Chemistry Practical Questions

[Chingoo]

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, Na₂S₂O₃.5H₂O. 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, I₂ which can be titrated with sodium thiosulphate. You are to use this reaction to show that CrO^-4 ion is reduced to Cr³⁺ during this reaction.
{FB 1 is diluted to form FB 3}
Pipette 25.0 cm³ of FB 2 into a conical flask and add, from a measuring cylinder, 10 cm³ of dilute sulphuric acid and 10 cm³ 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 cm³ 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 Cr³⁺.
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.

[Chingoo]

Bump.

I'm not sure if this is against the rules, but since it seemed beyond the scope of the rules I think I'll consider it the benefit of doubt.

[Chingoo]

I don't really wanna do it again, but bump.

Please, I really could use the help--I have a practical tomorrow!

[Ykov]

For the first, it's a redox reaction, reduction I believe and you can characterize it by the black/brown deposits which is usually iodine

[Chingoo]

I guess that's good for a start =D +rep

[Ykov]

 * 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? Basically, the color gets paler as the reaction proceeds so you need an indicator to get an accurate value for the exact titre needed. The reaction was not yet completed when the starch indicator was added so Iodine is still present, even in minute amounts. The exact time the blue-black colour disappears = the exact time the reaction is completed
    * Wouldn't the solution turn brown yellow even if we do not titrate, because of the reaction with chromate in an acidic medium?
10 cm3 of dilute sulphuric acid and 10 cm3 of 5% aqueous potassium iodide, KI.

When you add KI, the Cromium has been reduced to Cr3+, leading to the pale blue colour mentioned, masked by the brown colour of the iodine solution
    * Doesn't the addition of starch and hence formation of a starch-iodine complex, hamper the reaction in any way? 

This possibly has to do with substrate-product chemistry, so I'm guessing the Thiosulphate ions displace the starch and reacts with Iodine

[Chingoo]

Thank you again Ykov! I'm really greatly, and can't double rep but I would if I could =D