Trioxocarbonates: Preparation, Physical and Chemical Properties, Tests and Uses of Trioxocarbonates

Trioxocarbonates(IV)

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Trioxocarbonate(IV), (CO₃²⁻), is the modern (IUPAC) systematic name for the carbonate ion  "tri-oxo-carbonate," meaning three oxygen atoms bonded to carbon, with carbon in the +4 oxidation state. Salts containing this ion are called trioxocarbonate(IV) salts (commonly still called "carbonates"). The related hydrogen trioxocarbonate(IV) ion, (HCO₃⁻), is the systematic name for the bicarbonate ion.

 

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Names and Formulae of Trioxocarbonates(IV) and Hydrogentrioxocarbonates(IV)

 

Name	Formula
Sodium trioxocarbonate(IV)	Na₂CO₃
Sodium hydrogen trioxocarbonate(IV)	NaHCO₃
Potassium trioxocarbonate(IV)	K₂CO₃
Calcium trioxocarbonate(IV)	CaCO₃
Calcium hydrogen trioxocarbonate(IV)	Ca(HCO₃)₂
Magnesium trioxocarbonate(IV)	MgCO₃
Magnesium hydrogen trioxocarbonate(IV)	Mg(HCO₃)₂
Ammonium trioxocarbonate(IV)	(NH₄)₂CO₃
Zinc trioxocarbonate(IV)	ZnCO₃
Lead(II) trioxocarbonate(IV)	PbCO₃
Copper(II) trioxocarbonate(IV	CuCO₃
Iron(II) trioxocarbonate(IV)	FeCO₃

 

Please read on carbon and its compounds (oxides of carbon - carbon(II) oxide) here.

 

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Preparation of Trioxocarbonates(IV)

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Most trioxocarbonates(IV) are insoluble in water. However, the trioxocarbonates(IV) of sodium (Na₂CO₃), potassium (K₂CO₃) and ammonium - (NH₄)₂CO₃ are soluble. 

Soluble and insoluble trioxocarbonates(IV) can be prepared via different methods in the laboratory.

 

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Lab. Preparation of Soluble Carbonates

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Examples of soluble trioxocarbonates(IV) include Na₂CO₃, K₂CO₃ and (NH₄)₂CO₃.

They are prepared by passing excess CO₂ into a solution of the respective metal hydroxide. This reaction forms the hydrogencarbonate first, then it is heated (or an appropriate stoichiometry is used) to get the normal carbonate. This reactions are shown via the equations below:

 

KOH_(aq) + excess CO_2(g) → KHCO_3(aq)

2KHCO_3(aq) + Heat → K₂CO_3(s) + CO_2(g) + H₂O_(l)

NaOH_(aq) + excess CO_2(g) → NaHCO_3(aq)

2NaHCO_3(aq) + Heat → Na₂CO_3(s) + CO_2(g) + H₂O_(l)

Note: The NaHCO₍₃₎ (formed above) is left as a white powder. It crystallizes from its solution with ten molecules of water of crystallization. See the equation below:

• Na₂CO_3(s) + 10H₂O_(l) → Na₂CO_3.10H₂O_(s)

Please read more on salts, their preparation, solubility and water of crystallization here.

 

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Lab. Preparation of Insoluble Carbonates

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Examples on insoluble trioxocarbonates(IV) include CaCO₃, ZnCO₃, PbCO₃, MgCO₃, ZnCO₃, CuCO₃, FeCO₃ and so on.

Insoluble trioxocarbonates(IV) are prepared by double decomposition (or precipitation).

A double decomposition (also called a metathesis or double displacement reaction) is a chemical reaction where two compounds exchange their ions or component parts to form two new, different compounds.

The preparation of insoluble salts (through double decomposition) involves mixing a solution of a soluble salt of the metal with a solution of a soluble carbonate (e.g. Na₂CO₃). See the equations below:

•  CaCl₂_(aq) + Na₂CO₃_(aq) → CaCO₃_(s)↓ + 2NaCl_(aq)

•  Pb(NO₃)₂_(aq) + Na₂CO₃_(aq) → PbCO₃_(s)↓ + 2NaNO₃_(aq)

The precipitate (i.e,  insoluble products in the above reactions - CaCO₃_(s)↓ and  PbCO₃_(s)↓) is filtered, washed with distilled water, and dried.

Please read on salts in chemistry, types properties and uses of salts here.

