This compound is best prepared by mixing equal volumes of the trifluoride and chlorine confined over mercury, and allowing the mixture to stand for some days. The volume diminishes to one half, and the composition and molecular weight of the product are thus determined by Gay-Lussac's law and by the density, viz. 5.4 (air = l):—
PF3 + Cl2 = PF3Cl2
It is a colourless gas with an unpleasant odour, and does not burn or support combustion. It condenses to a liquid at -8° C. When heated to over 200° C. it decomposes according to the equation
5PF3Cl2 = 3PF5 + 2PCl5
It is much more easily reduced than the pentafluoride. Hydrogen combines with the chlorine at 250° C. leaving PF3. Many metals, e.g. aluminium, magnesium, tin, lead and iron, decompose it similarly, forming chlorides but not decomposing the trifluoride. Sodium absorbs it completely. Sulphur combines with the phosphorus and the chlorine, giving PSF3 and S2Cl2. Hydrolysis with a limited amount of water splits off the chlorine first, thus:—
PF3Cl2 + H2O = POF3 + 2HCl |