Solubility product refers to the dissolution equilibrium constant of precipitationKspIt indicates that the solubility product reflects the solubility of insoluble electrolyte.The solubility product constant is only applicable to the saturated solution of insoluble electrolyte, not applicable to the soluble electrolyte.At a certain temperature, K of each insoluble salt compoundspAll are a specific value.
At a certain temperature, there is a balance of precipitation dissolution and generation between insoluble electrolyte crystals and ions dissolved in solution, which is called precipitation dissolution balance.When equilibrium is established and the solids are not yet fully dissolved, the solution is said to be saturated.The concentration of solute in saturated solution is called solubility.The unit of solubility can be mole (mol dm − 3) or expressed as mass per unit volume, such as μ gmL − 1.The solubility is related to temperature, so the solubility product is also related to temperature.
Put the insoluble electrolyte AgCI into water, and a part of Ag on the solid surface+And Cl-Under the continuous action of water molecules, they break away from AgCl solids and combine with water molecules to form hydrated ions into solutions. This process is called precipitation dissolution;At the same time, the hydrated Ag in the solution+And Cl-With continuous movement, some of them will be attracted by ions with opposite charges on the surface of AgCl solid, and then they will recombine to form solid AgCI. This process is called precipitation generation.The dissolution and formation of insoluble electrolytes are reversible processes.After a period of time, when the dissolution rate of the insoluble electrolyte is equal to the generation rate, the concentration of each ion in the solution will not change, and the precipitation dissolution balance between the insoluble electrolyte solid and the hydrated ions in the solution will be established:
AgCI(s)
Ag+(aq)+Cl-(aq)
The equilibrium constant expression of the reaction is as follows:
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Facts have proved that any insoluble electrolyte is always more or less dissolved in water, and absolutely insoluble substances do not exist.Generally, the substance whose solubility is less than 0.01g in 100g water is called insoluble substance.The part of insoluble electrolyte dissolved in water is completely dissociated, that is, the amount dissolved will be dissociated.
For example, the dissociation equilibrium of AgCl is as follows:
The solution when dissolution reaches equilibrium is called saturated solution.Solubility product of AgCl:
In the formula, the concentration of each substance is the concentration at dissolution equilibrium, and the solid concentration does not appear in the expression.cIs the standard concentration, generally defined as 1mol/L, used for offsetKspUnits of.
The size of the solubility product reflects the solubility of the material.It will be affected by the change of temperature and the dissolution of other electrolytes, so the value usually given is measured by a single electrolyte at a specific temperature.
For PbCltwo，Cathree(POfour)twoAnd Mg (OH)twoFor insoluble electrolytes that can be dissociated into two or more identical ions equally, the expression should be written in the same way as the equilibrium constant. The coefficient of the ion in the dissociation equation is taken as the index for each ion concentration:
It can be measured by experiments and theoretically inferred from some chemical thermodynamic formulas.
Solubility product（Ksp）And solubility（S）Both can be used to measure the solubility of a insoluble substance, and they can be converted to each other.
If an electrolyte molecule is formed after dissolutionpCation,qAnions, then
solubility product principle
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takeQThe size ofKspBy comparison, according to the principle of chemical balance movement, it can be seen that:
WhenQ>KspWhen the solution is supersaturated, the equilibrium moves to the left and precipitates;
WhenQ<KspThe solution is unsaturated. If there is still precipitation in the solution, the balance moves to the right and the precipitation is dissolved;
WhenQ=KspThe solution is saturated and in the equilibrium state of precipitation and dissolution. There is neither precipitation generation nor precipitation dissolution.
The above rules can be used to judge the occurrence of precipitation formation and dissolution, calledsolubility product principle [1]
