Landolt Reaction - Iodine Clock Kinetics

Demonstration Experiment on Video

Landolt Reaction - Iodine Clock Kinetics

Objective: Effect of Concentration on the Rate of a Reaction

Peter Keusch

German version

Chemicals:
sodium disulfite
potassium iodate
ethanol
starch, soluble
conc. sulfuric acid

Apparatus and glass wares:
magnetic stirrer
hot plate
2 magnetic stirring bars
stirring bar remover
2 volumetric flasks 1000 mL
volumetric flask 100 mL>
2 beakers 250 mL
3 beakers 100 mL
2 beakers 50 mL
beaker 10 mL
1 volumetric pipet 10 mL
2 volumetric pipets 25 mL
3 volumetric pipets 50 mL
3 pipette bulbs
stop-watch

Hazards and safety precautions:

Sodium disulfite may cause irritation and may be harmful if swallowed. May cause allergic reaction.skin and eye irritant.

Potassium iodate is harmful if swallowed. May be harmful by inhalation or through skin absorption. Irritant.

Conc. sulfuric acid is highly toxic. Causes severe burns. May be fatal if swallowed. May cause cancer through inhalation. Very destructive of mucous membranes.

Ethanol is highly flammable.

Safety glasses and safety gloves must be worn. Suitable ventilation!

Preparation of the solutions:

Solution A: 1.16 g of sodium metabisulfite are dissolved in a 1000 mL volumetric flask containing deionized water. 10 mL of ethanol and 4 g of conc. sulfuric acid are added to the aqueous solution. The 1000 mL volumetric flask is made up to the mark with deionized water.

Solution B: 4.3 g of potassium iodate are dissolved in a 1000 mL volumetric flask containig deionized water. The 1000 mL volumetric flask is made up to the mark.

Solution C: 2 g of starch are dissolved in a 100 mL beaker containing 50 mL of boiling water. The solution is filled into a 100 mL volumetric flask. The flask is made up to the mark with deionized water.

The solutions above should be freshly prepared.

Experimental procedure:

Beaker 1 and 2:
Two 250 mL beakers are filled with a mixture of 50 mL of solution A (metabisulfite), 50 mL of dist. H₂O and 10 mL of solution C (starch).

Beaker 3 and 4:
One 100 mL beaker is filled with 50 mL of solution B (iodate) and 50 mL of dest. H₂O. Another 100 mL beaker is filled with 25 mL of solution B and 75 mL of deionized water.

The solution from beaker 3 is poured into beaker 1 while stirring and afterwards solution of beaker 4 is mixed with the solution in
beaker 2.

Using a stop watch the reaction is timed from the moment of mixing. It is convenient to start the stop watch when one half of the reactant has been added to the sulfite solution.

Results:

When the two colorless solutions are mixed, there is no apparent reaction. After 11 and 22 seconds, respectively, the colorless mixture suddenly turns blue.

Landolt

Videoclip (Download RealPlayer .rm file)

Discussion:

The experiment shows the effect of the interaction between chemical reactions that have different rates that are dependent on the concentrations of the reagents involved in the reaction. Doubling the concentration increases the rate by a factor of 2

The reaction can be described by the following mechanism:

Reaction step 1: First bisulfite reduces iodate to form iodide (Bisulfite ions are formed when metabisulfite is dissolved in water).

This reaction proceeds very slowly. It is the rate determining step.

Reaction step 2: The iodide formed is oxidized by the iodate excess under formation of iodine. Iodine reacts with starch to form a blue iodine-starch complex.

The reaction proceeds fast.

Reaction step 3: Iodine liberated is instantaneously retransformed by bisulfite to iodide.

This is an "immeasurable fast" reaction. The blue starch iodine complex becomes visible after complete consumption of the bisulfite ions, since then a reduction of the iodine formed cannot take place according to equation (3) any longer.

The collision theory provides a successful basis for understanding the concentration effect. Doubling the concentration doubles the number of collisions, which in turn doubles how fast the reaction goes.

References:
Demonstration Experiment on Video Cocktail 'Old Nassau' - Modified Landolt Reaction
Demonstration Experiment on Video 'One Cola and one Beer, please' - Modified Landolt Reaction
Demonstration Experiment on Video Starch - widely available in many Foods

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