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METALLIC TRIOXOCARBONATES, OCCURRENCES, PREPARATION AND USES METALLIC

All trioxocarbonate(IV) salts are insoluble in water except soluble. Hence, they are prepared by double decomposition method, using aqueous Na2CO3 e.g.

Na2CO3(aq) + CuSO4(aq)(      →           CuCO3(s) + Na2SO4(aq)

Na2CO3(aq)+ ZnCl2(aq)  →     ZnCO3(s) + 2NaCl(aq

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METHOD 2: Ca(OH)2(aq) + CO2(g)         →        CaCO3(s) + H2O(l)

PROPERTIES OF SALTS

  1. All trioxocarbonate(IV) salts are insoluble in water, except those of Na, K and NH4 which are soluble.
  2. They dissolve in water to produce alkaline solutions. This is because they are salts of strong bases and weak acid, hence, they hydrolysed in water to produce alkaline

Solutions’

e.g K2CO3(s) + 2H2O      →       2KOH(aq) + H2CO3(aq)

  1. All trioxocarbonate(IV) salts except those of potassium and Sodium, decompose on heating, to liberate CO2(g).

e.g. ZnCO3(s)   →     ZnO(s) + CO2(g)

(NH4)2CO3(s)   →       2NH3(g) + H2O(g) + CO2(g)

Note: Na2CO3 and K2CO3 are not affected by heating, to liberate carbon(IV) oxide e.g.

ZnCO3(s) + 2HCl(aq)        →       ZnCl2(aq) + H2O(l) + CO2(g)

Ionically: CO3(s)2-+ 2H+(aq      →          H2O(l) + CO2(g)

TRIOXOCARBONATE(IV) ACID

Trioxocarbonate(IV) [carbonic acid], H2 CO3 is a very weak dibasic acid. It is obtained when carbon(IV) oxide dissolves in water. It is unsuitable; hence never isolated.

CO2(g) + H2O⇌      H2CO3(aq    ⇌  2H+(aq) + CO2-3(aq)

The acid forms both the normal and acid salts; i.e the acid ionizes into two stages;

  1. H2CO3(aq) ⇌         H2CO3(aq)     ⇌         H+(aq) +  HCO3(aq)
  2. HCO3(aq)               H+(aq) + CO2-3(aq)

Uses of trioxocarbonate(IV) salts

  1. Na2CO3 is used in softening hard water, in making soap and glass, and as an alkali in acid-base titration.
  2. CaCO3 is used in making quick lime, cement and glass; in agriculture [in liming the soil – to reduce acidity]

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