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# Equilibria, Energetics and Elements (F325) - June 2011 Exam.

Chemistry exam discussion - share revision tips in preparation for GCSE, A Level and other chemistry exams and discuss how they went afterwards.

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1. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by blush.ox)
this is exaclty how we did it at our school...
seems its much quicker to do module 3 after the AS exams, as its easier/faster in terms of actual content...

we've just got about half of module 2 to do
and like a spread on module 1
which leaves almost no revsion time in lessons im guessing to go over stuff again

find this alot harder then organic chemistry...
We did this as well! Module 3, 1,2! Worked out well as we've finished... I think my teachers rushed it a bit though, he's missed out a bit :/
2. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by haydyb123)
Going to finish of Cell potentials and half-cells today, then onto transition elements it's back to the good ol' stereoisomerism...
Guys I need some help!

The equation is 2N0+02--------------->2N02

The rate equation is; Rate=[NO]^2

Now surely the mechanism would be 2N0 + 2N0--------->2N02+N2
02+ N2--------------> 2N0

Cancelling that down you get 2N0+02-------> 2N02

Is that wrong? The text book says its this: NO +NO--------> N202
02+N202--------> N02

If the rate equation says its R=K[NO]^2 that means there are 2 molocues of No, but the equation says there is 2NO already? Im so confused
3. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
contrary to what some people in this thread have said i'm actually rather pleased that this is the bigger paper. I absolutely hate organic so having some theoretical chemistry is a nice break :-)
4. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by J DOT A)
Guys I need some help!

The equation is 2N0+02--------------->2N02

The rate equation is; Rate=[NO]^2

Now surely the mechanism would be 2N0 + 2N0--------->2N02+N2
02+ N2--------------> 2N0

Cancelling that down you get 2N0+02-------> 2N02

Is that wrong? The text book says its this: NO +NO--------> N202
02+N202--------> N02

If the rate equation says its R=K[NO]^2 that means there are 2 molocues of No, but the equation says there is 2NO already? Im so confused
rate equation says thats theres 2 moles of NO but what you've put is 2NO + 2NO which is of course 4NO.

it should be NO + NO ---> N2O2 (or N2 +O2 since either is acceptable)
then you just react with more O2 for the 2nd step

N2O2 + O2 ----> 2NO2

and that should give the overall equation of 2NO + O2 ----> 2NO2

EDIT: i see why you're first equation is wrong, there are 4Ns on the LHS and only 2Ns on the right
Last edited by Jtking3000; 15-04-2011 at 20:30.
5. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
Got transition metals to do by this weekend, but need to go over all my notes and stuff. Don't know how I'm going to get an A in this.
6. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by Jtking3000)
rate equation says thats theres 2 moles of NO but what you've put is 2NO + 2NO which is of course 4NO.

it should be NO + NO ---> N2O2 (or N2 +O2 since either is acceptable)
then you just react with more O2 for the 2nd step

N2O2 + O2 ----> 2NO2

and that should give the overall equation of 2NO + O2 ----> 2NO2

EDIT: i see why you're first equation is wrong, there are 4Ns on the LHS and only 2Ns on the right
I still dont understand...
The equation says 2N0+ 02---------> 2N02
The rate= K[NO]^2

From the equation, we can't tell anything about the RDS.... but the rate equation tells us that there are 2 molocules of NO? so surely there will have to be
2N0+2N0? lol im confused.
7. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
I am going to hate this exam so much -.-

I've finished the course (well, my college has) and now revising, except I haven't done much for it. Thats what this next week of my easter hols are for Along with all my other exam :'(
8. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
*subscribes*

I've been doing a lot of revision for this exam these easter- pretty sure I know most of the stuff but some things still catch me out. Like pH calculations- I can do them but sometimes I do the wrong thing, or I'm right but really slow. I guess I just need to do a load of practice!

I'm also retaking 2 modules, which doesn't help... Does anyone know any threads on here for Rings Polymers Analysis or Chains Resources and Energy?

