TRIOXOCARBONATE (iv) ACID

H₂CO₃ is formed when CO_2(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)

 

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 CO₃^2- 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)     +    CO_2(g)          K₂CO_3(aq)      +      H₂O_(l)

Decomposition of corresponding hydrogen trioxocarbonates (iv).

2KHCO_3(s)            K₂CO_3(aq)       +       H₂O_(l)     +     CO_2(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.

CaCl_2(aq)      +     Na₂CO_3(aq)       CaCO_3(s)    +   2NaCl_(aq)

CaCl_2(aq)    +    2NaHCO_3(aq)      CaCO_3(s)   +   2NaCl_(aq)  +  H2O_(l)  +  CO_2(g) 2AgNO_3(aq)   +  Na₂CO_3(aq)          Ag₂CO_3(s)  +   2NaNO_3(aq)

2AgNO_3(aq)  +  2NaHCO_3(aq)       Ag₂CO_3(s)   +   2NaNO_3(aq)  + H₂O_(l)  + CO_2(g)

Note: When preparing the CO₃^2- of the less electropositive metals like Cu, use NaHCO₃

 

Properties of CO₃^2- Salts

Solubility: The trioxocarbonate (iv) of alkali metal and NH₄⁺ are soluble while the other trioxocarbonate (iv) are insoluble in water.

Na₂CO_3(s)   +  2H₂O_(l)        2NaOH_(aq)   +     H₂CO_3(aq)

2. Action of heat: The trioxocarbonate (iv) of Na, K and Barium cannot be decomposed by heat while all other CO₃^2- decompose on heating to liberate CO_2.

ZnCO_3(s)            ZnO(s)   +   CO_2(g)

2Ag₂CO_3(s)           4Ag_(s)       +   2CO_2(g)     +     O_2(g)

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

1. Reaction with dilute acids: All trioxocarbonates (iv) react with dilute acids to form CO₂, H₂O and a salt.

Na₂CO_3(aq)  +  H₂SO_4(aq)     Na₂SO_4(aq)  +  H₂O_(l)  +  CO_2(g)

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

Metal	Solubility/effect of heat	Reaction with acids
K, Na	Soluble in water. Does not decompose on heating	These trioxocarbonate (iv) react with dilute acids to give a salt, water and carbon (iv) oxide.
Ca, Mg, Al, Zn Fe, Sn Pb, Cu	Insoluble in water. Decompose to yield the oxide and carbon (iv) oxide. Al2(CO3)3 does not exist.
Hg, Ag Au	Insoluble in water. Decomposed to the metal, CO2 and oxygen

 

Test for any CO₃^2-

The unknown substance is placed in a test-tube and dilute trioxonitrate (v) acid is added into the test tube. If a CO₃^2- is present, there will be effervescence and the gas which evolved will turn calcium hydroxide solution (lime water) milky.

CO₃^2-_(s)     +    2H⁺_(aq)                    H₂O_(l)     +       CO_2(g)

 

EVALUATION

1. Describe the laboratory preparation of soluble trioxocarbonate (iv) salts
2. Giving suitable equation, state two properties of trioxocarbonate (iv) salts.

 

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 CO₃^2-.

1. 2OH^–_(aq) + CO_2(g)                       CO₃^2-_(aq)       +    H₂O_(l)
2. CO₃^2-_(aq) + CO_{2 (g)}      +       H₂O _(l)                    2HCO₃^–_(aq)

 

Properties of HCO₃^–

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

2NaHCO_3(s)                      Na₂CO_3(s)   +   H₂O_(l)      +     CO_2(g)

3. Reaction with dilute acids: All HCO₃^– reacts with dilute acid to produce CO₂, H₂O and a salt. 2NaHCO_{3 (aq)} + H₂SO_{4 (aq)}        Na₂SO_4(aq)   +    2H₂O_(l)  +  2CO_2(g)

NOTE: This reaction is used to test for HCO₃^–

 

EVALUATION

1. Describe the laboratory preparation of HCO₃^–
2. 2. List the properties of HCO₃^–

 

GENERAL EVALUATION/REVISION

1. State Boyle’s law and Charles’ law
2. A certain mass of gas occupies 300cm³ at 35^oC. At what temperature will it have its volume reduced by half, assuming its pressure remains constant
3. An enclosed vessel contains 2.8g of nitrogen and 14.2g of chlorine at atmospheric pressure and 0^oC. What will be the partial pressure of nitrogen, if the temperature is raised to 180^oC?
4. A certain mass of hydrogen gas collected over water at 10^oC and 760mmHg pressure has a volume of 37cm³. Calculate the volume when it is dry at s.t.p (saturated vapour pressure of water at 10^oC = 9.2mmHg)
5. State Graham’s law of diffusion

 

WEEKEND ASSIGNMENT

1. All are decomposed by heat except (a) BaCO₃ (b) CuCO₃ (c) ZnCO₃ (d) Ag₂CO₃
2. The liquid from a bee sting is found to be acidic. Which of the following solutions will neutralize the bee sting? (a) Solution of pH 9 (b) Solution of pH 7 (c) Solution of pH 5 (d) Solution of pH 1

3. What is the IUPAC name of K₄Fe(CN)₆? (a) Potassium hexacyanoferrate (ii) (b) Potassium hexacyanoferrate (iv) (c) Potassium hexacyanoferrate (vi) (d) Potassium hexacyanoferrate (vii)

4. Which of the following trioxocarbonates (iv) are not decomposed by heat? (a) Calcium and sodium trioxocarbonate (iv) (b) Potassium and zinc trioxocarbonate (iv) (c) Magnesium and potassium trioxocarbonate (iv) (d) Sodium and potassium trioxocarbonate (iv)
5. An aqueous solution of CaCl₂ is (a) acidic solution. (b) alkaline solution (c) a buffer solution (d) a neutral solution

 

THEORY

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).

 

See also

COAL AND FUEL GASES

OXIDE OF CARBON

CARBON: OCCURRENCE, ALLOTROPES, USES & PROPERTIES

BASES AND ALKALIS

ACIDS: CLASSES, BASICITY, PHYSICAL & CHEMICAL PROPERTIES