Why can't I find information about certain mercurous compounds in any texts?

What does mercurous carbonate look like and why can't I find in chemistry books?
Ciske 1/19/99

The mercury(I) ion, Hg22+, is an unusual ion of a very peculiar metal. Most mercury(I) compounds are unstable, especially when exposed to water or light, or when heated. Because they are rather uncommon they are mentioned only in passing in most introductory texts.

Mercury (I) (or mercurous) compounds decompose on exposure to water into liquid mercury and mercury(II) (or mercuric) compounds. The equilibrium constant for the disproportionation of mercury(I) ion

Hg22+(aq) = Hg(ell) + Hg2+(aq)

is 6.0×10-3. So while the equilibrium normally lies to the left, the balance is rather delicate. Addition of any anion that precipitates or complexes Hg2+ more than Hg22+ shifts the equilibrium to the right. The result is disproportionation rather than formation of a mercury(I) compound. For example, mercury(I) sulfide decomposes in the presence of water because mercury(II) sulfide (cinnabar) is almost completely insoluble:

Hg22+(aq) + S2-(aq) rightarrow Hg(ell) + HgS(s)

A partial list of mercurous compounds, taken from the Merck Index and the CRC Handbook of chemistry and physics.
Name Formula Appearance Decomposes on
exposure to:
mercury(I) acetate Hg2(C2H3O2)2 flat sheetlike crystals light, water
mercury(I) azide Hg2(N3)2 white crystals shock, light, heat
(explosive!)
mercury(I) bromate Hg2(BrO3)2 crystals water, heat
mercury(I) bromide Hg2Br2 white tetrahedral crystals light
mercury(I) carbonate Hg2CO3 yellow-brown crystals light, heat, water
mercury(I) chloride
(calomel)		 Hg2Cl2 white tetrahedral crystals light
mercury(I) chlorate Hg2(ClO3)2 white crystals light
mercury(I) chromate Hg2CrO4 red needles or powder heat
mercury(I) fluoride Hg2F2 yellow cubic crystals light, heat (as liquid),
water
mercury(I) formate Hg2(CHO2)2 shiny scales heat, hot water
mercury(I) iodate Hg2(IO3)2 yellow powder heat (at 250°C)
mercury(I) iodide Hg2I2 yellow tetrahedral crystals light, heat (at 290°C)
mercury(I) nitrate Hg2(NO3)2·2H2O monoclinic crystals light, water
mercury(I) nitrite Hg2(NO2)2 yellow heat (at 100°C), water
mercury(I) oxide Hg2O black or brown-black powder light, heat (at 100°C)
mercury(I) sulfate Hg2SO4 white-yellow powder light, heat, water
mercury(I) sulfide Hg2S black solid heat, water

References

F. A. Cotton and G. Wilkinson in Advanced Inorganic Chemistry, Wiley Interscience, New York, 1988.
The Merck Index, 8th ed., Merck & Co., 1968.
The CRC Handbook of Chemistry and Physics, 78th ed., eds. D. L. Lide and H.P.R. Frederikse, CRC Press, 1998

Resources

Mercury (J. P. Birk, Arizona State University)
The qualitative analysis and characteristic reactions of mercury(I) and mercury(II) cations are described. Includes photographs of precipitates.
http://www.public.asu.edu/~jpbirk/Qual/QUALANAL/Mercury.htm (01/19/99)

Author: Fred Senese senese@antoine.frostburg.edu


General Chemistry Online! Why can't I find information about certain mercurous compounds in any texts?

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