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Solubility Equilibria. Will it all dissolve, and if not, how much will?. SOLUBILITY EQUILIBRIA. Solubility : Relative term used to describe how much of a particular substance dissolves in a certain amount of solvent. Substances that dissolve very well are said to be soluble

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solubility equilibria

Solubility Equilibria

Will it all dissolve, and if not, how much will?

slide2
SOLUBILITY EQUILIBRIA
  • Solubility: Relative term used to describe how

much of a particular substance dissolves in a

certain amount of solvent.

  • Substances that dissolve very well are said to

be soluble

  • Insoluble species don’t dissolve well.
  • All substances are “soluble” to some extent
  • We will look at slightly soluble substances
slide3
SOLUBILITY EQUILIBRIA
  • All dissolving is an equilibrium.
  • If there is not much solid it will all dissolve.
  • As more solid is added the solution will become saturated.
  • Solid ↔ dissolved
  • The solid will precipitate as fast as it dissolves, forming an equilibrium.
watch out
Watch out
  • Solubility is not the same as solubility product.
  • Solubility product is an equilibrium constant.
  • It doesn’t change except with temperature.
  • Solubility is an equilibrium position for how much can dissolve.
  • A common ion can change this.
slide6
SOLUBILITY PRODUCT CONSTANTS
  • Consider the following reaction
  • The equilibrium constant expression is

Ksp = [Pb2+][Cl-]2

  • Ksp is called the solubility product constant or

simply solubility product

  • For a compound of general formula, MyXz (next page)
slide7
Ksp = [Mz+]y[Xy-]z

Ksp = [Mg2+][NH4+][PO43-]

Ksp = [Zn2+][OH-]2

Ksp = [Ca2+]3[PO43-]2

slide8
Molar solubility: the number of moles that

dissolve to give 1 liter of saturated solution

  • As with any equilibrium constant the numerical

value must be determined from experiment

  • The Ksp expression is useful because it applies

to all saturated solutions

- the origins of the ions are not relevant

  • Consider that @ 25C Ksp AgI = 1.5 x 10-16
slide9
Solving Solubility Problems

For the salt AgI at 25C, Ksp = 1.5 x 10-16

AgI(s)  Ag+(aq) + I-(aq)

O

O

+x

+x

x

x

1.5 x 10-16 = x2

x = solubility of AgI in mol/L = 1.2 x 10-8 M

slide10
Solving Solubility Problems

For the salt PbCl2 at 25C, Ksp = 1.6 x 10-5

PbCl2(s)  Pb2+(aq) + 2Cl-(aq)

O

O

+2x

+x

2x

x

1.6 x 10-5 = (x)(2x)2 = 4x3

x = solubility of PbCl2 in mol/L = 1.6 x 10-2 M

relative solubilities
Relative Solubilities
  • Ksp will only allow us to compare the solubility of solids the that fall apart into the same number of ions.
  • The bigger the Ksp of those the more soluble.
  • If they fall apart into different number of pieces you have to do the math.
the common ion effect
The Common Ion Effect
  • When the salt with the anion of a weak acid is added to that acid:
    • it reverses the dissociation of the acid.
    • lowers the percent dissociation of the acid.
  • The same principle applies to salts with the cation of a weak base..
  • The calculations are the same as with acid base equilibrium.
slide13
Solving Solubility with a Common Ion

For the salt AgI at 25C, Ksp = 1.5 x 10-16

What is its solubility in 0.05 M NaI?

AgI(s)  Ag+(aq) + I-(aq)

0.05

O

+x

+x

0.05+x

x

1.5 x 10-16 = (x)(0.05+x)  (x)(0.05)

x = solubility of AgI in mol/L = 3.0 x 10-15 M

ph and solubility
pH and solubility
  • OH- can be a common ion.
  • More soluble in acid.
  • For other anions if they come from a weak acid they are more soluble in a acidic solution than in water.
  • CaC2O4↔Ca+2 + C2O4-2
  • H+ + C2O4-2 ↔HC2O4-
  • Reduces C2O4-2 in acidic solution.
precipitation
Precipitation
  • The reaction quotient (called ion product) may be applied to solubility equilibria - determines if a substance will precipitate from solution
  • Ion Product, Q =[M+]a[Nm-]b
  • If Ksp
  • If Ksp=Q equilibrium solution is just saturated
  • If Ksp>Q No precipitate, forward process occurs
precipitation example
Precipitation Example
  • A solution of 75.0 mL of 0.020 M BaCl2 is added to 125.0 mL of 0.040 M Na2SO4. Will a precipitate form? (Ksp= 1.5 x 10-9M BaSO4)

BaSO4 could form if Ksp

For Q you need initial concentrations:

[Ba2+] = mmol Ba2+ / total mL

= (0.0750L)(0.020 M)/(0.0750L + 0.125L) = 0.0075 M

[SO42-] = mmol SO42- / total mL

= (0.1250L)(0.040 M)/(0.0750L + 0.125L) = 0.025 M

Q = [Ba2+] [SO42-] = (0.0075 M)(0.025 M) = 1.9 x 10-4

Ksp

To figure out concentrations set up an ice table.

slide17
Complex Ions

A Complex ion is a charged species composed of:

1. A metallic cation

2. Ligands – Lewis bases that have a lone electron pair that can form a covalent bond with an empty orbital belonging to the metallic cation

the addition of each ligand has its own equilibrium
The Addition Of Each Ligand Has Its Own Equilibrium
  • Usually the ligand is in large excess.
  • And the individual K’s will be large so we can treat them as if they go to equilibrium.
  • The complex ion will be the biggest ion in solution.
slide20
Coordination Number
  • Coordination number refers to the number of ligands attached to the cation
  • 2, 4, and 6 are the most common coordination numbers
slide21
Complex Ions and Solubility

AgCl(s)  Ag+ + Cl- Ksp = 1.6 x 10-10

Ag+ + NH3 Ag(NH3)+ K1 = 1.2 x 10-3

Ag(NH3)+ NH3 Ag(NH3)2+ K2 = 9.6 x 10-4

K = KspK1K2

AgCl + 2NH3 Ag(NH3)2+ + Cl-

slide22
Dim the lights…

it’s showtime!

slide24
Pamukkale is one of the extraordinary natural wonders of Turkey.

The great attraction is the white immensity of the cliff with sculptured basins full of water and congealed waterfalls; they seem done of snow, cloud, cotton.

slide25
The scientific explanation is the hot thermal places that lie under the mount provoke the calcium carbonate spill, that makes the forms as solid as travertino marble.
slide27
It is a protecting landscape that fascinates, as the action of the mineral waters that contains calcium oxides left fantastic marks in the structures.
slide28
The resultant effect is spectacular: the waters spill on a series of steps, forming solid cascades and pools.
slide29
As much the cascades of calcium carbonate as the water change color in accordance with changes of the solar light that illuminates them, and the effect is surprising.
slide31
The continuous dynamics of the erosion and the transformation of the natural landscape result in an unusual environment.
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