Thursday, June 2, 2011

IB Chemistry, Chemistry IA, Kinetics, Iodine Clock Reaction

IB Chemistry, Chemistry IA, Kinetic, Iodine Clock, Visible Spectrophotometer
Visible Spectrophotometer to measure the Rate of Reaction ( Iodine Clock )

Conventional Way ( X cross Method )
  • Time for triiodide/starch to obscure a mark “X”.(subject to lots of Human Error)
Visible Spectrophotometer (Change in Absorbance at 608.7nm)
  • Time for triiodide/starch to form, causing an increase in absorbance.(Accurate)
Comparative study on both Methods can be done by students
...................................................................................................................................................
Iodine Clock Introduction.



H2O2 oxidises I- into I3 and I3 will react with starch to form blue black.
H2O2 + 3 I  + 2 H+ → I3 + 2 H2O.
Triiodide formed will be reconverted to iodide by the thiosulfate. (Delaying mechanism) .
I3(aq) + 2 S2O32−(aq) → 3 I(aq) + S4O62−(aq)


Reaction Mechanism







Absorbance MUST increase immediately when blue black colouration forms.
Research Question?

How changing KI conc affect the rate of Iodine Clock ( by the sudden increase in absorbance ) over time.

Steps to follow
1. Prepare 2 fold serial dil for 0.1M KI
2. Add (0.5ml, 0.1M S2O3) , (0.5ml, 0.5M KI), (0.1ml 1% starch), (0.1ml, 0.1M HCI) into a cuvette.

3. Set visible spec at 608.7nm
4. Set data collection to 300s (Abs vs time)
5. Place cuvette containing solution in step 2
6. Add 1ml of 3%H2O2 and press collect button
7. Take time for absorbance to increase
8. See 2min video tutorial for clarification



Data done by IB students
Rate is measured by time it takes for an increase in absorbance using different KI Conc










Video, Iodine clock reaction using Visible Spec


Advantage of Visible Spec and Tips to make it work
  • MUST produce a sharp increase in absorbance once blue black form
  • Reduce human error ( X method )
  • Micro volume is used, save reagents (total volume 2.5ml )
  • DO NOT NEED TO CALIBRATE but ensure wavelength at 608.7nm
  • MUST add H2O2 into cuvette and start data collection simultaneously
  • Uncertainty can be measured easily ( see picture )
  • Sulphur clock may be adopted ( sulphur formation ) but may NOT BE ACCURATE cause sulpur formation may not produce a sharp increase in absorbance



Uncertainty in time when absorbance changes
Increase Accuracy if increase from 1s to 0.1s per sample ( see picture )






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Iodine Clock Reaction, Peroxodisulphite S2O82− and Potassium iodide, KI
Iodine is generated:
2I + S2O82− → I2  + 2SO42−
I2  + 2S2O32− → 2I + S4O62−
Same Method but using S2O82− as oxidising agent not H2O2
Steps to Follow

1. Prepare 2 fold serial dilution for 0.1M
2. Add 0.5ml 0.1M KI, 50ul 1% starch, 50ul 0.02M S2O3 into cuvette

3. Set visible spec at 608.7nm
4. Set data collection to 300s (Abs vs time)
5. Place cuvette containing solution in step 2
6. Add 0.5ml of 0.05M S2O82-and press collect 
7. Take time for absorbance to increase
8. See 2min video tutorial for clarification

IB Student data, Iodine clock with different conc KI

Results:

  • Visible is accurate if it shows a sharp increase in absorbance
  • H2O2 is better compared S2O82-
  • Comparative study between X cross method, visible and light sensor can be investigate and error analysis can be done




Video, Iodine Clock using visible spectrophotometer
To all IB students,
  • Try out new techniques and Embrace ICT in your data collection. 
  • Be Risk takers and choose path less travel
  • Time to move away from X cross method
  • Embrace New techniques and Explore new ways of doing things
  • Innovate and be Creative as that is the path to success.............Lawrence Kok

1 comment:

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