A2 OCR: Chemistry Plan Watch

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cool_angel
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#21
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#21
hi guys.
i was also wonderin if this reaction with zinc is the only one. what about another reaction?
thanx
tc
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ChemGeez
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#22
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#22
Hi cool angel.
Are you also wondering on which metal to use? Zn seems to be the preferred choice amongst most people, to displace the copper! I mentioned using Mg (for the reasons in one of my previous replies), but angel dude said that i shouldnt.
However i spoke to my Chemistry tutor about this and he said that using Mg would not be that much of a problem as the CuCl2 is in an aqueous state.

But just to be safe i would use Zn.
Is this what you meant when you said 'what about another reaction?' If not let me know!
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cool_angel
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#23
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#23
u know on the sheet they say a "displacement reaction and another type of reaction occurs". there must be another reaction other than just this one with zinc. i just cant think!!!! oh its so depressing. got any suggestions.
thanx
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cool_angel
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#24
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#24
is that how u do it?
did any of u find any data to do with it. like enthalpy of formation/reaction data. and which one do u use. r they all like reactions in a cup. what about Mg. would that not be better to use than zinc.
thanx
tc
cu
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ChemGeez
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#25
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#25
I think that you would only have to use polystyrene cups or another form of an insulating beaker, although it depends on the volumes that you intend to use. I wouldnt advise using large volumes as it would be a pain to measure it all out and it might make room for error.

In theory Mg would be the better choice as it is more reactive than Zn. One proble with Zn is that it forms an oxide layer. This would mean that you would have to scrape off the oxide layer before you use it.
However like i said before, people are concerned about the temperature rise being too great. Mg does cause violent reactions with many copper compounds, but i think that because the CuCl2 is aqueous that it is less danger. For example mixing Mg powder (solid) with CuCl2 (solid) and then heating together would be crazy thing to do as it is far too dangerous.

If i were you i wouldnt do enthalpy change of formations at all as this will just complicate things. e.g. for HCl(aq) you would first need to produce
HCl(g) from H2(g) and Cl2(g). Then once you have the HCl(g), you would have to dissolve it in distilled water to produce HCl(aq).
This would be stupidly hard to do and measure results at the same time.

If you still want to use Mg as it is better in theory, then you would have to say in your plan that you intend to carry out some preliminary experiments and if the temp rise is too great then you could just use a less concentrated solution of CuCl2. Although you would need calculations to back up actions.

Hope some of it helps!!!
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cool_angel
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#26
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#26
chemgeez

do u know any websites that are useful for experiements here like temperature changes. my teacher is so stupid. he gave it to us to do over easter and wants it back first day back. how the hell can we do research and prelims in the lab!!! so useless. might as well have given us a day to to do it. books are useless, cant find anything on internet.
pleez help.
thanx
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ChemGeez
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#27
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#27
Yeah i Know what you mean, mine is also due first day back, but luckily i have a chemistry tutor. I've also been looking for some good sites but most of them are too advanced or way off the point.

I'll have have a look today for some good sites and let you this evening!

A good website for revision questions is www.mp-docker.demon.co.uk.
All you need todo is go into the NEW OCR syllabus. It has all the modules for AS and A2, apart from the separate units like 'Methods of Analysis and Detection'. the only one of those that it has is 'Biochemistry'.

Let you Know tonight about the site!
Cheers!
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cool_angel
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#28
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#28
hey how would u draw a hess's law cycle for the reaction. im not sure if arrows should go down coz were tryin to form cucl2. also will final answer be endothermic/exothermic.
eg. calculation would be helpful if ne ideas.
thankx
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ChemGeez
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#29
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#29
Hi Cool Angel!

In your Hess's Law cycle both of your arrows should be pointing downwards as you say. This means that you will in the end be subtracting the delta H that you get from your displacement reaction from your delta H of the reaction of Mg(or Zn) with HCl.
If you use Mg, the delta H of the displacement reaction is going to be quite high. This value you will be subtrating from your other value which should end up as a negative value which is exothermic. Dont quote me on this but i think it is right.(it all depends on the value that you get with the reaction of Mg(s) + HCl(aq)).
Did you want calculations on whether or not it is exothermic or on the quatities of the chemicals?
I havent had enough time to look for a good site yet but i will asap.
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cool_angel
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#30
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#30
hey where did u get the values from. which reaction is more exothermic?
this is so head racking? isnt the final answer endothermic/exothermic?
no idea.
thanx
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ChemGeez
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#31
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#31
The values that you get will be from your actual experiment, which we probably wont be doing in the practical exam. To find delta H you need the following equation:
DH = mcDT

(DH = enthalpy change, m = mass of water(liquid),
c = specific heat capacity of water = 4.18 Jg-1 K-1, DT = temperature change).

the mass of your liquid will be the amount of HCl or CuCl2 you decide to use in grams, and 'c' is constant = 4.18. Therefore for your plan you sub in the values for 'm' and 'c', but leave DT = DT. You can then say that once you have done the expt. you can sub in DT to get your value of DH(enthalpy change).
you wont actually have a final value for your plan cause you havent done the expt yet.
I think that the displacement reaction will be the most exothermic, but really not sure if final Enthalpy change will exo or endo!
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Tidus24kmc
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#32
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#32
Hello everyone, We have read the forum and decided that all of you iare in need of a lot of assistance. So keith and kyle your friendly neighbourhood Chemistry genius’ have come to help. By the way, you can’t have both reactants in excess because that is both chemically and physically impossible.

Below is an internet sit, itt explains how you would go about the whole thing and everyone will find it most helpful, the only thing that you lot are doing is making things worse by discussing it. When you discuss it, your muddling up fundamental points. So this internet site should help you all out

Thanks and good luck in the summer


Chem Info
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Baralian
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#33
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#33
The website is really good and tells a lot of stuff, which is quite helpful but one problem though ... in the site u keep refering to "Nuffield Book of Data" , where do i get that to get the exact figures?!

thanks.
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sam j
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#34
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#34
Hi, i am doing the same planning exercise about enthalpy change and i dont know how to get started. could someone help me by giving me a good opening paragraph that i could look at to get the right idea.
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fluffysheep2000
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#35
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#35
Can someone give me some kinda help on the whole calculation side of things cos I havent got a clue on the whole of this planning thing.............. help i feel so stupid I'm gonna fail
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fluffysheep2000
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#36
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#36
please please please will some1 give me some clue on where to go with the calculations, i'm not expecting you to give me the answers just help me in the right direction as i havent even started yet
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sam j
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#37
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#37
What does the Hess' Law cycle look like. Please help
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cheesecakegirl
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#38
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#38
I have no clue about the calculations for this and it has to be in friday!I think hess' law cycle should be the main reaction along the top and the other two reactions coming off either end to join in the middle underneath the main reaction.Similar examples should be in the course text book though i havent checked yet.Also i heard it was better to have an excess of HCl in the displacement reaction because its more realiable,is this true?Grrrrr i hate chemistry now,its just so annoying!I thought the biology plan was quite hard but its nothing compared to this!

xxx
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wizzkid
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#39
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#39
what has to actually be included in the word count? are the calculations and equations included? would u lose marks for going over the word count by say a 100 words?
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cobra
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#40
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#40
(Original post by wizzkid)
what has to actually be included in the word count? are the calculations and equations included? would u lose marks for going over the word count by say a 100 words?
It tells you on your instructions, all large blocks of text, not labels not equations or calculations and not in tables. it doesnt matter if you go over, our teacher who is an examiner reconed that you could go up to bout 700 without being penolised but if you went much further they might take 1 mark off for bad communication
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