Solubility Equilibria

Question 1

Define solubility of a salt.

Answer 1

Solubility of a salt, s, is the amount of that can be dissolved in 1dm3 of a given solute to form a saturated solution at a given temperature.

Question 2

What is a soluble salt?

Answer 2

Soluble salts dissociate fully in solution into their constituent ions.

Question 3

What is a sparingly soluble salt?

Answer 3

When a large amount of sparingly soluble salt is dissolved in a solvent, the solution eventually becomes saturated (ie it contains the maximum amount of dissolved solute at that particular temperature in the presence of undissolved solute). At this point, the ions in the saturated solution are in dynamic equilibrium with the excess undissolved solid.

Question 4

State the equilibrium constant Ksp.

Answer 4

Ksp of MaXb(s) = [Mm+(aq)]^a[Xn-(aq)]^b
units: (moldm^-3)^a+b

Question 5

Define solubility product.

Answer 5

Solubility product, Ksp, os the product of the molar concentrations of the constituent ions in a saturated solution of a sparingly soluble salt, raised to the powers as indicated by the stoichiometric coefficients in the balanced equation for the equilibrium, at a given temperature.

Ksp only changes with temperature

Question 6

When can the solubility product of the two salts can be directly compared?

Answer 6

Only when the two salta give the same total number of ions in solution. The larger the Ksp value the higher the solubility of the salt.

Question 7

Define ionic product.

Answer 7

The ionic product of a sparingly soluble salt is the product of the molar concentrations of the constituent ions in the solution, raised to the appropriate powers as indicated by the stoichiometric coefficients in the balanced equation.

When considering ionic product, the solution may or may not be saturated

Question 8

What does the value of the ionic product compared to the Ksp imply?

Answer 8

1. Ionic product < Ksp, solution is unsaturated and precipitation will not occur
2. Ionic product = Ksp, solution is satured and prepcipitation will not occur
3. Ionic product > Ksp, the solution is saturated and precipitation will occur

Question 9

How does the common ion effect affect solubility?

Answer 9

When a common ion is added via a second solute and the concentration of that ion in the solution increases, the ionic product of the first solute increases and exceeds the solubility product of the first solute. Due to common ion effect the solubility of the sparingly soluble salt inan aqueous solution containing a common ion is less than its solubility in water, however Ksp does not change.

Question 10

How does the formation of a complex ion affect solubility?

Answer 10

Formation of the complex ion causes the concentration of one of the constituent ions to decrease, this results ina decrease in the ionic product of the sparingly soluble salt. When ionic product decreases below the Ksp, the ppt of the sparing soluble salt dissolves by LCP, the POE is shifted right to produce more of the constituent ion. Complex ion formation increases the solubility of the sparingly soluble salt.

Question 11

Explain why AgCl is soluble in dilute NH3, AgBr is only soluble in concentrated NH3 and AgI is insoluble in both.

Answer 11

For AgCl, the addition of dilute NH3 is sufficient to lower [Ag+] such that ionic product becomes smaller than Ksp of AgCl. Hence AgCl dissolves completely in dilute NH3.
For AgBr, the addition of dilute NH3 is not sufficient as Ksp of AgBr is lower than AgCl. Only when concentrated NH33 is added will it sufficiently lower [Ag+] such that ionic product is smaller than the Ksp for AgBr to dissolve.
For AgI, the value of Ksp is extremely low, thus regardless of the concentration of NH3 added, the ionic product og AgI remains greater than the its Ksp, hence AgI does not dissolve.