EDEXCEL January 2003 Practical 6A/01 Question 3
We have here an Inorganic Chemistry question. Let us follow through it.
Use the EDEXCEL User Guide as we go along.
You can tell a lot from appearance. If coloured then possible transition element/metal compound.
If colourless then s-block of metals.
But you would see of course brownish with yellow somewhat.
So by writing down under observations brown with some yellow will get you one mark.
By inferring presence of transition metal compound will get you the second mark.
Using the user guide book you can even attempt boldly to suggest two candidate names of the compound for the second mark instead. Two because you are not sure but suggesting. Let say PbO2 and Fe2O3.
Interesting to note that there are undissolved solid. But we know it is not a mixture. So possibly a saturated solution is prepared. That means as strong a reaction you can possibly get.
But why sodium carbonate solution?
From your notes you will learn that aqueous sodium carbonate will form pecipitates with ALL d-block cations.
So following from your inference in the first exercise you are following a lead.
What would you get?
Brownish or orangey ppt. But you notice strange reaction. Some what effervescence. Should you write down? Yes because that gets you your second mark. Report the effervescence.
What can you infer? Iron (III) of course. You write Fe(III) or Fe3+
What is happening here?
Well dilute sulphuric acid delivers H+ and zinc as we know is used as a reducing metal. Together we have prepared a reducing reagent. Things need warming, the usual reason increase rate of reaction by increasing collision rate and effective collisions etc.
So what happens?
The original yellow colour slowly turns pale green. You get 2 marks for reporting before and after colours.
A clear case of Fe(III) been reduced (that would give you 1 mark) to Fe(II) (this gets you the second mark).
At this stage we have undisputedly confirmed that the original cation is Fe(III)!!!
Let us move on to look at the anion.
Well there can only be two sets of possibilities because lead can only produce compounds of 2 possible colours. White or yellow.
We do the test and we get white!! Hopefully all the anions have been mopped up by the lead ions which we hope would be in excess.
So white! You must suggest two anions to get 2 marks. In other words you as a professional chemist must offer your client 2 possible choices at least. I would say that a real chemist will suggest all possibilities. Suggest chloride and carbonate.
So is it chloride?
You perform the test and found no ppt.!!
So you worry because there was no ppt!!
Something is wrong.
You obviously did not get the right salt.
Because they asked you to perform the test so it must be white ppt and sulphate was present.
You write it down and of course you lose marks!!
No!
You right what you see.
No ppt or no change!
And say absence of either sulphate or carbonate.
(You just eliminated these two so it can only be chloride or bromide.)
What is going on here? What test is this?
Well give what the EXAMINER wants!! Do NOT question!!
Okay you see orange solution.
Weird eh?
Well look at it this way.
Water is H-OH and methanol is CH3-OH
Get the resemblance? Methanol is organic with the –OH.
We know that really organic chemicals can dissolve in organic solvents and highly ionic salts will not like sodium chloride. Difficult to prove unless further tests made.
Okay so this Fe(III) salt possibly Chloride or Bromide does indeed dissolve!! In both water (but maybe not easily because we say solid undissolved) and now in methanol an organic solvent.
So what does it mean?? Suggest.
Well what is opposite of IONIC??
COVALENT of course.
So write down that the salt has significant covalent bonding
You have to decide whether significant or not from the yellow or orangey colour.
Was it more than that in water?
So what were you given??
Fortunately you were not asked to identify.
So YOU WILL NEVER KNOW!!!