Unit 11SolutionsEssential Questions:•What factors determine the rate at which a solute dissolves?
Recall the TermsSolute
◦The substance being dissolved (the one that changes phase)
Solvent◦The substance doing the dissolving
Universal solvent is waterIf it is a solution of 2 liquids or 2
solids, the solvent is the one in large quantity
Solution formationNature of the solute and the solvent
◦Whether a substance will dissolve◦How much will dissolve
Factors determining rate of solution...◦stirred or shaken (agitation)◦particles are made smaller◦temperature is increased
Why?
Making solutionsIn order to dissolve, the solvent molecules must come in contact with the solute.
Stirring moves fresh solvent next to the solute.
The solvent touches the surface of the solute.
Smaller pieces increase the amount of surface area of the solute.
Temperature and SolutionsHigher temperature makes the molecules of the solvent move around faster and contact the solute harder and more often.◦Speeds up dissolving.
Usually increases the amount that will dissolve (exception is gases)
Gases In LiquidsHenry’s Law - says the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid◦Think of a bottle of soda◦Removing the lid releases pres.
Equation: S1 S2
P1 P2=
LiquidsRecall from Unit 9Miscible means that two liquids can dissolve in each other◦water and antifreeze, water and ethanol
Immiscible means they can’t◦oil and vinegar
How Much?Solubility-The maximum amount of
substance that will dissolve at a specific temperature (g solute/100 g solvent)
Saturated solution◦Contains the maximum amount of solute dissolved
Unsaturated solution◦Can still dissolve more solute
Supersaturated◦Solution that is holding more than it theoretically can
Read a Solubility Curve
Solubility SummaryFor solids in liquids, as the temperature goes up-the solubility usually goes up
For gases in a liquid, as the temperature goes up-the solubility goes down
For gases in a liquid, as the pressure goes up-the solubility goes up
Concentration Is…• A measure of the amount of solute
dissolved in a given quantity of solvent
• A concentrated solution has a large amount of solute
• A dilute solution has a small amount of solute–Thus, only qualitative descriptions
• But, there are ways to express solution concentration quantitatively
Concentration of Solutions◦Recall from Unit #4MolarityMolalityMole fraction% by weight% by volumeRecall how to make solutions
What is equal at Equilibrium?• Rates are equal• Concentrations are not.• Rates are determined by
concentrations and activation energy.
• The concentrations do not change at equilibrium.
• or if the reaction is verrrry slooooow.
Law of Mass Action• For any reaction • jA + kB lC + mD• K = [C]l[D]m PRODUCTSpower
[A]j[B]k REACTANTSpower
• K is called the equilibrium constant.• is how we indicate a
reversible reaction• Ignore pure solids and liquids
Law of Mass ActionWrite the equilibrium constant
expression for the reaction equation:
NH3(aq) + HCl (aq) NH4+(aq) +
Cl-(aq)
What is K when [NH3] = 0.100M, [HCl] = 1.00M, [NH4
+] = 0.200M, [Cl-] = 0.100M
KspSolubility product constantWrite dissociation equationNaCl (s) Na+ (aq) + Cl-(aq)Ksp= [Na+] [Cl-]No denominator since it will
always be a solid
KspIf your Ksp calculated value is =
to actual value the solution is saturated
If your Ksp calculated value is greater than the actual value the solution is supersaturated
If your Ksp calculated value is less than the actual value the solution is unsaturated
Colligative Properties Depend only on the number of
dissolved particlesNot on what kind of particle
Vapor Pressure decreasedThe bonds between molecules keep molecules from escaping.
In a solution, some of the solvent is busy keeping the solute dissolved.
Lowers the vapor pressureElectrolytes form ions when they are dissolved = more pieces.
NaCl ® Na+ + Cl- (= 2 pieces)
More pieces = bigger effect
Boiling Point Elevation• The vapor pressure determines the boiling point.
• Lower vapor pressure = higher boiling point.
• Salt water boils above 100ºC• The number of dissolved particles determines how much, as well as the solvent itself.
Freezing Point Depression• Solids form when molecules make an orderly pattern.
• The solute molecules break up the orderly pattern.
• Makes the freezing point lower.• Salt water freezes below 0ºC• How much depends on the number of solute particles dissolved.
Why Molality?The size of the change in boiling
point is determined by the molality.DTb = Kb x m x nDTb is the change in the boiling pointKb is a constant determined by the
solvent m is the molality of the solution.n is the number of pieces it falls into
when it dissolves.
What about Freezing?The size of the change in freezing
point is also determined by molality.DTf = -Kf x m x nDTf is the change in freezing pointKf is a constant determined by the
solvent m is the molality of the solution.n is the number of pieces it falls
into when it dissolves.
Molar MassWe can use changes in boiling and freezing to calculate the molar mass of a substance
Find: 1) molality 2) moles, and then 3) molar mass