Water as a Catalyst

The presence of water has considerable influence on the course of some chemical reactions; thus an alcoholic solution of potassium acetate reacts with carbon dioxide, yielding a precipitate of potassium carbonate, a result which is in striking contrast with the interaction of acetic acid and potassium carbonate in aqueous solution. Also silver nitrate and hydrogen chloride fail to react in anhydrous benzene or ether. Many such precipitation and other reactions fail in the absence of water.

Water-vapour is capable of functioning as a catalyst in many reactions, attention has already been directed to the combustion of hydrogen and carbon monoxide in oxygen, the presence of small quantities of water being essential. Carbonyl sulphide, COS, will not combine either with oxygen or nitrous oxide when perfectly pure and free from water. Thoroughly dry carbon dioxide is not reduced by dry carbon even at red heat, and no visible combustion of pure sugar charcoal is observed even at bright-red heat in oxygen, although carbon monoxide is being slowly produced. Sulphur may be distilled in dry oxygen without visible combustion, and the same appears to be true for sodium and potassium. Calcium is not oxidised by dry oxygen. Boron and red and yellow phosphorus do not burn in the dried gas. Indeed, amorphous phosphorus is not even converted to the red variety when heated to 278° C. in pure, dry nitrogen, although normally the conversion takes place with considerable violence just above 260° C.

Water-vapour exerts a considerable influence upon the dissociation of nitrogen trioxide. The effect of prolonged drying of the liquid is apparently to reduce the number of NO₂ and NO molecules normally present by causing them to combine to N₄O₆ molecules. On vaporisation, these latter molecules dissociate to a mixture of N₂O₃, NO, and NO₂ molecules, the N₂O₃ not undergoing further dissociation. Thus

N₄O₆ ⇔ N₂O₃ + NO, + NO₂.

The activity of the halogens is greatly influenced by the presence of traces of water. Thus, dry chlorine does not attack copper, and dry hydrogen chloride does not unite with ammonia. Although dry chlorine attacks mercury, mercurous chloride does not dissociate upon heating, when thoroughly dried. The same is true for ammonium chloride at 350° C. and phosphorus pentachloride.

Hydrogen and chlorine do not explode in sunlight when dry; only slow combination takes place. Similarly ammonium chloride and lime do not interact.

Nitric oxide does not unite with perfectly dry oxygen. Experiments indicate that perfectly dry sulphur dioxide and oxygen will not unite in contact with platinised pumice, whilst sulphur trioxide has no action on calcium oxide.

On the other hand, selenium, tellurium, arsenic, antimony, and carbon disulphide appear to burn readily in dry oxygen. Dry ozone is decomposed on warming, but may be prepared in the usual way from dry oxygen. Iron yields its characteristic " tempering " colours when heated in dry air or oxygen. Both lead nitrate and potassium chlorate decompose when dry.