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

Trioxocarbonate (IV) acid, H₂CO₃, is a weak dibasic acid formed when carbon (IV) oxide dissolves in water.

Behavioural objectives

1. State the two series of salts formed by H₂CO₃.
2. Describe how soluble and insoluble trioxocarbonates (IV) are prepared in the laboratory.
3. State the properties of trioxocarbonates (IV) and hydrogen trioxocarbonates (IV).
4. Describe the laboratory test for a trioxocarbonate (IV).
5. Write balanced equations for the preparation and reactions of these salts.

What is trioxocarbonate (IV) acid?

Trioxocarbonate (IV) acid is the weak acid, H₂CO₃, formed when CO₂(g) dissolves in water.

• Formation: CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq).
• Weak acid: it ionises only slightly in water.
• Dibasic acid: each molecule has two replaceable hydrogen atoms.
• Stability: it exists only in solution and breaks down easily to CO₂ and water.

What are the two series of salts formed by H₂CO₃?

H₂CO₃ forms two series of salts: normal trioxocarbonates (IV) and acidic hydrogen trioxocarbonates (IV).

• Normal trioxocarbonates (IV): both hydrogen atoms are replaced by a metal or cationic radical. They contain CO₃^2-, as in Na₂CO₃.
• Hydrogen trioxocarbonates (IV): only one hydrogen atom is replaced. They contain HCO₃^–, as in NaHCO₃.

What are normal trioxocarbonates (IV)?

Normal trioxocarbonates (IV) are salts derived from H₂CO₃ by complete replacement of the hydrogen by a metal or cationic radical.

How are soluble trioxocarbonates (IV) prepared?

The CO₃^2- salts of Na⁺, K⁺ and NH₄⁺ are soluble in water and are prepared by two methods.

1. From an alkali: bubble CO₂ through a solution of the corresponding alkali.
   • 2KOH(aq) + CO₂(g) → K₂CO₃(aq) + H₂O(l)
2. By heating the hydrogen salt: decompose the corresponding hydrogen trioxocarbonate (IV).
   • 2KHCO₃(s) → K₂CO₃(s) + H₂O(l) + CO₂(g)

How are insoluble trioxocarbonates (IV) prepared?

Insoluble metallic trioxocarbonates (IV) are 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: for the CO₃^2- salts of less electropositive metals like Cu, use NaHCO₃.
• Method: the insoluble solid forms as a precipitate, which is filtered, washed and dried.

What are the properties of CO₃^2- salts?

The main properties of CO₃^2- salts are their solubility, their action of heat and their reaction with dilute acids.

Solubility

• Soluble: the trioxocarbonates (IV) of sodium, potassium and ammonium dissolve in water.
• Insoluble: the other trioxocarbonates (IV) do not dissolve, for example CaCO₃ and ZnCO₃.
• Alkaline solution: soluble CO₃^2- salts react slightly with water, so the solution is alkaline.
• Equation: CO₃^2-(aq) + H₂O(l) ⇌ HCO₃^–(aq) + OH^–(aq)

Action of heat

Most CO₃^2- salts decompose on heating to release CO₂, but sodium and potassium trioxocarbonates (IV) do not.

Reaction with dilute acids

All trioxocarbonates (IV) react with dilute acids to form a salt, water and CO₂.

• 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?

A CO₃^2- salt gives effervescence with dilute acid, and the gas turns lime water milky.

1. Place the unknown substance in a test tube.
2. Add dilute trioxonitrate (V) acid.
3. Watch for effervescence (bubbles of gas).
4. Pass the gas into calcium hydroxide solution (lime water).
5. The lime water turns milky, which shows that the gas is CO₂.

• Ionic equation: CO₃^2- + 2H⁺(aq) → H₂O(l) + CO₂(g)

What are hydrogen trioxocarbonates (IV)?

Hydrogen trioxocarbonates (IV) are salts containing HCO₃^–, derived from H₂CO₃ by partial replacement of the hydrogen by a metal or cationic radical.

How is HCO₃^– prepared?

HCO₃^– is prepared by passing excess 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)

What are the properties of HCO₃^– salts?

• Solubility: all hydrogen trioxocarbonates (IV) are soluble in water.
• Action of heat: all of them decompose on heating to give the normal salt, water and CO₂.
  • 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g)
• Reaction with dilute acids: all of them give a salt, water and CO₂.
  • 2NaHCO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g)
• Note: effervescence with dilute acid shows the presence of HCO₃^– or CO₃^2-.

What is the difference between CO₃^2- and HCO₃^– salts?

CO₃^2- salts are normal salts with no replaceable hydrogen, while HCO₃^– salts are acidic salts with one hydrogen left.

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. Write balanced equations for the following:
   1. the decomposition of potassium hydrogen trioxocarbonate (IV) by heat;
   2. the preparation of CaCO₃ from CaCl₂ and Na₂CO₃;
   3. the reaction of ZnCO₃ with dilute hydrochloric acid.