Unit 11 Solutions Essential Questions: What factors determine the rate at which a solute dissolves?

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