 

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Industrial Preparation of Na2CO3

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Industrially, sodium trioxocarbonate(IV), (Na₂CO₃, washing soda or soda ash) is manufactured by the Solvay process (or ammonia-soda process), using brine (conc. NaCl), ammonia, and CO₂ (from heating limestone). In this process, brine is saturated with ammonia gas (NH₃) + H₂O to form a mixture termed ammoniacal brine.

• NaCl_(aq) + NH_3(g)​​​​ + H₂O_(l) → NaCl_(aq) + NH_3(aq) (Ammoniacal brine).

The ammoniacal brine (NaCl + NH₃ in water) is sprayed down the Solvay tower, and CO₂ is forced up through the Solvay tower. Excess CO₂ leaves at the top of the tower.

The Solvay process is summarized via the equations below.

• NH_3(g) + CO_2(g) + H₂O_(l) → NH₄HCO_3(aq)

• NH₄HCO_3(aq) + NaCl_(aq) →NaHCO_3(s) + NH₄Cl_(aq)

The image below shows the Solvay process.

 

Note: NaHCO₃ dissolves only moderately in water, about 96 grams per liter (or roughly 9.3 to 9.6 grams per 100 mL) at room temperature (20 °C). In this regard, the insoluble NaHCO₃ is filtered, washed and heated to yield anhydrous Na₂CO_3(s) (or soda ash), steam - H₂O_(g) and CO_2(g). See the equation below:

• 2NaHCO_3(aq) → Na₂CO_3(s) + H₂O_(g) + CO_2(g)

The NaHCO₃ (formed above) is left as a white powder. It crystallizes from its solution with 10 molecules of water of crystallization (Na₂CO₃ or washing soda).

• Na₂CO_3(s) + 10H₂O_(l) → Na₂CO₃.10H₂O_(s)

You can read on relative acidity and alkalinity (pH scale) and calculations on pH here.

 

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Preparation of Hydrogen trioxocarbonates(IV), (HCO₃)

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Hydrogen trioxocarbonates(IV) is formed by passing excess CO₂ through a suspension (or solution) of the carbonate or hydroxide in water: This is shown via the equations below:

•  CaCO₃_(s) + H₂O_(l) + CO₂_(g) → Ca(HCO₃)₂_(aq)

•  NaOH_(aq) + CO₂_(g) [excess] → NaHCO₃_(aq)

Examples of hydrogen trioxocarbonates(IV) include Ca(HCO₃)₂, NaHCO₃ (or baking soda), KHCO₃, and Mg(HCO₃)₂.

You can read on acids, basicity and types of acid here.

 

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Physical Properties of Trioxocarbonates(IV)

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• Most metal trioxocarbonates(IV) are white crystalline solids (or fine white powders) at room temperature, (except those of transition metals, which are often colored; e.g. CuCO₃ is green (or blue-green), FeCO₃ is pale green/buff, and MnCO₃ is pink).

• Soluble carbonates (that is, sodium, potassium, and ammonium trioxocarbonates) form alkaline solutions in water, (pH > 7) due to hydrolysis of the CO₃²⁻ ion.

• Most other trioxocarbonates (such as calcium, magnesium, zinc, and barium trioxocarbonates) are insoluble in water.

• Insoluble carbonates are chemically stable solids at room temperature.

• Most trioxocarbonates have high melting points.

• Trioxocarbonates of sodium, potassium, and barium are thermally stable and do not decompose easily upon heating. However, most other trioxocarbonates decompose on heating to yield the metal oxide and carbon(IV) oxide gas (CO₂).

• They are generally odorless.

Please read on oxide of carbon (carbon(IV) oxide) here.

 

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Chemical Properties of Trioxocarbonates(IV)

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1. Reaction with dilute acids

All trioxocarbonates(IV) react with dilute acids to liberate CO₂ gas. This is a key general (diagnostic) reaction.

• CaCO₃_(s) + 2HCl_(aq) → CaCl₂_(aq) + H₂O_(l) + CO₂_(g)

• Na₂CO₃_(aq) + H₂SO₄_(aq) → Na₂SO₄_(aq) + H₂O_(l) + CO₂_(g)

 

2. Thermal decomposition (effect of heat)

Most metal carbonates (except those of Group I metals, e.g Na and K) decompose on strong heating to give the metal oxide and CO₂:

•  CaCO₃_(s) --(heat) → CaO_(s) + CO₂_(g)

•  ZnCO₃_(s) --(heat) → ZnO_(s) + CO₂_(g)

Group I carbonates (e.g. Na₂CO₃, K₂CO₃) are thermally stable, that is; they do not decompose on ordinary heating. This is a useful distinguishing property between Group I and other metal carbonates).

