Cracking of water

Definition

The cracking of water is a process allowing obtaining Hydrogène and Oxygène by dissociating the Atome S composing the Molécule of water H2O. It is about a reaction Thermochimique occurring at high temperature (between 850°C and 900°C).

It is about the one of the tracks under consideration for the production in very great quantity of hydrogen bound for the combustible batteries, in particular for portable applications (car, cellphone, laptop, etc)

Energy assessment

Assessment of the decomposition of a water molecule:

H₂O → H₂ + ½ O₂

The water H2O molecule consists of 2 connections O-H and each connection has a molar energy of 460 kJ, which represents 2 X 460 = 920 kJ for a mole of water.

From where the rupture of connections O-H of the water molecules for a mole of water requires the contribution of 920 kJ (left side of the equation).

However the recombining of the atoms of hydrogen H in H2 (hydrogen gas) will produce a contribution of energy:

H-H → H₂

This recombining brings 432 kJ .

In the same way for the recombining of the oxygen atoms:

½ O-O → ½ O₂

This reaction will release ½ X 494 kj is 247 kJ .

Balance operation:

920 - 432 - 247 = 241 kJ

Thus the manufacture of 2 G hydrogen by cracking of a mole of water (without taking account of the losses) requires the contribution of 241 kJ , that is to say 120.500 kJ to manufacture 1 kg hydrogen.

Implementation

As mentioned above, this phenomenon, which still waits to arrive at a stage of industrial and economic maturity, is one of the tracks under consideration for the manufacture of hydrogen in great quantity to be used as vector of clean energy for the future years. The use being envisaged mainly in the mobile units like the cellphones, the computers, the cars, etc

The energy production necessary to cracking, in order to be a true projection on the ecological level, will have to be done by a clean primary energy source and not rejecting a Gaz with greenhouse effect.

It is the die Nucléaire which offers the most promising track here. Indeed, the nuclear reactors of new generation, still on the drawing board, could allow a double use of fission at ends of traditional electric production and hydrogen. The nuclear reactors of type HTR (High Reactor Temperature | High-temperature reactor) or HTGR (Hight Temperature Gas Cooled Reactor | Engine High Temperature cools with Gas) will make it possible, if they are produced, to reach the temperatures about 900°C necessary to the cracking of the water molecule.

The advantage of the HTGR is that it burns the fissionable matters most frequent, of which the Plutonium. The other favors nuclear energy utilization lies in the fact that waste is produced in very small quantities for an unequalled energy production, and that those can be stored on standby hiding, treatment or another process. The storage of waste being one of the properties of the nuclear power which is specific for him. This waste is extremely polluting and over very a long period, it would be preferable to find an alternative energy more " propre".

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