Lab: Microscale Electrolysis
Introduction:
Chemical reactions are sometimes conducted by converting electrical energy into chemical
potential energy. The generic name for these reactions is electrolysis. Whenever a direct current
passes through an electrolyte solution for a sustained period of time, electrolysis reactions take
place.
In this experiment, several electrolysis reactions are studied.
Discussion
Electrolysis is a technique for converting electrical energy into chemical potential energy. The
products of an electrolysis have greater chemical potential energy than do the reactants. In a sense,
electrical energy provides a way of undoing a chemical reaction.
In order for electrolysis to occur, two conditions must prevail. First, the solution (or system)
electrolyzed must conduct electric current. In water solutions this is accomplished by having ions
move through the solution toward electrodes.
Second, at the electrode, electrons must pass from the electrode into the solution or vice versa.
Nearly always this process is accompanied by a chemical reaction. At one of the electrodes, called
the cathode, electrons enter the solution. An example of a cathode reduction is:
2 H2O + 2e- ----> H2(g) + 2OH-
At the other electrode, electrons are removed from the solution and enter the electrode. This
electrode is called the anode, and an example of an anode oxidation is:
2 H2O -----> O2 + 4H+ + 4e-
Water will always pass an electric current and support these reactions. Adding a soluble ionic
compound, a salt such as sodium sulfate, greatly enhances the extent of reaction, however, since it
allows more current to flow. This happens even though neither of the ions (sodium cation or
sulfate anion) is involved in an electrode reaction.
Some compounds provide species, usually cations or anions, that react at the electrode in
preference to water. These are used as examples here. Stannous ion is reduced to tin metal; iodide
ion is oxidized to iodine.
Gas bubbles observed during electrolysis are usually hydrogen at cathodes or oxygen at anodes.
Several other solutions, such as Na2SO4 and NaCl, may be tried. For example, electrolysis of
solutions of ordinary table salt lead to the production of chlorine. The chlorine is concentrated
enough to detect by odor, and may be used to bleach selected colored fibers that may be projected
on the overhead projector.
Safety:
Potassium iodide, stannous chloride, silver nitrate and phenolphthalein are toxic. Do not ingest
toxic chemicals. Silver nitrate causes black stains on skin. Wash all spilled chemicals immediately
with large amounts of water. Wash hands before leaving the laboratory.
Procedures:
Build the apparatus shown. Sharpen both ends of 2 pencils. Clip the alligator clips to one end of
each
pencil.
1. Place U tube in a beaker and then fill the U tube with a solution of potassium iodide. In one end of the U tube put
a small amount of soluble starch solution and add 1-2 drops of phenolphhalein to the other end. Stir each end
with a toothpick. Place the electrode attached to the positive end of the battery or power source in the end of the
U tube to which you added starch solution. Place the electrode attached to the negative end of the battery in the
other end of the U tube. Note any evidence for reaction. If no reaction occurs reverse the electrodes. Rinse
your electrodes and wipe with a paper towel. Wash out the U tube and keep it for part 2
2. Place U tube in a beaker and then fill the U tube with a solution of sodium sulfate. In each end of the U tube put a
couple of drops of bromthymol blue solution. Stir each end with a toothpick.Place the electrodes \in the ends of
the U tube and watch for a chemical reaction. If no reaction occurs reverse the electrodes. Rinse your
electrodes and wipe with a paper towel. Wash out the U tube and keep it for part 2
3.
Fill a well in a 24 well plate about two third full with a solution of copper sulfate. Place your carbon electrodes
in the well and look for evidence of a chemical reaction. Reverse the electrodes by switching the alligator clips
and observe the results. Record your observations
4. Fill a well in a 24 well plate about two third full with a solution of copper sulfate. Replace your carbon
electrodes with two fine strips of copper metal. Place the copper electrodes in the well making sure that they do
not touch each other and look for evidence of a chemical reaction. Reverse the electrodes by switching the
alligator clips and observe the results. Record your observations
5.
Fill a well in a 24 well plate about two third full with a 0.1 M NaCl solution. Place the electrodes in the solution,
and note any evidence for reaction. Add a drop or two of food coloring. Stir with a toothpick. Place the
electrodes in the solution. Note any evidence for reaction. After a few moments, use a wafting technique to sniff
the gas produced. Rinse the electrodes and wipe with a paper towel.
6.
Fill a well in a 24 well plate about two third full with a 0.1 M 0.1 M silver nitrate solution. Place the electrodes
in the solution, and note any evidence for reaction. Note any evidence for reaction. Rinse the electrodes and
wipe with a paper towel.
7.
Fill a well in a 24 well plate about two third full with freshly prepare 0.1 M stannous chloride solution. Using
your carbon pencil electrodes again, place the electrodes in the solution . Note any evidence for reaction. Rinse
the electrodes and wipe with a paper towel.