If a reaction forms a precipitate using CO2 gas, is it also possible using carbonate in solution?

Question

A precipitate containing barium is formed when CO2 gas is passed through a saturated soln of Ba(OH)2. The CO2 gas is produced when carbonate ions react with hydrogen ions. If a solution with a carbonate ion is reacted with Ba(OH)2, does it also form a precipitate?

If not, why does the state matter? If so, why doesn't the state matter?

Actually, you two are both wrong. :(
Richard: carbonic acid is not formed.
science teacher: it is not double replacement; the carbonate ion is actually used to create CO2 when it is reacted with two hydrogen ions. A byproduct of this is water.

Then, the carbon dioxide reacts with the barium ion and two hydroxide ions to form a precipitate of BaCO3 as well as water.

In this experiment, it is also given that in the confirmation of sulfate (this is qualitative analysis of anions), the original solution (which also contains the carbonate ion) is added to a solution of BaCl2. However, only a precipitate of BaSO4 remains.

I guess that invalidates my question. However, I would still like to know why the precipitate is formed with carbon dioxide and not with the carbonate ion in solution.

Answer 1

Yes, you could.

The Carbon dioxide reacts with the water to form carbonic acid:

CO2 + H20 --> H2CO3

This can dissociate into Bicarbonate [HCO3-] and Carbonate [CO3-2]. It is the Carbonate anion which reacts with the Barium cation to form the solid Barium carbonate.

You could have used any soluble carbonate, like Sodium carbonate or Potassium carbonate. The net ionic equation is:

Ba+2 (aq) + CO3-2 (aq) --> BaCO3 (s)

edited later:
The saturated solution of Barium hydroxide in water would contain about 5.6g of Barium hydroxide for every 100g of water at room temperature. This would contain Barium cations and Hydroxide anions both solvated with water molecules. But there is far more water than either ion.

The Carbon dioxide may have been formed by reacting an acid with a carbonate or it may have come from your breath (blowing through a straw) or it could have come from a tank of Carbon dioxide. It does not make any difference.

At the surface of the Carbon dioxide bubbles, the Carbon dioxide reacts with the water. Most of just dissolves. Some of it chemically reacts:

H2O (l) + CO2 (g) --> H2CO3 (aq)

The equilibrium constant at 25°C is Kh= 1.70×10−3 . This means that the majority of the carbon dioxide is not converted into carbonic acid and stays as CO2 molecules.

Of those which do chemically react, some dissociate into H+ and HCO3-. The H+ can react with the Hydroxide in solution:

H+ (aq) + OH- (aq) --> H2O (l)

The Hydrogen carbonate (Bicarbonate) anion can react with the Ba+2 to give

Ba+2 (aq) + HCO3- (aq) --> BaHCO3+ (aq)

which loses a proton (H+) to give BaCO3 (s).

BaHCO3+ (aq) + OH- (aq) --> BaCO3 (s) + H2O (l)

The Barium Carbonate is very insoluble in water.

YES the BaCO3 could have been made directly from adding a water soluble Carbonate to the the Ba+2 solution.

Ba+2 (aq) + (CO3)-2 (aq) --> BaCO3 (s)

The statement you added about the Sulfate ion is confusing because it is actually a different problem entirely. If a solution containing soluble Barium cations (like a solution of BaCl2) to a solution which contains both Carbonate and Sulfate, both Barium carbonate and Barium sulfate would precipitate out. However, the Barium chloride solution used to confirm Sulfate is usually acidic. This neutralizes and disolves the Barium carbonate:

2 H+ (aq) + BaCO3 (s) --> Ba+2 (aq) + H2O (l) + CO2 (g)

The Barium cation in solution reacts with the sulfate:

Ba+2 (aq) + SO4-2 (aq) --> BaSO4 (s)

Answer 2

Any carbonate will react in a double displacement reaction with Barium hydroxide ,forming a preciptate. All materals are trying to get to a stage of less energy. Ba(OH)2 is more stableas a precipitate and it will form if a carbonate is present.