23.3 iodine/thiosulfate redox titrations

Question 1

what is the oxidation and reuction and overall equation of iodine/thisulfate

Answer 1

oxidation: 2S2O3 2- -> S4O6 2- + 2e-
reduction: I2 + 2e- 2I-
overall : I2 (aq) + 2 S2O32- (aq) 🡪 2I- (aq) + S4O62- (aq)

Question 2

what are the four steps of a iodine thiosulphate titrations

Answer 2

There are four steps involved in this titration:
Oxidise the iodide ions to iodine
Titrate the iodine solution with sodium thiosulfate
Calculate the number of moles of iodine present
Calculate the concentration of the oxidising agent

Question 3

Oxidise the iodide ions to iodine

Answer 3

Measure out a certain volume of potassium iodate(V) solution (KIO3) (the oxidising agent) – say 25cm3.
Add this to excess acidified potassium iodide solution (KI)
The iodate(V) ions in the potassium iodate(V) solution oxidise to some of the iodide ions to iodine (solution will be brown-yellow)

IO3- (aq) + 5I- (aq) + 6H+ (aq) 🡪 3I2 (aq) + 3H2O (l)

Question 4

Titrate the iodine solution with sodium thiosulfate

Answer 4

Titrate the solution formed in step 1 with sodium thiosulfate (Na2S2O3) of a known concentration
The iodine in the solution reduces back to I- ions :
I2 (aq) + 2 S2O32- (aq) 🡪 2I- (aq) + S4O62- (aq)
the brown fades quite gradually maing an end point quite dificult- so we solve this with a starch indicator

Carrying out the titration:
Take the flask containing the solution that was produced in stage 1
From a burette, add sodium thiosulfate solution to the flask drop by drop
It’s hard to see the end point, so when the iodine colour fades to a pale yellow, add 2cm3 of starch solution to (detect the presence of iodine) the solution in the conical flask will go dark blue, showing there’s some iodine there
Add sodium thiosulfate one drop at a time until the blue colour disappears
When this happens, it means all the iodine has been reacted
Jot down the volume of sodium thiosulfate added to the solution

Question 5

Calculate the number of moles of iodine present

Answer 5

To do this you need to…
Calculate the number of moles of thiosulfate ions used in the titration
Then use the 1:2 molar ratio of I2 : S2O32- in the balanced equation for the reaction to work out how many moles of iodine were present in the solution.

Question 6

Calculate the concentration of the oxidising agent

Answer 6

This is the same as the last few steps for an acid-base titration
Use the 3:1 molar ratio of I2 : IO3- from the balanced equation in first step to work out how many moles of potassium iodate(V) were present in the solution
You then use this to work out the concentration of potassium iodate(V)

Question 7

the active ingredient in bleach

Answer 7

ClO-

Question 8

oxidation of copper

Answer 8

Cu(s) → Cu2+(aq) + 2e-

Question 9

brass is

Answer 9

copper and zinc

Question 10

bronze

Answer 10

copper and tin