Carbon and It’S Compounds V: Trioxocarbonate (IV) Acid – SS1 Chemistry Lesson Note (Third Term)

TRIOXOCARBONATE (iv) ACID

H2CO3 is formed when CO2(g) is dissolved in water. H2CO3 is a weak dibasic acid. It forms two series of salts:

1. Normal trioxocarbonate (iv)
2. Acidic hydrogen trioxocarbonate (iv)
3. Normal trioxocarbonate (iv)

Normal trioxocarbonate (iv) may be regarded as salts derived from H2CO3 by the complete replacement of the hydrogen by a metal or cationic radical.

Preparation of soluble trioxocarbonates (iv)

The CO32- of Na+, K+, and NH4+ are soluble in water. They are prepared in the laboratory by:

Bubbling CO2 through a solution of corresponding alkali.

2KOH(aq) + CO2(g) > K2CO3(aq) + H2O(l)

Decomposition of corresponding hydrogen trioxocarbonates (iv).

2KHCO3(s) > K2CO3(aq) + H2O(l) + CO2(g)

Preparation of insoluble trioxocarbonates (iv)

The insoluble metallic trioxocarbonates (iv) can be prepared by adding a solution of Na2CO3 or NaHCO3 to a solution of the corresponding metallic salt.

1. CaCl2(aq) + Na2CO3(aq) > CaCO3(s) + 2NaCl(aq)
2. CaCl2(aq) + 2NaHCO3(aq) > CaCO3(s) + 2NaCl(aq) + H2O(l) + CO2(g). 2AgNO3(aq)  + Na2CO3(aq) > Ag2CO3(s)  +   2NaNO3(aq)
3. 2AgNO3(aq) + 2NaHCO3(aq) > Ag2CO3(s) + 2NaNO3(aq) + H2O(l) + CO2(g)

Note: When preparing the CO32- of the less electropositive metals like Cu, use NaHCO3

Properties of co32- salts

1. Solubility: The trioxocarbonate (iv) of alkali metal and NH4+ are soluble while the other trioxocarbonate (iv) is insoluble in water. Na2CO3(s)   +  2H2O(l) > 2NaOH(aq)   +     H2CO3(aq)
2. Action of heat: The trioxocarbonate (iv) of Na, K and Barium cannot be decomposed by heat while all other CO32- decompose on heating to liberate CO2. ZnCO3(s) > ZnO(s)  + CO2(g) > 2Ag2CO3(s) > 4Ag(s) + 2CO2(g) +     O2(g)

(NH4)2CO3(s) > 2NH3(g) + CO2(g) + H2O(l)

3. Reaction with dilute acids: All trioxocarbonates (iv) react with dilute acids to form CO2, H2O and a salt. Na2CO3(aq) + H2SO4(aq) > Na2SO4(aq) + H2O(l) + CO2(g)
4. ZnCO3(s) + 2HCl(aq) > ZnCl2(aq)   + H2O(l) + CO2(g)

TEST FOR ANY CO32–

The unknown substance is placed in a test tube and dilute trinitrate (v) acid is added to the e. If a CO32- is present, there will be effervescence and the gas which evolved will turn calcium hydroxide solution (lime water) milky.

CO32-(s) + 2H+(aq) > H2O(l) + CO2(g)

• HYDROGEN TRIOXOCARBONATE (iv).

HCO3- may also be regarded as salts derived from H2CO3 by the partial replacement of the hydrogen by a metal or cationic radical.

Preparation of hco3-

HCO3- can be prepared by passing CO2 through a cold solution of the corresponding OH- or CO32-.

1. 2OH-(aq) + CO2(g) > CO32-(aq) + H2O(l)
2. CO32-(aq) + CO2 (g)l + H2O (l) > 2HCO3-(aq)

Properties of hco3-

1. Solubility: All hydrogen trioxocarbonate (IV) is soluble in water.
2. Action of heat: They can all be decomposed by heat.

2NaHCO3(s) > Na2CO3(s) + H2O(l) + CO2(g)

3. Reaction with dilute acids: All HCO3- reacts with dilute acid to produce CO2, H2O and a salt. 2NaHCO3 (aq) + H2SO4 (aq) > Na2SO4(aq) + 2H2O(l) + 2CO2(g)

NOTE: This reaction is used to test for HCO3-

ASSIGNMENT

1. State the function of each of the following substances in the laboratory preparation of dry carbon (iv) oxide: 

(a) potassium hydrogen trioxocarbonate (iv) solution; 

(b) fused calcium chloride.

2(a) Draw a labelled diagram for the laboratory preparation of dry carbon (iv) oxide. 

(b) Write a balanced equation for the decomposition of potassium hydrogen trioxocarbonate (iv).