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

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Reply 1900

Uh it appears to be 2.39 to my eyes?

Oh - Im so sorry - I read that as 2.39 * 10 ^ -3 because of the dm^-3

That makes so much more sense

Sorry again - I sometimes read things wrong

Thanks for your help

Reply 1901

RUNSran

can anyone please explain the difference between amine and amide?

Reply 1902

C0balt

Original post by Don Pedro K.

what the heck o.O 78....on that evil paper? You're sorted lol.

I'm going for 100% in A level Chemistry man, I'm not too sorted for that lol

Original post by the-anonymous-me

Oh - Im so sorry - I read that as 2.39 * 10 ^ -3 because of the dm^-3

That makes so much more sense

Sorry again - I sometimes read things wrong

Thanks for your help

Hahaha no worries, I read things wrong all the time, including that unit 4 exam we sat XD

Reply 1903

blueynuey

Could someone please explain recrystalisation including why each step is done thank you :smile:

Reply 1904

Don Pedro K.

Original post by RUNSran

can anyone please explain the difference between amine and amide?

An amide has the -CONH2 functional group whereas amines just have NH2/NH/N

Reply 1905

MacroniCheese

Original post by blueynuey

Could someone please explain recrystalisation including why each step is done thank you :smile:

Here goes!
1) Dissolve impure compound in a minimum amount of hot solvent. It's a minimum amount so that you obtain a saturated solution. If you use an excess some crystals may remain in solution

2) Hot filtration - hot so you avoid premature formation of crystals and it's filtered to remove insoluble impurities

3) Cool the filtered solution to ensure maximum amount of solid crystallises

4) Suction filtration with Büchner funnel to remove any soluble impurities and then wash with distilled water to dry

Reply 1906

RUNSran

Original post by Don Pedro K.

An amide has the -CONH2 functional group whereas amines just have NH2/NH/N

oh I see, thanks

Reply 1907

username1210827

Original post by blueynuey

Could someone please explain recrystalisation including why each step is done thank you :smile:

Add minimum hot solvent
minimum so crystals form

Hot filtration
to remove insoluble impurities

Cool the filtrate so crystals can form

Filter crystals through Buchner funnel then wash on filter paper using cold solvent
to remove soluble impurities

Dry in desiccator or oven
to remove water
Desiccator is better for substnaces with low melting points or substances that decompose on heating

Reply 1908

username1210827

I posted this a while back but can someone please explain solvent extraction??

Reply 1909

Rahatara Sadique

Original post by C0balt

Everyone's fav MCQ :tongue:

20cm3 contracted because water condensed as it was carried out at room temp
10cm3 of the hydrocarbon therefore produced 20cm3 of water. Water has 2 H atoms per molecule so from the hydrocarbon : water ratio you know that a mole of the hydrocarbon produces two moles of water, which means a mole of the hydrocarbon has four moles of H atoms.

Thankyou!

Reply 1910

Wunderbarr

Original post by ilovecake123

oh god won't there be a legacy paper?

Would seem veeery unlikely.

Reply 1911

Don Pedro K.

For recrystallisation, why it important that the product is very soluble at high temps and almost insoluble at low temps?

Reply 1912

tayloryeah

Original post by the-anonymous-me

I posted this a while back but can someone please explain solvent extraction??

1. put the distalle of the impure product into a separating funnel
2. wash with NaHCO3 to neutralise acid
3. allow separation to occur
4. run the organic liquid into a conical flask and place with anhydrous sodium sulphate
(the drying agent must not react with the organic liquid and should be insoluble)
5. decant the liquid into a distillation flask
6. distill to obtain pure product

Reply 1913

C0balt

Original post by Rahatara Sadique

Thankyou!

No problem, gl tomorrow

Reply 1914

Wunderbarr

Original post by Don Pedro K.

For recrystallisation, why it important that the product is very soluble at high temps and almost insoluble at low temps?

Heat the solution until it all dissolves.

Cool the solution until that which is insoluble at low temperatures will "crash out" of the solution. Hence you can throw away the solution containing all the impurities, and be left with the product (followed by that buchner funnel stuff).

Reply 1915

username1210827

Original post by tayloryeah

Thanks - I couldn't find a simple step-by-step explanation anywhere

Reply 1916

Don Pedro K.

[QUOTE="Wunderbarr;65991421"]

Original post by RUNSran

x /QUOTE]

I think you can just look at Cr²⁺ oxidising straight to Cr⁶⁺.

If you look at the oxidation of Cr²⁺ by MnO₄^-, then you can formulate some equations with:

Cr²⁺-> Cr⁶⁺
MnO₄^- -> Mn²⁺

then balance these by balancing the electrons and add them together (for now I'm just ignoring all the other junk that actually appear in the equations).

We know MnO₄^-goes to Mn²⁺and Cr²⁺ goes to Cr⁶⁺:

MnO₄^- + 5e^--> Mn²⁺ + stuff
Cr²⁺-> Cr⁶⁺ + 4e^-

Therefore: 4MnO₄^- + 5Cr²⁺ -> 4Mn²⁺+ 5Cr⁶⁺

So mole ratio of MnO₄^- to Cr²⁺ is 4:5 or the moles of MnO₄^- will be 4/5 times the moles of Cr²⁺.

You were right with dividing the moles given by 10 to get 2.66x10^-4 but then you need to multiply this number by 8/5 to get the moles of MnO₄^-.

Why 8/5 instead of the just mentioned 4/5? Because there are twice as many Cr²⁺ to oxidise.

Finish it for me baby.

What do you mean?

Reply 1917

Wunderbarr

Original post by Don Pedro K.

What do you mean?

I just dealt with one Cr²⁺but can you see that the original complex ion (top) has two Cr atoms in its formula?

Reply 1918

Don Pedro K.

Original post by Wunderbarr

I just dealt with one Cr²⁺but can you see that the original complex ion (top) has two Cr atoms in its formula?

Ah of course :smile:

Thanks! Also, how can you tell when something won't react with 2,4-DNP? I know that carboxylic acids don't, but do acyl chlorides/amides for example? Or is it purely aldehydes and ketones?

Reply 1919

Wunderbarr

Original post by Don Pedro K.

Ah of course :smile:

Thanks! Also, how can you tell when something won't react with 2,4-DNPH? I know that carboxylic acids don't, but do acyl chlorides/amides for example? Or is it purely aldehydes and ketones?

I'm just going to say yes to aldehydes and ketones xD

Someone else may be able to clarify because I'm at the point of using the old tactic of hoping some things won't come up tomorrow :tongue: