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The preceding examples show that a state of equilibrium may be disturbed by withdrawing one or more of the interacting substances from the sphere of action. In examining the proportions of substances existing in equilibrium, it must be ensured that the reverse reaction does not take place when the conditions are changed. If hydrogen iodide is heated until equilibrium is attained: 2HI <=> H2 + I2, the proportions of HI, H2, and I2 may be determined after rapidly cooling the mixture, when very little reaction occurs. In some cases, e.g., the dissociation of steam: 2H2O <=> 2H2 + O2, this cooling must be performed exceedingly quickly, otherwise the reverse reaction occurs and no trace of the products of dissociation can be discovered.
Grove (1847) heated a platinum wire in steam by an electric current. In contact with the hot wire, dissociation occurred and the products at once passed into the diluting atmosphere of steam, which prevented their recombination by separating them and by cooling. If a heated platinum wire (the temperature of which can be measured from its electrical resistance) is allowed to remain for a sufficient length of time in a flask of steam, the products of dissociation and the unchanged steam are continually brought in contact with the heated wire by diffusion, and a state of equilibrium is ultimately attained, corresponding with the temperature of the wire.
Deville (1864) demonstrated the dissociation of gases at high temperatures by means of the apparatus shown in
 | Fig: Devilles experiment on dissociation |
A wide tube of glazed porcelain, with a narrower tube of unglazed porcelain supported axially inside, was heated strongly in a furnace. Water vapour was passed through the inner tube and carbon dioxide through the annular space, and the gases from both were collected over potash solution, which absorbed the carbon dioxide. The steam was dissociated and the hydrogen passed out by diffusion through the porous tube into the annular space, leaving most of the oxygen in the inner tube. If the two gases were passed to the same receiver, i c.c. of detonating gas (2H2 + O2) was collected for every gram of water passed through the apparatus. If carbon dioxide was passed rapidly through a glazed porcelain tube packed with fragments of porcelain heated in a furnace to 1200°-1300°, dissociation occurred: 2CO2 <=> 2CO + O2. When the gas was collected over caustic potash, a small volume of a mixture of carbon monoxide and oxygen was obtained.
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