2 Solubility product rule
Comparing the size of the reaction quotient Q Θ and the solubility product constant Ksp Θ system can determine the direction of the reaction in the poorly soluble strong electrolyte solution
For the precipitation and dissolution equilibrium in a solution
The reaction quotient Q Θ of the precipitation and dissolution reaction at a certain moment is
When Q Θ >Ksp Θ , the solution is supersaturated, and the reaction direction is that precipitation separates out of the solution;
When Q Θ = Ksp Θ , the solution is saturated and the reaction is in equilibrium;
When Q Θ <Ksp Θ , unsaturated solution, the reaction direction is the dissolution of the precipitate
This is the solubility product rule, which is often used to judge the formation and dissolution of precipitates
[Example 7-9] In 0.
50mol·dm -3 MgCl 2 solution, add an equal volume of 0.
10 mol·dm -3 ammonia water, this ammonia water also contains 0.
020 mol·dm -3 NH 4 Cl, Mg(OH) 2 Can it be precipitated? If precipitation occurs, how many grams of solid NH 4 CI need to be added to each liter of ammonia water to prevent Mg(OH) 2 from precipitation? It is known that Ksp Θ , Mg(OH) 2 =5.
6×10 -12 , Kb Θ , NH 3 , H 2 O=1.
Solution The concentration of each substance after mixing the two solutions in equal volume is
In NH 3 ·H 2 O-NH4 + mixed solution
Since Q Θ >Ksp Θ , Mg(OH) 2 precipitation can be generated
To make the precipitation of Mg(OH) 2 just not formed or just formed, by [Mg 2+ ][OH - ]=Ksp Θ ,Mg(OH) 2 , we get
The [NH 4 + ] obtained is the [NH 4 Cl] in the solution when the Mg(OH) 2 precipitation just does not form in the mixed system .
While the original ammonia
The mass of solid NH4CI that should be added to each liter of solution is
5g·mol -1 ×(0.
There are many factors that affect the precipitation and dissolution balance, among which the effect of the same ion effect is very obvious, that is, adding a soluble and strong electrolyte with the same ion as the system to the balance system will move the balance to the direction of precipitation, and the solubility of the insoluble strong electrolyte will decrease.
In addition, the salt effect also has an impact on the precipitation and dissolution balance, which will slightly increase the solubility of insoluble substances, but this effect is generally small