Hydrogentrioxocarbonates(IV) decompose much more readily on gentle heating, even those of Group I metals:

• 2NaHCO₃_(aq) --(heat) → Na₂CO₃_(s) + H₂O_(l) + CO₂_(g)

• Ca(HCO₃)₂_(aq) --(heat) → CaCO₃_(s) + H₂O_(l) + CO₂_(g)

 

3. Reaction with water (hydrolysis)

Soluble carbonates like Na₂CO₃ and K₂CO₃ hydrolyze in water to give a mildly alkaline solution:

•  CO₃²⁻(aq) + H₂O(l) ⇌ HCO₃⁻(aq) + OH⁻(aq)

The OH^- produced in the hydrolysis of soluble trioxocarbonate makes the solution alkaline.

Please read on bases and alkalis, their classification, physical and chemical properties here.

 

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General Solubility Pattern of Trioxocarbonate(IV) 

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1. Soluble trioxocarbonates(IV)

Only those of Group I metals (Na₂CO₃, K₂CO₃) and ammonium (NH₄)₂CO₃ are appreciably soluble in water.

2. Insoluble trioxocarbonates(IV)

Apart from group I metal carbonate and ammonium carbonate, all other metal carbonates (BaCO₃, CaCO₃, MgCO₃, ZnCO₃, PbCO₃, CuCO₃, FeCO₃, etc.) are insoluble in water.

 

Note: In contrast to trioxocarbonates(IV), nearly all hydrogen trioxocarbonates(IV) (bicarbonates), are soluble in water, even those of metals whose normal carbonates are insoluble.

Many hydrogen trioxocarbonates(IV) only exist stably in solution (e.g. Ca(HCO₃)₂, and as a result, cannot be isolated as a solid since they decomposes back to their metallic carbonate when heated (or evaporated).

CaCO₃ → H₂O + CO₂ 

You can read on the allotropes of carbon here.

 

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Test for CO₃²⁻ (Trioxocarbonate(IV) Ion)

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Standard confirmatory test

Test: Add a few drops (or a small amount) of a dilute acid (e.g. dilute HCl or dilute H₂SO₄) to the solid or solution suspected of containing CO₃²⁻.

Observation: Brisk effervescence (bubbling) occurs, releasing a colorless, odorless gas.

Confirmation: Confirm the gas is CO₂ by passing it through (or bubbling it into) freshly prepared limewater - (CaOH)₂. The limewater turns milky (or cloudy).

This above test for CO₃²⁻ is shown via the equation below:

• CaCO₃_(s) + 2HCl_(aq) → CaCl₂_(aq) + H₂O_(l) + CO₂_(g)

• CO₂_(g) + Ca(OH)₂_(aq) → CaCO₃_(s)↓ + H₂O_(l)

 

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Distinguishing Trioxocarbonate ion (CO₃²⁻) from Hydrogen trioxocarbonate ion (HCO₃⁻)

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Both give the same acid test, that is, effervescence, and the liberated CO₂ gas turns limewater milky. Therefore, this test alone does not distinguish them. Both ions can be distinguished through:

1. Magnesium sulfate (MgSO₄) test

CO₃²⁻ gives a white precipitate with MgSO₄ solution in the cold (without heating or boiling), while HCO₃⁻ only gives a precipitate on heating (or boiling) the solution. (Or HCO₃⁻ gives no precipitate in the cold).

 

2. Heating the CO₃²⁻ and HCO₃⁻ of Group I

Alternatively, solid carbonates and hydrogen carbonates can sometimes be distinguished by heating.

Group I carbonates do not decompose easily on heating, while group I hydrogen carbonates readily decompose to release CO₂ and water, even at moderate heat. See the equation below:

• 2NaHCO_3(s) + Heat → Na₂CO_3(s) + H₂O_(g) + CO_2(g)

You can read on families of elements in the periodic table here.

