1)
Reference CIE May/June 2003 Practical Paper (Paper 3) Q2. (c)
Question Ion/Cation Detection
Statement(s) To 2 cm depth of the filtrate in a test-tube, add 2 cm depth of aqueous hydrogen peroxide followed by 1 cm depth of dilute sulphuric acid
Answer in markscheme Observation: Yellow-brown, orange-brown, red-brown, brown solution OR Grey or black ppt or Iodine (formed/liberated)
Queries- Which reaction takes place when the reagents are added to a solution of (supposedly) iodide ions?
- Is the above reaction in our syllabus (anymore)? If so, please provide evidence from the syllabus itself.
- If it is not in the syllabus, how do we deduce (later on in the question) that it is iodine? Is it because of the instinctive color change?
2)
Reference CIE May/June 2003 Practical Paper (Paper 5) Q1
Question Titration
Statement(s) FB 1 is an aqueous solution containing 100.00 gdm
-3 of sodium thiosulphate, Na
2S
2O
3.5H
2O.
FB 2 is an aqueous solution containing 0.023 moldm
-3 of the chromate ion, CrO
-4. Chromate ions, CrO
-4, oxidise I
-, in the presence of an acid, H
+, and produce aqueous iodine, I
2 which can be titrated with sodium thiosulphate. You are to use this reaction to show that CrO
-4 ion is reduced to Cr
3+ during this reaction.
{
FB 1 is diluted to form
FB 3}
Pipette 25.0 cm
3 of
FB 2 into a conical flask and add, from a measuring cylinder, 10 cm
3 of dilute sulphuric acid and 10 cm
3 of 5% aqueous potassium iodide, KI.
Titrate the contents of the conical flask with
FB 3 until the color of the iodine solution has faded to alight orange/yellow color. Add 1 cm
3 of starch indicator and continue the titration until the blue-black color of the starch-iodine complex disappears leaving the transparent pale blue color of Cr
3+.
Answer in markscheme N/A
Queries- Please explain, in detail, what is happening during the reaction(s), specifically focusing on:
- Why is iodine still present in the solution during titration with sodium thiosulphate?
- Wouldn't the solution turn brown yellow even if we do not titrate, because of the reaction with chromate in an acidic medium?
- Doesn't the addition of starch and hence formation of a starch-iodine complex, hamper the reaction in any way?
It would be really, really, REALLY nice of someone to explain this to me, at least the first question as the second question is mostly unimportant in performing the experiment itself. Thanks in advance.