Solubility

The solubility of an ionic compound or molecular, called Aqueous solution, is the maximum quantity of mole S of this compound which one can dissolve or dissociate, at a given temperature, in a Liter of Solvant. The solution thus obtained is saturated.

The solvent more the current is water. The aqueous solution can be a gas (example: air in water), a liquid (example: alcohol in water) or a solid (example: cooking salt NaCl in water). Water is one of the most effective solvents to dissolve the ionic compounds or polar.

The simple dissolution of a compound is endothermic: for example the dissolution of sugar (solid molecular) in water. However, when other reactions intervene during dissolution, the total phenomenon can be Exothermique: example dissolution in the water of the sulphuric acid H2SO4 is very exothermic because of the Solvatation of the ions obtained H3O+ and SO42-.

In the case of an ionic solid compound, the constant of balance of the reaction of dissolution is called Produit solubility and is noted Ks (T) . It depends only on the temperature T and in general it increases with this one. Solubility S is function of this product of solubility and varies in the same direction.

Expression of the product of solubility

Let us consider the following reaction:

A_ {m} B_ {N} (S) \ Longleftrightarrow m.A^ {n+} (aq) + n.B^ {m} (aq) ~

The product of solubility Ks is given by the relation: Ks = m X n

The expressions has (An+) and has (Bm-) correspond to the Activité S of the ionic species. With the weak concentrations one can confuse activity and concentration. The weaker the Ks is, the more the product is insoluble.

Relation between product of solubility and solubility

Let us consider the following reaction:

A_ {m} B_ {N} (S) \ Longleftrightarrow m.A^ {n+} (aq) + n.B^ {m} (aq) ~

With:

One thus has:

Ks = m X n = (m.s) m X (n.s) n = mm X nn X sn X sm

From where:

The solubility S of a solution is expressed by: S = (Ks/(nn X mm)) 1 (n+m)

Parameters influencing solubility

Temperature

In the general case, when the temperature of the solution increases, solubility increases. Examples: PbCl2; NaCl .

Effect of common ion

The solubility of a salt is decreased if an ion of this salt is already present in the solution: one calls that the effect of common ion .

Example: dissolution of KCl in a solution where the ions Cl- (common ions) are already introduced.

See too

  • Produces solubility
  • Solution
  • Soluble

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