Why can't we experiment with mercury?

Prof. Blum's tip of the month March 2007 (tip no. 117)

Hard times have come for hands-on chemistry classes. Much everyday is forbidden. As in industry, a hazard sheet must be drawn up for each chemical. No wonder that teachers no longer want to experiment, so they dispose of all chemicals prophylactically and only teach the students to learn to learn.

But don't dispose of everything! For example, save some mercury. You can make a nice attempt with this. A steady hand is a prerequisite for this!

Experiment 1: The chemical heart
A large drop of mercury (diameter about 2 cm) is placed on a watch glass (diameter 12 cm). Then a solution of potassium permanganate (w = 1%) in sulfuric acid (c = 2 mol / l) is prepared. This solution is poured onto the watch glass until the drop is just covered.

If you now touch the drop in the solution with a clean, degreased iron nail (not a steel nail!), The drop will contract. The latter loses contact with the nail and therefore expands again. As a result, it touches the nail again (etc.).

Every now and then, potassium permanganate solution has to be added, because the potassium permanganate is obviously used up. In addition, the nail should be brushed with filter paper from time to time because it will turn black.

The most common mistake is to poke the nail too deep into the mercury drop.

Prof. Blume makes the mercury heart beat
(Photo: Daggi)
There is a film about this (5.2 MB)
Click here

The result of the movement of the mercury drop is a rhythmic movement of the drop. Since triangular structures are formed, this is reminiscent of the beating of a heart. In general, unfortunately, most experimenters only get a ventricular fibrillation ...

Mercury and iron are different precious metals
When they touch, the more noble mercury attracts electrons, and as a positive pole, it charges itself increasingly negatively. We could thus build a battery from the experimental setup.
Let's find the tension between the two metals.

Experiment 2: voltage measurement in the mercury heart
First the watch glass is prepared as described in experiment 1. We attach a cable over a clean steel alligator clip on the iron nail. We dip a second cable - also with a steel alligator clip - into the mercury. Then we measure the voltage with a measuring device. It was 0.84 volts for us.
When the two metals are short-circuited, the voltage collapses.

Mercury is the cathode and iron is the anode.

The iron forms iron (II) ions because of the release of electrons to balance the charge.

Fe ———> Fe2++ 2 e-

The formation of iron (II) ions is promoted by the fact that the iron is quickly coated with black iron oxide. The oxygen donor is potassium permanganate.

The fact that mercury is also chemically active can be recognized by the fact that light-colored coatings briefly form on the surface, which pull over the metal, but quickly disappear again. These are hydrogen bubbles. So everything speaks in favor of catalyzed corrosion of the iron. (The fact that hydrogen bubbles are occasionally observed on iron nails is due to the simple acidic effect of the very concentrated sulfuric acid, which is known to be able to decompose iron even without catalysis.)

But are the protons of sulfuric acid really the oxidizing agent that turns the iron nail into an anode?

The following observation puts us on the right track to answer the question: The potassium permanganate discolours around the mercury. Instead of the hydrogen ions, it takes electrons from the mercury and forms colorless manganese (II) ions.

MnO4- + 8 H.+ + 5 e- ———> Mn2+ + 4 H.2O

The protons are extremely helpful here.

The overall reaction equation is:

How is it now with the moving drop of mercury?
The electrons flowing in from the iron mainly occupy the surface of the mercury. This increases the surface tension of the liquid metal. (One speaks of electrical surface polarization.)
The drop contracts and loses contact with the electron donating iron.
Now the permanganate becomes active: It frees the mercury from the excess electrons. Therefore the drop expands again and comes into contact with the iron nail again. It is reloaded - the oscillation starts.

With this tip I say goodbye to retirement! But don't worry: the server will continue to exist. Because my heart continues to beat for chemistry. I hope it will do that for a long time ...

R diger flower

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