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All Chemistry A2 students, the planning question this summer...

Kieny

HEY!

We've gotta write a plan ... so when we say we're gonna filter off the ppt do we just say - using filter paper in funnel we filter off the solution retsining the ppt... is that gonna be enough when we have to describe the method!!!

Seriously!

Hoe are we gonna get all the marks that are designated for the practical by saying we filter off the solution - and wash and dry it. Can anyone give a detailed explanation of the filtration process so that we can formulate an articulate response to the method.

P.S - Dont forget there are always marks for safety and improvement and stuff - so we've gotta think of stuff like handle the NaOH carefully (or whatever) and maybe u could use the Buchner Funnel as an improvement if you say filter paper in the main part of the method.

Keep it coming guys...

P.S.S - I've heard we're gonna get a transition metal in the inorganic analysis - possibly Fe 3+ as the cation :wink:

Reply 41

LbreetaM

For Q. 3 it will be the Siwoloboff method, becuase it is the method used if only a small quantity of liquid availiable.
I did this in lesson last week;
At first, the bubbles escape bout 1 every few seconds, but as get closer to the bp, the bubbles escape in a continuous stream (far too many to count).
When get to this stage, allow apparatus to cool (stop heating!) The liquid then cools, and the point at which the liquid goes up the capillary tube is the assumed to be the bp. It is really hard to see tho, so innacurate method.
A better way would be to use fractional distillation.
Sometimes questions ask for problems of this method:
A few are:
1) may not be a pure sample
2) temp of the water is higher than the temp of the liquid becuase glass not a good conductor of heat.
3) boiling temp must be below 100 degrees, else oil needs to be used.

We looked at some past exam practical papers...it seems that they ask for the temp at which the bubbles start, then when there's continuos stream, and temp that liquid taken up the melting pt. tube.

Hope that's of some help

Reply 42

awakening_D

i guess safety measures could be normal stuff such as "wear goggles", "beware of alkalis" "label solutions A, B and C to avoid mixing up" etc etc

Reply 43

Smashingdude

OP

4

a question guys..
wht is a Buchner Funnel ?
Seriously never came across it.
Anyways the general idea of the plan is simple. They will either give NaOH or NH3. I think wht they wud as the first part wud be to identify each.(like they did in AS). So this is easy. Add NaOH or NH3 and observe the ppt. Compare with table. If necessary add excess to see if dissolves(i seriously dont remeber anyone of the tests now but i will go thru these in an hour and then post the Whole proposed plan)
The second i think they wud say 2 of the solutions are now mixed. Plan how wud u separte those. Now i am confused here. That means we have to write plan for all possible combinations (Al,Zn or Zn,Mg or Al,Mg) So in each case we ppt it out using NaOH/NH3. One will dissolve and the other wont in excess.
After this its all separation mumbojumbo(u know filteration/Recrystallization)
An alteration to the xp which they might give is that they might tell us which one was mixed. So now we just have to write one plan.

Getting more info soon...
And thanks for the info on Fe +3 . is it confirmed?
Keep it shooting....

Reply 44

Smashingdude

OP

4

Silver mirror is probably coming ............

Reply 45

har1s

read page 117 of nelson advanced science @ smashing dude. it has info abt the Buchner Funnel there.

Reply 46

spacko101

Question about recrystallisation, so it does work with inorganic compunds as well as organic, right?

Also, about this ppt thing we got going on here:
1. the two solutions are mixed.
2. we add NaOH/NH3.
3. One should redissolve whereas the other one is insoluble.
4. Then we use a Buchner funnel and Vacuum Flask. Don't forget to place a piece of filter paper in the funnel.
5. Wet the paper with small amount of solvent.
6. Pour the solution with the ppt onto the centre of the paper carefully
7. Clean the ppt using cold solvent
8. Now we got our ppt.

I think thats about it. Corrections are welcome of course!

PS. do we have to say a specific solvent. If so, im f**ked. Need some suggestions.

EDIT: Oh and i missed out the recrystallisation part. (if thats needed) :biggrin:

Reply 47

LbreetaM

Will you need to acidify after recrystalisation etc. so that it is an ion? :confused:

Reply 48

har1s

LbreetaM

When get to this stage, allow apparatus to cool (stop heating!) The liquid then cools, and the point at which the liquid goes up the capillary tube is the assumed to be the bp. It is really hard to see tho, so innacurate method.
A better way would be to use fractional distillation.

