SS1 Chemistry Lesson Note on Metallic Trioxocarbonates

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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.

Na₂CO_3(aq) + CuSO_{4(aq)(      →}           CuCO_3(s) + Na₂SO_4(aq)

Na₂CO_3(aq)+ ZnCl_2(aq)  →     ZnCO_3(s) + 2NaCl_(aq

White

METHOD 2: Ca(OH)_2(aq) + CO_2(g)         →        CaCO_3(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 K₂CO_3(s) + 2H₂O      →       2KOH_(aq) + H₂CO_3(aq)

3. All trioxocarbonate(IV) salts except those of potassium and Sodium, decompose on heating, to liberate CO_2(g).

e.g. ZnCO_3(s)   →     ZnO_(s) + CO_2(g)

(NH₄)₂CO_3(s)   →       2NH_3(g) + H₂O_(g) + CO_2(g)

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

ZnCO_3(s) + 2HCl_{(aq)        →}       ZnCl_2(aq) + H2O_(l) + CO_2(g)

Ionically: CO_3(s)^2-+ 2H+_(aq      →          H₂O_(l) + CO_2(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.

CO_2(g) + H₂O⇌      H₂CO_3(aq    ⇌  2H⁺_(aq) + CO^2-_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) +  HCO^–_3(aq)
2. HCO^–_3(aq) ⇌ H+(aq) + CO^2-_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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