Modern inorganic chemistry

GROUP IV 185

I H III

O" O O~

O=

All the carbon-oxygen bonds are found to be of equal length and

intermediate between carbon-oxygen single and double bond length.

STABILITY OF CARBONATES AND HYDROGENCARBONATES TO HEAT

The stability to heat of metal carbonates is related to the size and
charge of the cation present. Carbonates formed by metal ions with
large radius :charge ratios, for example, Na+, K+, Ba^24 , are stable
to heat at high temperatures whilst those ions with low radius : charge
ratios, for example, Li*, Zn^2 + , Cu2+ form carbonates which are
relatively easily decomposed by heat, the effect being so marked with
Fe3+ and A13+ that neither of these ions is able to form a carbonate
stable at room temperature. These changes in stability have been
attributed to the amount of distortion of the carbonate ion that
the metal ion causes; the greater this distortion the lower the stability
of the carbonate. The hydrogencarbonate ion is unstable and decom-
poses on heating in either solid or solution thus:

2HCOa -* H 2 O + CO 2 | + CO§~

(If the hydrogencarbonate is in solution and the cation is Ca^2 + or
Mg^2 +, the insoluble carbonate is precipitated; this reaction may be
used, therefore, to remove hardness in water by precipitation of
Ca^2 + or Mg^2 + ions.) The ease of decomposition of hydrogencar-
bonates affords a test to distinguish between a hydrogencarbonate
and a carbonate; carbon dioxide is evolved by a hydrogencarbonate,
but not by a carbonate, if it is heated, either as the solid or in solution,
on a boiling water bath.

Other oxides of carbon

Carbon forms a number of oxides in addition to carbon monoxide

and dioxide but they are beyond the scope of this book.

Oxides of silicon

SILICON MONOXIDE, SiO

When silica (silicon dioxide) and silicon are heated in vacuo to 1700 K,