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SolubilityUnit III
Unit Intro • Our focus is on solutions of aqueous ions
• As you know; acids, bases and salts form ionic solutions.
• This unit is only concerned with salts.
Review
Electrolytes: substances that dissolve to give electrically conducting solutions that contain ions.
Ex:
Review
Non-electrolytes: a substance that dissolves to give non-conducting solutions containing only neutral molecules.
Ex:
Molecular vs Ionic solutions
Ionic solutionsmetals and non-metals
compounds that contain polyatomic ions
Molecular compounds
They are covalent compoundsNon-metal and non-metal
especially organic compounds
Useful hint
When it comes to our chem12 course, you are only going to deal with ionic compounds that have one type of positive ion and one type of negative ion only.
Ionic Solutions Molecular/Covalent Solutions NaCl(aq) C6H12O6(aq) metal or polyatomic ion
Ca(OH)2(aq) C12H22O11(aq) nonmetal or carbon
(NH4)3PO4(aq) CH3OH(aq)
Ca(CH3COO)2(aq) O2(aq)
H2SO4(aq) N2H4(aq)
Conduct electricity Do not conduct electricity
Write equations to show the dissolving of the following substances in water
NaCl(s) Na+ + Cl-
C6H12O6(s) C6H12O6(aq)
Ca(OH)2(s) Ca2+ + 2OH-
C12H22O11(s) C12H22O11(aq)
(NH4)3PO4(s) 3NH4+ + PO4
3-
CH3OH(l) CH3OH(aq)
Back to solubility
In chem 11, you learnt that… The solubility of a substance is the maximum amount of the substance which can dissolve in a given amount of solvent at a given temperature.
Equilibrium Solubility
The solubility of a substance is the equilibrium concentration of the substance in solution at a given temperature.
when expressed in moles/L it’s called Molar Solubility.
Solubility at equilibirum
Solid MgCl2 dissolves and enters solutiondissolving reaction
Mg+2 + Cl-1 ions come together to form MgCl2
crystallization reaction
Solubility at equilibrium
When the rate of dissolving reaction equals the rate of crystallization reaction, we have equilibrium.
– A solution at equilibrium is called a saturated solution.
Saturation exists when…
• Equilibrium exists between the dissolved (ions) and the undissolved material (solid)
• Some undissolved material is still present (crystal solids)
How to saturate a solution & determine solubility
• To saturate a solution, add weighed portions of your solid to a volume of (solvent) water and stir until full.
– A bit of excess solid will always be present at equilibrium saturation though.
• In order to determine the solubility, you must completely
fill or saturate the solution!
Mg2+ Cl-
Amount MgCl2 DissolvedRate of dissolving > Rate of crystallization
Determining The Solubility of MgCl2
Add measured portions of MgCl2 to 100.0 mL and stir to dissolve
MgCl2
100.0 mL
10.0 gMgCl2 10.0 gMgCl2
10.0 g
MgCl2
3.0 g
MgCl2
0.0 g
Rate of dissolving = Rate of crystallization
slowvery slow
33.0 g equilibrium
unsaturated
saturated
MgCl2(s)
Calculate the solubility in units of g/L and mole/L
= 3.46 M
0.100 L
95.3g
x 1 mole33.0 g
=
Molar Solubility = Moles/L
Solubility =33.0 g
0.100 L= 330. g/L
Equilibrium Equation MgCl2(s) ⇌ Mg2+ + 2Cl-
Expression: Keq = [Mg2+][Cl-]2
do not use the solid
The Ksp or (solubility product) is used for saturated solutions at equilibrium
Ksp = [Mg2+][Cl-]2
Un-Saturated, Saturated and Super-satured Review
The rate of dissolving > the rate of crystallizing
Not at equilibrium
Not full – ( more solid can dissolve if you add it)
Unsaturated Solutions
How does it look? Clear solution!
The rate of dissolving = the rate of crystallizing
At equilibrium
Full- ( adding more solid will not dissolve )
How does it look? it always has crystals/solids in the solution.
Saturated Solutions
The rate of dissolving < the rate of crystallizing
Not at equilibrium
Over full – ( adding more solid causes precipitation)How does it look? Clear solution!
Supersaturated Solutions
Supersaturated video
Predicting the solubility of salts
Page 332 in your textbook has the table
“ Solubility of common compounds in water” Will be provided for you
Use it to predict solubility ( high or low )
Use it to predict if a precipitate will form.
Ag2SO4
CuSO4
CuCl2
CuCl
BaS
K2CO3
FeSO4
CaSO4
Ca(NO3)2
Na3PO4 High
Low
High Solubility means > .1 MLow Solubility means .1M
High
High
High
High
Low
High
Low
High
Precipitate Questions
Will a precipitate form when 0.2 M solutions of CaS and Na2SO4 are mixed?
Write the equation for equilibrium present in a saturated solution of Al2(SO4)3(s)
solution.
Ksp = [Al3+]2[SO42-]3
Equilibrium Expression
Al2(SO4)3(s) ⇌ 2Al3+ + 3SO42-
Write the equation for equilibrium present in a saturated solution of Ca3(PO4)2(s) solution.
Ksp = [Ca2+]3[PO43-]2
Ca3(PO4)2(s) ⇌ 3Ca2+ + 2PO43-
Equilibrium Expression
pg 74 #1-2, pg 76 #3-7, pg 77 #8-11Pg 83 #21,22 Pg 84 #24.
Practice Time