Lab #20 - Electrolysis of Aqueous Potassium iodide
|Introduction: Recall that when water
undergoes electrolysis, the oxidation occurring at the anode produced oxygen
gas while the reduction at the cathode produced hydrogen gas. Electrolysis
of some aqueous salt solutions, however, may lead to the oxidation or reduction
of the ions from the salt - if these ions are more easily oxidized or reduced
than water itself. In this experiment you will electrolyze
an aqueous solution of potassium iodide and then identify the products that
are formed at the electrodes.
|Problem: What are the products
of the electrolysis of aqueous potassium iodide?
|Apparatus: U-tube, carbon electrodes, connecting
wires, DC power source.
|Materials: aqueous potassium iodide,
TTE or some other organic solvent.
||Set up the apparatus as shown in the figure above.
||Add enough 0.5 M potassium iodide solution to fill the U-tube about
three-quarters full. Make the electrical connections and allow the electrolysis
to proceed for about 15 minutes.
||Note and record any observable products and colour changes that occur
at the electrode attached to the positive terminal of the power supply (the
anode). Make the same observations for the changes occurring at the
electrode connected to the negative terminal of the power source (the cathode)
||When you complete the electrolysis, carefully remove the anode and
note its odor. Using a medicine dropper, transfer about 2 mL of the
dark brown liquid from the anode side into a small test tube. Add about
1 mL of an organic solvent. Stopper and shake the test tube for a few
seconds. Allow the more dense organic solvent layer to settle and note
the colours of the two liquid layers. What material is present in the
solution around the positive electrode (anode?)
||Using a pipette, transfer about 2 mL of the solution from the
cathode side into another test tube. Add a few drops of phenolphthalein
indicator to find the approximate hydrogen ion concentration of the solution.
Add 1 mL of the organic solvent and shake the tube. The add a few mL
of 0.1 M iron(III) chloride solution drop by drop (with shaking between drops),
and note the results.
||Write the equation for the half reaction occurring at the anode.
||As iodine is produced at the anode, it forms the brown, complex ion,
I3-1 with the iodide ion of the electrolytic solution.
a) Write the equation for the reaction.
b) What effect did the organic solvent in Step
4 have brown liquid? Use your observations on the colour changes in
the two layers to explain the effect.
||a) Is the solution around the cathode acidic or basic?
Write the equation for the half-cell reaction occurring at the cathode.
b) Write the equation for the reaction of 0.1
M FeCl3 with the sample of solution from the cathode side.
||When iodine, I2, reacts with a basic solution, it undergoes
a self oxidation reaction for form iodide ions, I-1, and iodate
ions, IO3-1. Both of these ions are colourless
in aqueous solution. Give a plausible explanation for the appearance
of the sharp colour boundary noted near the bottom of the U-tube. Explain
in terms of your knowledge of the products at each electrode. Write
the equation for the reaction involved.