(Original post by haydyb123)
Going to finish of Cell potentials and half-cells today, then onto transition elements it's back to the good ol' stereoisomerism...
Hey, I remember you from the January F324 thread You seemed like you really knew your stuff! How is your preparation for this paper going?
Last edited by student777; 15-04-2011 at 22:39.
9. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by J DOT A)
I still dont understand...
The equation says 2N0+ 02---------> 2N02
The rate= K[NO]^2

From the equation, we can't tell anything about the RDS.... but the rate equation tells us that there are 2 molocules of NO? so surely there will have to be
2N0+2N0? lol im confused.
I did this question the other day, hopefully this will help. Let's start from the beginning...

The rate equation is rate= k[NO]^2
This means the rate determining step has 2 molecules of NO reacting together. I.e. NO + NO ---> something.

The overall equation is 2NO +O2 --> 2NO2

I think it says somewhere that it's a 2 step reaction right? And a rate-determining step must have a faster step later i.e the rate determining step is first.

The oxygen isn't in the RDS otherwise it would be in the rate equation. So it must be in step 2. And we know we form 2NO2 in the end.

So putting that together we have:

Step 1 NO + NO ---> ??
Step 2 ?? + O2 ---> NO2 + NO2
Overall 2NO +O2 ---> 2NO2

Now there must be an intermediate formed in step 1 that is used up in step 2. Playing around with the N and O gives us N2O2.

Does that make sense?
10. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by student777)
I did this question the other day, hopefully this will help. Let's start from the beginning...

The rate equation is rate= k[NO]^2
This means the rate determining step has 2 molecules of NO reacting together. I.e. NO + NO ---> something.

The overall equation is 2NO +O2 --> 2NO2

I think it says somewhere that it's a 2 step reaction right? And a rate-determining step must have a faster step later i.e the rate determining step is first.

The oxygen isn't in the RDS otherwise it would be in the rate equation. So it must be in step 2. And we know we form 2NO2 in the end.

So putting that together we have:
Step 1 NO + NO ---> ??
Step 2 ?? + O2 ---> NO2 + NO2
Overall 2NO +O2 ---> 2NO2

Now there must be an intermediate formed in step 1 that is used up in step 2. Playing around with the N and O gives us N2O2.

Does that make sense?
Thank you sooo much! Yes that makes sense! Basically the first line is RDS so no Oxygen, as that is NOT in the rate equation right?
So basically anything in the rate equation is part of the RDS? I got ya!
But one last thing...

The equation is 2NO+02----->2N02.

Do we discard that 2 in front of the NO in the equation? AND JUST look at the Rate equation to see how many molocules we need?
11. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by J DOT A)
Thank you sooo much! Yes that makes sense! Basically the first line is RDS so no Oxygen, as that is NOT in the rate equation right?
So basically anything in the rate equation is part of the RDS? I got ya!
But one last thing...

The equation is 2NO+02----->2N02.

Do we discard that 2 in front of the NO in the equation? AND JUST look at the Rate equation to see how many molocules we need?
Hmm, well I'm not really sure... but I would look at the rate equation first to see what molecules you need, and use the overall equation to check it.

But you may have something in the overall equation that isn't in the RDS. Like the O2 in that equation was in step 2, there may have been a NO in the step 2 that wasn't in the RDS.

I know I'm not being very clear here, but I don't really know myself, sorry
12. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
i actually dont understand the 'rate determining step' part at all

as far as i know, its normally just a 2 mark question, think it would be safe to maybe just pretend it doesnt exist
13. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by blush.ox)
i actually dont understand the 'rate determining step' part at all

as far as i know, its normally just a 2 mark question, think it would be safe to maybe just pretend it doesnt exist
LOL I was stuck at first but now I am a bit better thanks to one of the users on TSR that helped me on the question I posted above!

Lets say you have 5 filters, one above the other..... so like a waterfall, and you pour water into the first filter, this will go to the second, which would go to the third ect.
Now these filters are the same width, so the water will go straight down all of them at the same rate yes?
Now what if one of the filters was smaller than the other, lets say it was the 2nd filter. So you pour water into the first filter, and when it reaches the second the rate will all of a sudden slow down? Why? because obv the second filter is smaller then the others.... This is RDS. The rate of reaction is only as fast as its slowest step.

I.E If your family are getting ready to go out, and all 4 members are ready, but your sister(im being sexist/biased I know lol) is still getting changed, you are dependent on her to hurry the **** up!

