Factors Influencing Solute Dissolution Rates: Understanding Solutions and Concentration

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

2. Recall the Terms • Solute • The substance being dissolved (the one that changes phase) • Solvent • The substance doing the dissolving • Universal solvent is water • If it is a solution of 2 liquids or 2 solids, the solvent is the one in large quantity

3. Solution formation • Nature 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?

4. Making solutions • In 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.

5. Temperature and Solutions • Higher 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)

6. Gases In Liquids • Henry’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 =

7. Liquids • Recall from Unit 9 • Miscible means that two liquids can dissolve in each other • water and antifreeze, water and ethanol • Immiscible means they can’t • oil and vinegar

8. 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

9. Read a Solubility Curve

10. Solubility Summary • For 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

11. Concentration Is… • A measure of the amount of solute dissolved in a given quantity of solvent • Aconcentrated solution has a large amount of solute • Adilute solution has a small amount of solute • Thus, only qualitative descriptions • But, there are ways to express solution concentration quantitatively

12. Concentration of Solutions • Recall from Unit #4 • Molarity • Molality • Mole fraction • % by weight • % by volume • Recall how to make solutions

13. 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.

14. Law of Mass Action • For any reaction • jA + kBlC + mD • K = [C]l[D]mPRODUCTSpower [A]j[B]kREACTANTSpower • K is called the equilibrium constant. • is how we indicate a reversible reaction • Ignore pure solids and liquids

15. Law of Mass Action • Write 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

16. Ksp • Solubility product constant • Write dissociation equation • NaCl (s) Na+ (aq) + Cl-(aq) • Ksp= [Na+] [Cl-] • No denominator since it will always be a solid

17. Ksp • If 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

18. Colligative Properties • Depend only on the number of dissolved particles • Not on what kind of particle

19. Vapor Pressure decreased • The bonds between molecules keep molecules from escaping. • In a solution, some of the solvent is busy keeping the solute dissolved. • Lowers the vapor pressure • Electrolytes form ions when they are dissolved = more pieces. • NaCl ® Na+ + Cl- (= 2 pieces) • More pieces = bigger effect

20. 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.

21. 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.

22. Why Molality? • The size of the change in boiling point is determined by the molality. • DTb= Kbx m x n • DTb is the change in the boiling point • Kb 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.

23. What about Freezing? • The size of the change in freezing point is also determined by molality. • DTf= -Kfx m x n • DTf is the change in freezing point • Kf 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.

24. Molar Mass • We 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