AS Chemistry Planning Excercise!?!?! Watch

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#21
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#21
thats not a chemical test you reject
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#22
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#22
I would be inclined to do the test for the NO3- ion (in HNO3) and an esterification reaction.

OCR said that you have to give a safety precaution or risk assessment for one of the solutions; this might apply to conc. H2SO4, used as a catalyst in the esterification reaction.

I know it's not on the spec, but PCl5(s) will give a +ve test for an -OH group (alcohols and carboxylic acids). I was tempted to suggest this, as it gives off steamy white fumes of HCl(g).

CH3COOH + PCl5 ------> CH3COCl + POCl3 + HCl

Good luck,
J
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As the solutions are unlabelled, are you performing these tests on samples of each unknown solution, i.e. process of elimination? I am.
penny1985uk
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#24
You can't use process of elimination cos it says you must have a positive result for each solution - sorry.

Does anyone have a balanced equation for the test for ethanoic acid, adding calcium carbonate which produces carbon dioxide and decting the CO2 with limewater.

Cheers me dears :-)
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The proff
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#25
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#25
How can u guys be all so dense, the answer is to use silver nitrate to differenciate between the hydroxide chloride and bromide and then use the brown ring test for the nitrate and then react the ethanoic with PCl5 as sumone said.
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#26
You can't use process of elimination cos it says you must have a positive result for each solution - sorry.

If all that you're given are 5 bottles of solutions and the bottles are not labelled, the only way to detect the relevant ions present is to test each solution in turn, i.e. perform the same chemical test (say, the test for halide ions, Cl- and Br-) on samples of each solution. There is no way you can isolate a bottle and claim its identity, and then perform a confirmatory test.

In a real life situation, like a practical exam, you would have to perform the same test more than once to derive the correct identity, and then make your inferences.
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#27
how do you differentiate between the hydroxide, chloride and bromide with silver nitrate? i thought that could only be used to identify the chloride and bromide. my text book mentions something about dissolving the calcium hydroxide in water.. to produce lime water..? is this right? i'm really starting to panic now!
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#28
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#28
u dont need to add calcium hydroxide to water cos its already aqueous!! u just react an acid with a carbonate to get CO2 which turns only the CaOH cloudy cos it IS limewater.

silvernitrate test is a positive result for Cl- and Br-

for nitric acid, test for nitrate by doing brown ring test or boiling with sodium hydroxide and aluminium powder to give off ammonia which u can test for with a stopper from a bottle of Hcl (white fumes)

ethanoic acid is a carboxylic acid so test for that by reacting with sodium hydrogen carbonate to get CO2 which turns limewater cloudy

if u search carefully on the net all these simple tests are there...

gd luck

Bob
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#29
So am I correct in assuming that each chemical test has to be performed on samples of all the solutions mentioned, given that the bottles are unlabelled?
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#30
i dont think so. whats the point? if u know that only 1 solution will give the positive result for a test, the positive result confirms its identity so it doesnt need to be used again. id just say ud have to do each test on the REMAINING solutions....

Bob
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#31
has anyone added scientific knowledge about any of the tests to their coursework? such as why the reactions happen etc? it juz seems stupid to me that theyve set a topic like this. u cud juz copy it all from a text book. weird.
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#32
Hi.

I stumbled across this thread looking for info on all the tests being mentioned. I'm doing the same Chemistry planning task.

Here's what I'm doing -

I'm distinguishing between the acids and the other solutions first, by adding sodium carbonate to all of them. The two acids will effervesce.

This next bit seems to be causing a problem with all you guys. I've been told that there is a specific test for both the nitric and ethanoic acid. The nitric acid test would be the brown ring test, and ethanoic acid I'm assuming you can react with ethanol and see if you get the fruity smell. It says you have to do chemical reactions, and technically you are because you've changed it into an ester, so I guess that counts as okay. But I'm not sure.

What I'm doing, however, is adding sodium carbonate to both the acids and measuring the volume of carbon dioxide produced in a given time. The one that produces the most volume has to be the nitric acid, because it's a stronger acid (as has already been said.)

I asked my chemistry teacher about doing that, and he said that it was fine because it's positive testing for both the acids. Then he said 'but there are specific tests you could use!' In other words, he was hinting that I should probably use something better.

For the remaining three solutions, I'm going to add carbon dioxide to each of them, as the aqueous calcium hydroxide is lime water (again, as has been said) and this will turn cloudy.

