406 THE TRANSITION ELEMENTS
heating with sulphur compounds to give the sulphide, which is
roasted to form the oxide; the latter may be reduced directly by
heating with coke or dissolved to give a solution containing nickel(II)
from which the nickel can be deposited electrolytically. The metal
obtained by reduction can be purified by the Mond process, in
which it is heated to 320 K with carbon monoxide to give the pure,
volatile tetracarbonyl Ni(CO) 4 ; the latter when heated to 500 K
gives the pure metal and carbon monoxide is recovered:Ni + 4CO ^ Ni(CO) 4Nickel is a moderately lustrous, silvery metal, and is extensively
used in alloys (for example coinage, stainless steel) and for plating
where a durable resistant surface is required. It is also used as an
industrial catalyst, for example in the hydrogenation of unsaturated
organic compounds. It is attacked by dilute aqueous acids but not
by alkalis; it combines readily with many non-metals on heating.
In the chemistry of nickel, we observe the continuing tendency
for the higher oxidation states to decrease in stability along the
first transition series; unlike cobalt and iron, the -f 3 state is rare
and relatively unimportant for nickel and the +2 state is the only
important one.Oxidation state + 2
Nickel forms yellow anhydrous halides NiX 2 (X = F, CL Br) and a
black iodide NiI 2 : all these halides are made by direct combination
of the elements, and the chloride by reaction of sulphur dichloride
oxide with the hydrated salt. All dissolve in water to give green
solutions from which the hydrates can be crystallised; the solutions
contain the ion [Ni(H 2 O) 6 ]2+, and the chloride crystallises as
NiCl 2. 6H 2 O, nickel(II) chloride hexahydrate.
Addition of an alkali metal hydroxide solution to an aqueous
solution of a nickel(II) salt precipitates a finely-divided green
powder, nickeHII) hydroxide Ni(OH) 2 ; on heating this gives the
black oxide. NiO, which is also obtained by heating nickel(II)
carbonate or the hydrated nitrate. Black nickel(II) sulphide, NiS, is
obtained by passing hydrogen sulphide into a solution of a nickeHII)
salt.
Nickel forms a green hydrated sulphate NiSO 4. 7H 2 O and the
double sulphate (NH 4 ) 2 SO 4. NiSO 4. 6H,O (cf. iron, p. 396).