 

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Summary Table of Trioxocarbonate (CO₃²⁻) Salt and Hydrogen trioxocarbonate (HCO₃⁻ ) Salt

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Property	Normal Carbonate (CO₃²⁻) salts	Hydrogencarbonate (HCO₃⁻) salts
Solubility	Only Group I and NH₄⁺ are soluble; rest insoluble	Nearly all are soluble
Reaction with dilute acid	Effervescence, CO₂ released	Effervescence, CO₂ released
Thermal stability	Group I carbonates are stable; others decompose on heating	Decompose readily on gentle heating
Test with cold MgSO₄(aq)	White precipitate forms	No precipitate in cold (forms on heating)
Example	CaCO₃, Na₂CO₃	NaHCO₃, Ca(HCO₃)₂

 

You can read on the rate of chemical reaction here.

 

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Uses of Trioxocarbonate(IV) Compounds

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1. Calcium trioxocarbonate(IV) (CaCO₃)

CaCO₃ occurs naturally as limestone.

• As limestone, it is used in the construction industry (building stone) and as a raw material for making cement, glass, and quicklime (CaO).

• It is used to make CO₂ (industrially, via heating) and in the extraction of iron (as a flux in the blast furnace, to remove impurities (silica) as slag.

• It is used in agriculture to neutralize acidic soils.

• It is a component of eggshells; used medically as an antacid and calcium supplement.

 

2. Sodium trioxocarbonate(IV) (Na₂CO₃)

It is also known as washing soda (Na₂CO₃.10H₂O) or soda ash (Na₂CO₃). 

• Na₂CO₃ is used in the manufacture of glass.

• It is used in the manufacture of soap and detergents.

• It is used as a water softener (removes hardness by precipitating Ca²⁺/Mg²⁺ ions as insoluble carbonates).

• It is used in the paper and textile industries.

 

3. Sodium hydrogentrioxocarbonate(IV)

NaHCO₃ is also known as baking soda

• NaHCO₃ is used as a raising agent in baking. It releases CO₂ on heating, causing dough to rise).

• It is used as an antacid to relieve indigestion and heartburn.

• It is used in fire extinguishers. It releases CO₂ when it decomposes with heat, (or reacts with acid).

 

4. Magnesium trioxocarbonate(IV) (MgCO₃)

It occurs naturally as Magnesite (mineral form of anhydrous MgCO₃).

• MgCO₃ is used as an antacid and mild laxative.

• It is used in the manufacture of certain building materials and as a drying/anti-caking agent.

 

5. Calcium/magnesium hydrogentrioxocarbonate(IV)

It is naturally present in hard water. Their formation and decomposition explain limescale deposits in kettles and pipes.

Please read on the uses of chemistry here.

 

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Trioxocarbonates(IV) in Nature and Examples

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Trioxocarbonates(IV) are widespread in the Earth's crust, mostly as insoluble mineral deposits formed over geological time from marine organisms' shells/skeletons, and from precipitation in ancient seas.

Trioxocarbonates(IV) occurs naturally as:

• Calcium trioxocarbonate(IV), CaCO₃: It occurs naturally as limestone, marble, chalk, and calcite. It also occurs as the main component of eggshells, seashells, and coral.

• Magnesium trioxocarbonate(IV), MgCO₃: It occurs as magnesite. Together with CaCO₃, it forms dolomite, CaCO₃·MgCO₃ (or CaMg(CO₃)₂).

• Sodium trioxocarbonate(IV), Na₂CO₃: It occurs naturally in mineral deposits (natron/trona), and dissolved in some salt lakes.

• Sodium hydrogen trioxocarbonate(IV), NaHCO₃: It occurs in smaller natural deposits and dissolved in mineral springs.

• Zinc trioxocarbonate(IV), ZnCO₃: It occurs as the ore calamine/smithsonite.

• Iron(II) trioxocarbonate(IV), FeCO₃: It occurs as the ore siderite.

• Lead(II) trioxocarbonate(IV), PbCO₃: It occurs as the ore cerussite.

• Dissolved calcium and magnesium hydrogentrioxocarbonate(IV), Ca(HCO₃)₂ and Mg(HCO₃)₂: They are responsible for temporary hardness of water which is formed when rainwater (containing dissolved CO₂, i.e. weak carbonic acid) percolates through limestone rock (CaCO₃). See the equation below:

 CaCO₃_(s) + H₂O_(l) + CO₂_(g) → Ca(HCO₃)₂_(aq)

This above reaction, in reverse, is responsible for the formation of stalactites and stalagmites in caves.

Please read the next topic (Hydrocarbons, classifications of hydrocarbons, fractional distillation of crude oil and uses of the fractions of crude oil) here