. Therefore eqn 1 is flipped not eqn 2.

[Cetacea]

I'm sitting this. We've finished the content for Unit 5 now but I'm going over it using revision guides and textbooks. Personally I enjoy the Unit 5 content better than Unit 4, mainly because of the organic chemistry. Transition metals are also ok once you've got your head around the reactions with NaOH/NH₃, the shape of the complex ions etc.

[EffKayy]

(Original post by clownfish)
The Cu(OH)₂ precipitate will react with the ammonia and dissolve, whereas the Fe(OH)₃ precipitate will remain insoluble. Hence they can be seperated this way. It's in the syllabus for unit 5, section 2.J

----

In order to have Cr³⁺ as the starting material you need to flip the dichromate equation. What you did was that would actually happen, by looking at the SEP values, but they were asking you to make Cr³⁺ react with itself. You then combine to get:

8Cr³⁺ + 7H₂O --> 6Cr²⁺ + Cr₂O₇^2- +14H⁺

Then work out the E_cell = the equation that goes forwards - the equation that goes backwards = -0.41 - 1.33 = -1.74V As this is negative the reaction is not feasible.

For half equations, as a general rule the more positive one goes forwards and the less positive one goes backwards. Therefore eqn 1 is flipped not eqn 2.

Thank you so much. I get the ppt thing. Still consumed over half cell thing. I got it ll until the last sentence where you Said the first one is flipped not the second one? But you said above the second one is flipped.?

And also I thought to work out the Half cell it was the most positive one minus the negative one....??
Confused

[clownfish]

(Original post by EffKayy)
Thank you so much. I get the ppt thing. Still consumed over half cell thing. I got it ll until the last sentence where you Said the first one is flipped not the second one? But you said above the second one is flipped.?

And also I thought to work out the Half cell it was the most positive one minus the negative one....??
Confused

Sorry ignore the last bit! Left that in there when I was working it out as originally I thought the same as you.

For calculations you do the reaction that goes forwards - the reaction that goes backwards. Normally this would be the most positive minus the least positive, but in this case we have flipped the more positive one - which is why it is an unfeasible reaction. In other words, the question has asked you to do it wrong and then prove that what you did was wrong.

[EffKayy]

(Original post by clownfish)
Sorry ignore the last bit! Left that in there when I was working it out as originally I thought the same as you.

For calculations you do the reaction that goes forwards - the reaction that goes backwards. Normally this would be the most positive minus the least positive, but in this case we have flipped the more positive one - which is why it is an unfeasible reaction. In other words, the question has asked you to do it wrong and then prove that what you did was wrong.

Thank you so much!!!!
I need a D in this paper for an A but it seems impossible for me- phft
How did you do in unit 4?

[NutterFrutter]

(Original post by EffKayy)
Thank you so much. I get the ppt thing. Still consumed over half cell thing. I got it ll until the last sentence where you Said the first one is flipped not the second one? But you said above the second one is flipped.?

And also I thought to work out the Half cell it was the most positive one minus the negative one....??
Confused

You need to flip the second equation as the question is asking about the formation of Cr²⁺ and
Cr2O7^2–. Then all you need to do is add the Electrode potentials for the half-cell reduction equations. So, -0.41 + (-1.33) = -1.74V so it's not feasible.

EDIT: I see it's already been answered now.

[clownfish]

(Original post by EffKayy)
Thank you so much!!!!
I need a D in this paper for an A but it seems impossible for me- phft
How did you do in unit 4?

I didn't sit this exam, I'm currently helping some students with it.

I'd really recommend you work closely with the syllabus. It seems like a massive topic, but once you focus on what is in the syllabus only it gets a bit smaller. Personally I don't find it as bad as unit 4.

[EffKayy]

(Original post by clownfish)
x

(Original post by NutterFrutter)
x

(Original post by Cetacea)
x

Sorry but being a bit of a bug, but can you help me with:

http://www.edexcel.com/migrationdocu...e_20100628.pdf

Question 5.??
How do you work it out? Frustrating.

[NutterFrutter]

(Original post by EffKayy)
Sorry but being a bit of a bug, but can you help me with:

http://www.edexcel.com/migrationdocu...e_20100628.pdf

Question 5.??
How do you work it out? Frustrating.

Interesting question... firstly you can discount A and D because of the first ionisation energies. I'm not sure how you'd work it out for between B and C but if I had a guess I'd choose B because the first and second ionisation energies are more similar than those for C and there is a larger jump from the second to third ionisation energy. :/

[EffKayy]

(Original post by NutterFrutter)
Interesting question... firstly you can discount A and D because of the first ionisation energies. I'm not sure how you'd work it out for between B and C but if I had a guess I'd choose B because the first and second ionisation energies are more similar than those for C and there is a larger jump from the second to third ionisation energy. :/

Yeah I would have chosen B too.. but it was C \:

Ffs i need to know why ahah thanks though!

[NutterFrutter]

(Original post by EffKayy)
Yeah I would have chosen B too.. but it was C \:

Ffs i need to know why ahah thanks though!

Post it in the main chemistry forum, I'd be interested to know why it's C as well.

[1platinum]

(Original post by raheem94)
I don't think there are any 'dreadful' questions in chem5, the dreadful things can be the grade boundaries, like the one of chem4 last year, a paper which required 88/90 to score full UMS, that was really high.

Unit 5 has a lot less content compared to unit 4, but unit 4 is easier in my opinion if the grade boundaries are not like june 2011.

The best way is to try to score full in all the calculation questions, because in the other questions there is a chance that a person may miss writing a point so he may lose some marks but calculations provide a easy way to score well.

By the way, i am not doing any of chem exams, i have already got an A* in edexcel a-level chem in june 2011.

Good luck

Any tips for achieving an A*?

[raheem94]

(Original post by 1platinum)
Any tips for achieving an A*?

I first put effort on understanding all the concepts, learning the definitions and reactions(e.g. organic chemistry reactions). Then i did a decade of past papers for all units.

The only tip is to work harder.

[1platinum]

(Original post by raheem94)
I first put effort on understanding all the concepts, learning the definitions and reactions(e.g. organic chemistry reactions). Then i did a decade of past papers for all units.

The only tip is to work harder.

Was this the legacy specification? Where did you get the papers from? Also may I ask what your UMS was for Unit 4, Unit 5 and the practical aspect?

[raheem94]

(Original post by 1platinum)
Was this the legacy specification? Where did you get the papers from? Also may I ask what your UMS was for Unit 4, Unit 5 and the practical aspect?

I took the exam last year, it was edexcel GCE specification, the new one.

I got the past papers from my school, but i do think they will be available on many websites. I only have the hard copy of past papers.

My UMS: AS: 300/300, Unit 4: 104/120 Unit 5: 108/120 Unit 6: 60/60

Good luck