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Carbonic acid H 2 CO 3 is a product of CO 2 dissolving in water ( the solubility of CO 2 in water is about 0.
04 mol·dm -3 ), and the central atom C adopts sp 2 isotropic hybridization
.
Among them, three single-electron-containing hybrid orbitals, two single-electron orbitals of OH oxygen atoms and one single-electron-containing orbital of one terminal oxygen atom form three σ bonds
.
Carbonic acid reacts with strong base to form carbonate
.
The structure of carbonate CO 3 2- is shown in Figure 13-4(b).
The central atom C adopts sp² hybridization.
Figure 13-4 The structure of carbonic acid and carbonate
Carbonic acid is a weak dibasic acid (Ka1 Θ = 4.
45×10 -7 , Ka 2 Θ = 4.
69×10 -11 ), which can form carbonate and bicarbonate
For insoluble carbonates , the corresponding bicarbonate has a greater solubility
.
The main reason is that it is easier to overcome the force between +2-valent ion and -1 valent ion than the force between +2-valent ion and -2 valent ion during dissociation
.
Figure 13-5 Hydrogen bond dimer of bicarbonate
The aqueous solution of sodium carbonate is strongly alkaline.
There are two precipitation agents, CO 3 2- and OH - , which may form carbonate, basic carbonate or hydroxide precipitation with metal ions
(1) Ca 2+ , Sr 2+ , Ba 2+, etc.
, the solubility of carbonate is much lower than that of hydroxide, and carbonate precipitation is formed when mixed with sodium carbonate solution
.
(2) The solubility of hydroxides of Al 3+ , Fe 3+ , Cr 3+ and other high-valent metal ions is much lower than that of carbonates.
When mixed with sodium carbonate solution, hydroxide precipitates are formed
.
(3) Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Ag +, etc.
The solubility of hydroxide is not much different from that of carbonate, and is comparable to that of sodium carbonate When the solution is mixed, a basic carbonate precipitate, such as Mg(OH) 2 CO 3 , will be formed
.
In order to obtain the normal salt MgCO 3 , the alkalinity of the precipitation agent can be reduced, that is, sodium bicarbonate solution is used as the precipitation agent instead of sodium carbonate
.
Mg 2+ +HCO 3 -=MgCO 3 ↓+H +
The thermal stability of carbonate , bicarbonate, and carbonic acid decreases sequentially, which can be explained by the strong reverse polarization ability of H +
