It is not always necessary to use the free metal ; for chlorine also attacks
various compounds, producing chlorides. For example, very often the
metallic oxides are decomposed when heated in chlorine gas.
Silver oxide, Ag20, at ordinary temperatures, is converted into chloride
with liberation of oxygen, while the oxides of the alkaline earths, CaO, BaO,
SrO, are decomposed on heating, with the formation of the corresponding
chlorides and the liberation of oxygen gas, whose volume amounts to half the
volume of the absorbed chlorine.
CaO, BaO, SrO, when heated red-hot and plunged into chlorine, are
actually decomposed with the production of incandescence. MgO is also
decomposed when highly heated in chlorine gas, but without incandescence
appearing. Also the oxides of zinc, cadmium, lead, copper, and nickel are
easily decomposed by hot chlorine, while cobalt and iron oxides are
decomposed only with difficulty. Aluminium oxide and silica are not
decomposed, even at a white heat only small amounts of chlorides being
formed. The trioxides Cr2 8, Mo03, W03 form oxychlorides. SnO, Sb2 5,
As2 3 , when strongly ignited in .the gas, are completely converted into
chlorides.
Several oxides which are only slightly attacked in the presence of hot
chlorine are readily decomposed if mixed with charcoal, and then the mixture
is ignited in a stream of chlorine. For example, SiCl4, SnCl4, A1C13, ThCl4,
etc., may be prepared by this method from the corresponding chlorides.
Chlorine also acts on most bromides and iodides of metals, displacing
bromine or iodine and producing chlorides. Many sulphides are decomposed
when heated in a stream of chlorine, and this provides a method of producing
chlorides of many metals.
A modification of this method consists in passing a stream of chlorine
charged with vapours of disulphur dichloride over heated oxides and oxygen
salts. Many anhydrous chlorides have been obtained in this manner. Si02,
for example, at a dull red heat yields SiCl4 ; A12 3 gives A1C13, while Th02
at a red heat yields ThCl4. W03 gives W02C1 2 at a high temperature, and
WOC14 at a lower temperature. NiO, CoO at 400 give the chlorides, while
Cr2 3 and Fe2Cl3 give CrCl3 and FeCl3 even below redness.
Similarly the oxy-salts of calcium, strontium, and barium (e.g. the
sulphates), are transformed into chlorides by this method. In the application
of this, however, care must be taken not to have excess of disulphur
dichloride if the chlorides to be prepared are volatile. When, however,
the chlorides to be prepared are fairly non-volatile, the action is accelerated
by excess of the sulphur chloride. |