The other two have to be acidified and then have to have silver nitrate added to them. In my chemistry text book however, it says this test isn't all that clear - it suggests adding aqueous ammonia to the solutions. With a little investigating, I found that if you add DILUTE aqueous ammonia to both of them, the precipitate in the potassium bromide won't dissolve while the precipitate in the sodium chloride will disolve.

So yeah, don't know whether this helps anyone, guess you guys have figured out the rest already, and like you all, it's the two acids that have stubbed me a little. I'm tempted to keep mine like it is but I think using the brown ring test will more likely be the test that's preferred on the mark scheme. Just not so sure what they're looking for with the ethanoic acid.

I have written my up completely, ie, so it can be handed in and I have managed (just!) to keep to the word limit. Just now very confused over whether I should change my tests for the acids.

We will see.

L.
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#33
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#33
Theoretically, measuring the volume of gas being given off of the two acids, and comparing the two results, is not a valid test according to the Exam paper...
The tests to be carried out have to be positive... you are merely carrying out an elimination by carrying out this process. Therefore, how can this be a valid positive test?

Adding Ethanol to Ethanoic Acid is a perfectly valid test- this is clearly stated on the syllabus, due mainly to the fact that an Ester is being formed in the reaction between Ethanol and the Ethanoic Acid.

PDT2k
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Very true. I had thought of that. The way I was getting around it was because although you're comparing the two results, you are still doing a positive test on both the chemicals. It's not as if you're just doing a test on one of the acids and then going 'well, that is X so the other MUST be Y.' It's more 'from this experiment, the faster must be nitric acid as that's the stronger acid and the slower must be ethanoic acid because that's the weaker acid.'

So, in that way, it is positive testing. I talked to my chemistry teacher about it and he said it was, so I guess it is. Unless he's wrong. Which would be funny. Just, yeah, I do agree with you because I do think that it's a little risky. It's been bugging me, hehee. I'm very tempted to change my experiments for the acids. I would use the ring test first. If I've got this right, you have to add iron (II) sulphate solution and then pour concentrated sulphuric acid gentle down the side of the test tube. This forms a seperate layer at the bottom and the brown ring forms between that layer and the layer above.

The thing that amuses me however, is that the sulphuric acid is needed to change nitrite to nitric acid, but since it's *already* nitric acid, it's kind of not needed. But then where down the brown ring form? I'm guessing that's irrelevent and I add the sulphuric acid anyway. Just thinking it may look silly to an examiner to add something into a solution to form what that solution already is...



Thanks anyway, you're convincing me into changing my experiments for the acids.
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#35
YOU NOOBS THE CHEMISTRY PLANNING IS EASIER THAN EATING CHEESE
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#36
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#36
reduce the ethanoic acid back to an alcohol using lithium tetrahydridoaluminate (LiH4Al) and test for an alcohol. Check A-Level chem books if u don't believe me about the reducing agent.
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#37
I have no idea what any of you are talking about.
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how about getting a bottle of each of the 5 solutions that are labelled then tasting each bottle and the bottles of unknown solutions in turn, and match the test? there's bound to be some chemical reaction on the tastebuds and u must be allowed cos in THEORY it would work, regardless of whether or not a skool would carry out a practical involving tasting substances...

it'll keep ur word count down too
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to drranjid and all the other geeks who say this is easy, SCREW YOU, pathetic glasses wearing gimps. don't make me wrap you up in your lab coats and burn you.

are there many different ways to test for the chemicals? or just 1 test for each, my mind's a blank. i probably shouldn't have left this til the day before it has to be handed in
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#40
I think that contravenes Health & Safety regulations.

If in tomorrow's exam you were given 5 bottles of solutions, unlabelled, and told to test for X test(s) using the reagents provided, you would perform, say the AgNO3 test, first...to eliminate KBr & NaCl, and proceed with testing the remaining 3 solutions with the reagents you have. Remember: OCR said 'devise a logical sequence of chemical reactions'.

It is only a 500-word plan, so I won't be doing an introduction. The reagents & apparatus lists don't form any part of the final word count...that would be darn unfair. Mine's written up, and I'm getting an early night...very early, coz I'm cream crackered!!!!

Was the bio AS practical agreeable? Liked the cell section a whole lot, but the Benedict's was a real pain in the ass!!!

Good luck for 2morrow
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