History

Splitting up water by passing an electric current through it was first done by Nicholson and Carlisle in 1800. Water was the first substance to be "electrolysed". They used the battery which has just been invented by Signor Volta to produce the electric current (The generator was invented 30 years later).

 
   
Physics and chemistry

The equation for the cathode half-reaction in the electrolysis of water is:

4 H3O+ + 4 e- --> 2 H2 + 4 H2O reduction, at the cathode

The equation for the anode half-reaction in the electrolysis of water is:

6 H2O --> O2 + 4 H3O+ + 4 e- oxydation, at the anode

The equation for the net reaction is the sum of the half-reactions:

2 H2O (liquid) --> 2 H2 (gas) + O2 (gas) DG°rxn = + 474.4 kJ

Water electrolysis requires energy to break the oxygen-hydrogen-bond. The water-splitting reaction is driven uphill by the voltage applied across the electrodes. The electrochemical decomposition of water cannot occur unless the voltage is high enough to overcome the strength of the hydrogen-oxygen bonds. The minimum voltage necessary to electrochemically decompose a sample is called decomposition voltage. The minimum decomposition voltage for water at 25°C is about 1.2 V.

Hydrogen is a colourless, odourless and tasteless gas. Lighter than air and all other gases, it is the least dense of all substances (0.09 g/l). Hydrogen has a high energy content by weight, its energy density (33.3 kWh/kg at temperatures below -259.2 °C) being three times that of petrol. Hydrogen melts at -259.2 °C and boils at -259.77 °C, with only helium having a lower melting point. Solid hydrogen (first produced in 1996) exhibits metallic properties such as electrical conductivity.

 
   
H2 preconception: hydrogen is explosive

By itself hydrogen will not explode. Even a mixture of hydrogen with air or pure oxygen gives no reaction. But when these mixtures are ignited a fast exothermic reaction takes place (see applications).

 
   
Applications

Hydrogen and oxygen are used to power the biggest space rockets because they provide the most energy per ton of any fuel. In order to get plenty on board, the gases have to be liquefied and kept very cold until needed. Rockets need to take their own oxygen for burning the hydrogen; when we have hydrogen-powered cars, we can use the oxygen in the air.

 
   
Things you can try yourself

You can do your own electrolysis of water using two bits of a wire and a battery. If the water has a little salt in it, so much the better, because pure water is rather a bad conductor. You should be able to see the bubbles of hydrogen and oxygen rising from the two wires where they dip into the water.

References: Techniquest, the UK's Leading Discovery Centre, Cardiff.

 

 

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