Page:Outlines of Physical Chemistry - 1899.djvu/244

 226 OUTLINES OF PHYSICAL CHEMISTRY

stances aa' and bb' (elementary or compound) 1 can give rise by a double exchange to two new substances ab and a'b'.

The conditions are :

1. That the affinities or valencies of the atoms must be satisfied in the new compounds.

2. That the original molecules must be endowed with a sufficient mobility for them to come into close contact, This amounts to ' corpora non agunt nisi fluida,' and a transposition is only possible if at least one of the reacting

3. That the temperature be favourable to the reaction. Chemical reactions, even the most violent ones, do not take place except between certain temperature limits. Hydrogen and oxygen only combine with explosion above 620°O., 3 and hydrogen and chlorine likewise only unite under the influence of heat or of light. \

Experiments performed by Donny and Mareska (1845) and by Baoul Pictet (1892-1893) show that at -80°C. the most energetic reactions (at ordinary temperatures) with which we are acquainted do not take place at all : sulphuric acid has no action on barium chloride, hydro- chloric acid none on silver nitrate, sulphuric acid none on sodium, and sodium none on alcohol. Towards — 125°C. chemical activity seems to be entirely suspended. And the reason of this inertness is not difficult to understand. Let us notice that in the substances brought together the

��1 a and a' or b and b' may be identical or different atoms.

2 We are, however, acquainted with a few reactions between solids.

Barium sulphate and potassium carbonate (and inversely barium carbonate and potassium sulphate) enter into reaction without either of them being dissolved, it being only necessary to mix them together in a finely powdered state. Under the influence of a strong pressure (up to 6,000 atmospheres) the reaction is fairly energetic (Spring).

8 A mixture of hydrogen and oxygen explodes when heated to 620 - 680°C. (V. Meyer and MUmch).— Silent combination takes place slowly at 300°. —At 100° the reaction is so slow that it cannot be experimentally followed (V. Meyer and, Baum),

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