Remeber that RDS is SOLEY determined by the rate equation=K[x][y]
The overall equation does not tell you anything and catches alot of people out, dont let it trick you!

For example, lets take a look at H2+I2------->2HI
remeber that bonds need to be broken and then reararranged. This is the golden rule and if you master it you have done RDS.

So i) H2-------> H+H (as bonds need to be broken)
ii) I2--------> I+I (same as above)
Now what do you get when you add this together? 2HI!
this is a really simple example but one that helped me.
14. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
Guys I need help on the question!

A mixture was used preparing 2mol of Propanoic acid, 4 mol of ethanol and 4 mol of H20.

The given equation is CH3CH2COOH + CH3CH20H-----------> CH3CH2C00CH2CH3 +H20
At Eqm, there was 1.4 mol of propanoic acid. Calculate Kc.

So using the table....

components CH3CH2C00H CH3CH20H CH3CH2C00CH2CH3 H20
Intial 2 4 0 4
moles reacted -0.6 -0.6 +0.6 +0.6
EQM 1.4 3.4 0.6 4.6

Now my question is, why the **** does, for moles reacted, ethylpropanoate have +0.6 moles, and same for H20? HOW can 4 moles of H20 be used, as that is the forward reaction? How can you get 4 moles of H20 Initally?

Sorry guys the table didnt work Basically the values correspond to the data above. So for intial, 2 corresponds to CH3CH2C00H ect, and 4 is Ch3CH2OH.
Last edited by J DOT A; 16-04-2011 at 19:23.
15. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
Our school made us do this exam in January. Worked out very well since the Jan exam had very low grade boundaries (65% raw marks for an 120/150 A and 80% raw marks for 135/150). So don't get put of by people saying it was hard - if you understand the topics and how they relate its fine. Also I spent a lot of time learning this module well in the first place so I didn't need to do heavy revision before the exam, just needed to retrick my memory. I think it works well for big modules like this. Rather than rush it, forget stuff and then need to relearn it. Good luck everyone.
16. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by J DOT A)
LOL I was stuck at first but now I am a bit better thanks to one of the users on TSR that helped me on the question I posted above!

Lets say you have 5 filters, one above the other..... so like a waterfall, and you pour water into the first filter, this will go to the second, which would go to the third ect.
Now these filters are the same width, so the water will go straight down all of them at the same rate yes?
Now what if one of the filters was smaller than the other, lets say it was the 2nd filter. So you pour water into the first filter, and when it reaches the second the rate will all of a sudden slow down? Why? because obv the second filter is smaller then the others.... This is RDS. The rate of reaction is only as fast as its slowest step.

I.E If your family are getting ready to go out, and all 4 members are ready, but your sister(im being sexist/biased I know lol) is still getting changed, you are dependent on her to hurry the **** up!

Remeber that RDS is SOLEY determined by the rate equation=K[x][y]
The overall equation does not tell you anything and catches alot of people out, dont let it trick you!

For example, lets take a look at H2+I2-------&gt;2HI
remeber that bonds need to be broken and then reararranged. This is the golden rule and if you master it you have done RDS.

So i) H2-------&gt; H+H (as bonds need to be broken)
ii) I2--------&gt; I+I (same as above)
Now what do you get when you add this together? 2HI!
this is a really simple example but one that helped me.
have to admit the second example made me lol but yeh i understand the theory behind it, in terms of how it affects the overall reaction time and the rate eqn part and also the example you've given me...BUT what i cant do is actually WRITE the equations, like when the question says 'write/suggest a 2 step mechanism'
im completely lost when i see this
i wasnt taught how to do this in lessons and even though i've read the spread in the book a couple of times, i still cant grasp it...
17. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
what revision guides are you guys using?

i have the CGP one, was wondering if there are any better ones out there, maybe something like a student book, that can be used as practise for exam type questions...
18. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
Has everyone finished being taught content?
19. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
(Original post by INeedToRevise)
Has everyone finished being taught content?
Nope. We still have most of transition metals to go. I'm teaching myself most of that though, doesn't seem that hard (so far). And we had to basically teach ourselves fuel cells anyway.

20. Re: Equilibria, Energetics and Elements (F325) - June 2011 Exam.
We still have half of ENERGY to learn

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