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

Trioxocarbonate (IV) acid, H₂CO₃, is formed when carbon (IV) oxide dissolves in water. It is a weak dibasic acid, so it forms two series of salts: normal trioxocarbonates (IV), where all the hydrogen is replaced by a metal or cationic radical, and hydrogen trioxocarbonates (IV), where only part of the hydrogen is replaced.

Behavioural objectives

By the end of the lesson, students should be able to:

• state the two series of salts formed by H₂CO₃;
• describe how soluble and insoluble trioxocarbonates (IV) are prepared in the laboratory;
• state the properties of trioxocarbonates (IV) and hydrogen trioxocarbonates (IV);
• describe the laboratory test for a trioxocarbonate (IV);
• write balanced equations for the preparation and reactions of these salts.

What is 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 trioxocarbonates (IV)
2. Acidic hydrogen trioxocarbonates (IV)

What are normal trioxocarbonates (IV)?

Normal trioxocarbonates (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- salts of Na⁺, K⁺ and NH₄⁺ are soluble in water. They are prepared in the laboratory by:

1. Bubbling CO₂ through a solution of the corresponding alkali.
   2KOH(aq) + CO₂(g) → K₂CO₃(aq) + H₂O(l)
2. Decomposition of the corresponding hydrogen trioxocarbonate (IV).
   2KHCO₃(s) → K₂CO₃(s) + 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)
3. 2AgNO₃(aq) + Na₂CO₃(aq) → Ag₂CO₃(s) + 2NaNO₃(aq)
4. 2AgNO₃(aq) + 2NaHCO₃(aq) → Ag₂CO₃(s) + 2NaNO₃(aq) + H₂O(l) + CO₂(g)

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

Properties of CO₃^2- salts

1. Solubility: The trioxocarbonates (IV) of alkali metals and NH₄⁺ are soluble in water, while the other trioxocarbonates (IV) are insoluble.
   Na₂CO₃(s) + 2H₂O(l) → 2NaOH(aq) + H₂CO₃(aq)
2. Action of heat: The trioxocarbonates (IV) of Na, K and Ba cannot be decomposed by heat. All other CO₃^2- salts 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)
   • ZnCO₃(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l) + CO₂(g)

How do you test for a CO₃^2- salt?

Place the unknown substance in a test tube and add dilute trioxonitrate (V) acid. If a CO₃^2- is present, there will be effervescence, and the gas evolved will turn calcium hydroxide solution (lime water) milky.

CO₃^2-(s) + 2H⁺(aq) → H₂O(l) + CO₂(g)

What are hydrogen trioxocarbonates (IV)?

Hydrogen trioxocarbonates (IV), which contain HCO₃^–, may 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) + CO₂(g) → CO₃^2-(aq) + H₂O(l)
2. CO₃^2-(aq) + CO₂(g) + H₂O(l) → 2HCO₃^–(aq)

Properties of HCO₃^–

1. Solubility: All hydrogen trioxocarbonates (IV) are 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^– salts react with dilute acids 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^–.

Evaluation

1. Name the two series of salts formed by H₂CO₃.
2. Describe two ways of preparing a soluble trioxocarbonate (IV) such as K₂CO₃, with equations.
3. How would you prepare insoluble CaCO₃ from CaCl₂? Write the equation.
4. Which trioxocarbonates (IV) cannot be decomposed by heat? Write an equation for the action of heat on ZnCO₃.
5. Describe the laboratory test for a CO₃^2- salt.

Assignment

1. State the function of each of the following substances in the laboratory preparation of dry carbon (IV) oxide:
   1. potassium hydrogen trioxocarbonate (IV) solution;
   2. fused calcium chloride.
2. 1. Draw a labelled diagram for the laboratory preparation of dry carbon (IV) oxide.
   2. Write a balanced equation for the decomposition of potassium hydrogen trioxocarbonate (IV).