Many ppl say that a better estimate is obtained by the method you mentioned above. BUT, i think a LARGER sample is required for that method because of evaporation.

Reply 49

har1s

@ spacko.. sorry but what question was it? Is it question 2 you're talking about ?

Reply 50

RMIM

9

Smashingdude

Update again. This time ive got which metals are there in the planning.
They are Al , Zn, Mg ions. I would recheck and confirm this info later this night. Now guys, pump ur brains up and give us points how to separte these ions.
Also can anyone write the whole Filteration/Recrystallization/Distillation process?

Everyone was going on about Transition metals. Al and Mg are not even Transition metals (right?)

But I suppose the ppt methods/chemicals (NaOH/NH3) are the same.
Al+NaOH = white ppt.
Al+ excess NaOH = dissolves to colourless solution
but not the same with NH3(aq)
Al+NH3=white ptt
Al+excess NH3=ppt INSOLUBLE

Mg+NaOH=white ppt, insol in excess NaOH
Same for NH3(aq)

Zn+NaOH=white ppt, dissloves to a colourless solution in excess
same with NH3(aq)

Reply 51

spacko101

sorry, it was Q4, the planning

Reply 52

Kieny

RMIM

Everyone was going on about Transition metals. Al and Mg are not even Transition metals (right?)

But I suppose the ppt methods/chemicals (NaOH/NH3) are the same.

Al+NaOH=white ppt
Al+excess NaOH= insol ppt
same for NH3(aq)

Mg+NaOH=white ppt, insol in excess NaOH
Same for NH3(aq)

Zn+NaOH=white ppt, dissloves to a colourless solution in excess
same with NH3(aq)

Ye good point RMIM!

But ... if what u say is true u can only identify Zn as it dissolves in excess NH3 - how would u distinguish b/w Mg and Al. In addition if they give u NaOH and not NH3 - then you're screwed...

Reply 53

spacko101

RMIM

Al+NaOH=white ppt
Al+excess NaOH= insol ppt
same for NH3(aq)

Sorry but are u sure about the Al + excess NaOH, in my Davies and Kelly inorganic reactions book it says that Al in excess NaOH will redissolve.

Reply 54

shabwasim

Recrystallisation method:
1-dissolve impure solid in minimum amount of hot solvent
2-filter it hot using bucchner funnel and flask
3-allow filterate to cool slowly for crystals to form
4-filter it again
5-wash crystals with minimum amount of cold solventand dry it to obtain pure solid :smile:

Reply 55

RMIM

9

spacko101

Sorry but are u sure about the Al + excess NaOH, in my Davies and Kelly inorganic reactions book it says that Al in excess NaOH will redissolve.

Yes you are right. Thanks for spotting that. I will go back and change it.

Let me try again.

Al+NaOH = white ppt.
Al+ excess NaOH = dissolves to colourless solution

but not the same with NH3(aq)
Al+NH3=white ptt
Al+excess NH3=ppt INSOLUBLE

is it right this time?

Reply 56

Mustard-man

18

spacko101

Sorry but are u sure about the Al + excess NaOH, in my Davies and Kelly inorganic reactions book it says that Al in excess NaOH will redissolve.

He's wrong. Al3+ is soluble in excess NaOH

Edit: ok, TSR is a bit slow for me today.

Reply 57

Kieny

shabwasim

Recrystallisation method:
1-dissolve impure solid in minimum amount of hot solvent
2-filter it hot using bucchner funnel and flask
3-allow filterate to cool slowly for crystals to form
4-filter it again
5-wash crystals with minimum amount of cold solventand dry it to obtain pure solid :smile:

Thats Gr8!

But why would we need to recrystallise in our planning experiment - with these seperation of cations - surely filtration would always do the trick? :confused:

Reply 58

shabwasim

can any one tell me how can i get hold of any chemistry practical mark scheme

Reply 59

Need 6A Help

You shouldn't need to recrystalise as your product is not impure as only the metal ion will react with the NaOH or NH3 to form that particular precipitate. Just filter in a buchner funnel

All Chemistry A2 students, the planning question this summer...

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