Breaking Water

Water is amazing stuff.

Just one atom of oxygen and two of hydrogen. Splitting it into these parts turns out to be quite easy. If you’ve a couple of pieces of platinum to spare…Water undergoes a decomposition reaction under electrolysis.

The Hoffmann voltameter was the original apparatus for electrolysing water, invented by August Wilhelm von Hoffmann (1818–1892). It consists of three joined upright cylinders, usually glass. The middle cylinder is open at the top to allow addition of water and an ionic compound to improve conductivity, such as a small amount of sulphuric acid. A platinum electrode is placed inside the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of electricity. When current flows, gaseous oxygen forms at the anode and gaseous hydrogen at the cathode. Each gas displaces water and collects at the top of the two outer tubes. It can be tested in the usual way, relighting a glowing splint for oxygen and the pop test for hydrogen. Here’s a picture of one.

hoff1

Can we make one in school? Yes – here’s what to do.. Look at the diagram. Fill the large gas jar with 0.1 Mhoof2 sulphuric acid almost to the top and hang the electrode assembly over the rim. Lower the burettes into the acid with the taps open until the open ends rest on the bottom of the jar. Close the top and lift one burette to surround one electrode. Repeat with the other burette and support them in clips as shown. Connect the voltameter into a series circuit of rheostat, ammeter and DC supply. Switch on. A current will now flow. The rheostat can be adjusted to give a suitable current of about 0.5 A. Bubbles will be seen at both electrodes and gas can be collected in the inverted burettes.

Things to notice:

We get twice as much hydrogen as oxygen. Why?

What will the equations look like?

oxid

The complete equations can be found in this document


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