Solution & Solubility

1. Solution & Solubility By iTutor.com T- 1-855-694-8886 Email- [email protected]

2. Solution Solutions are homogeneous mixtures of two or more pure substances. In a solution, the solute is dispersed uniformly throughout the solvent . A homogenous mixture of a solute dissolved in a solvent. The solubility (ability to dissolve) of a solute in a solvent is dependent on the Temperature For solid solutes as temperature increases, solubility increases. For gas solutes :as temperature increases, solubility decreases.

3. Pressure For solid solutes as pressure increases, solubility remains the same. For gas solutes: as pressure increases, solubility increases Nature of Solute/Solvent “Like dissolves in like.” Solute Type Non Polar Solvent Polar Solvent Non Polar (Fat Grease) Soluble (Soap) Insoluble (Water) Polar Insoluble Soluble (Water) Ionic (Salt) Insoluble Soluble (Water) High Solubility-Soluble Low Solubility-insoluble

4. Solubility Maximum grams of solute that will dissolve in 100 g of solvent at a given temperature. Solubility = Varies with temperature Based on a saturated solution Gram of solute 100 g of Water The bond between solubility and temperature can be expressed by a solubility curve. The solubility curves of several compounds are shown on the right. Such curves disclose the maximum amount of solute that can be dissolved in 100 grams of water over a range of temperatures. The solubility of most compounds increases as temperature increases, although exceptions do exist.

5. Types of Solution 1. An unsaturated solution is a solution in which more solute can be dissolved at a given temperature. 2. A saturated solution is a solution containing the maximum amount of solute that will dissolve at a given temperature. 3. A supersaturated solution is a solution that contains more solute than would dissolve in a saturated solution at a given temperature. SATURATED SOLUTION no more solute dissolves UNSATURATED SOLUTION more solute dissolves SUPERSATURATED SOLUTION becomes unstable, crystals form increasing concentration

6. Saturated Solvent holds as much solute as is possible at that temperature. Dissolved solute is in dynamic equilibrium with solid solute particles. Types of Solution Unsaturated Less than the maximum amount of solute for that temperature is dissolved in the solvent. Saturated Supersaturated Solvent holds more solute than is normally possible at that temperature. These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

7. Solubility Rules Not all ionic compounds are water soluble. There are some general rules for compounds that are water soluble: Group 1 ionic compounds and ammonium (NH4+) are always water soluble Group 17 ionic compounds are water soluble except when paired with Ag, Pb, and Hg ions A salt is soluble in water if it contains any one of the following ions: NH 4 + Li + Na + K + or NO 3 - Examples: Soluble salts LiCl Na 2 SO 4 KBr Ca(NO 3 ) 2

8. Solubility Rules 2. Salts with Cl - are soluble, but not if the positive ion is : Ag + , Pb 2+ , or Hg 2 2+ . Examples: Soluble Insoluble MgCl 2 AgCl , PbCl 2 3. Salts with SO 4 2- are soluble, but not if the positive ion is Ba 2+ , Pb 2+ , Hg 2+ or Ca 2+ . Examples: S oluble Not soluble MgSO 4 BaSO 4 PbSO 4

9. The stronger the attraction between solute and solvent molecules, the greater the solubility. Like dissolves like (the substances have similar intermolecular attractive forces.) Polar substances tend to dissolve in polar solvents. Non-polar substances do not dissolve in polar solvents. Factors Affecting Solubility

10. Chemists use the axiom “ like dissolves like ”: Polar substances tend to dissolve in polar solvents. Nonpolar substances tend to dissolve in nonpolar solvents. Factors Affecting Solubility The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

11. Glucose (which has hydrogen bonding) is very soluble in water, while cyclohexane (which only has dispersion forces) is not. Factors Affecting Solubility

12. Vitamin A is soluble in nonpolar compounds (like fats). Vitamin C is soluble in water. Factors Affecting Solubility

13. Concentration Concentration of a solution can be expressed either quantitatively or qualitatively (dilute or concentrated). Methods of Expressing Concentrations of Solutions Mass Percentage Mass percentage is expressed using the equation: Mass % of component = X Mass of component in solution T otal mass of solution 100 Parts per Million (ppm) ppm = mass of A in solution total mass of solution  10 6 Parts per Billion ( ppb) ppb = mass of A in solution total mass of solution  10 9

14. Mole Fraction This is how many moles of substance are in the solution. Mole fraction of component = Moles of component Total moles of all components Molarity Relates the volume of solution to the quantity of solute that it contains. Molarity = Moles solute Liters solution Molality(m) mol of solute kg of solvent m = Because both moles and mass do not change with temperature, molality (unlike molarity) is not temperature dependent.

15. Changing Molarity to Molality If we know the density of the solution, we can calculate the molality from the molarity , and vice versa.

16. Raoult’s Law P soln = Vapor pressure of the solution  solvent = Mole fraction of the solvent P  solvent = Vapor pressure of the pure solvent The presence of a nonvolatile solute lowers the vapor pressure of a solvent . P soln =  solvent P  solvent The presence of a nonvolatile solute lowers the concentration of solvent molecules in the solution, thus lowering the vapor pressure. Demonstration of Raoult’s Law

17. Water is transported to the solution due to difference in vapor pressure. An Aqueous Solution and Pure Water in a Closed Environment Demonstration of Raoult’s Law

18. Ideal Solution ...a solution that obeys Raoult’s law P soln =  solvent P  solvent Vapor Pressure of a Solution Containing a Nonvolatile Solute Ideal solutions obey Raoult’s law exactly solute-solute, solvent-solvent, and solute-solvent interactions are identical do not exist Real Solutions are nonideal may approach ideal behavior if solute and solvent are similar Example: sucrose in water

19. Vapor Pressure of an Ideal Solution Two Volatile Solvents P total = P A + P B = X A P A + X B P B

20. Raoult’s Law Vapor Pressure for a Solution of Two Volatile Liquids IDEAL SOLUTION NONIDEAL SOLUTIONS Positive Deviation Negative Deviation

21. Colligative Properties Depend only on the number, not on the identity, of the solute particles in an ideal solution. Boiling point elevation Freezing point depression Osmotic pressure Boiling Point Elevation K b = Molal boiling point elevation constant m = Molality of the solute A nonvolatile solute elevates the boiling point of the solvent.  T = K b m solute

22. Freezing Point Depression K f = Molal freezing point depression constant m = Molality of the solute A nonvolatile solute depresses the freezing point of the solvent.  T = K f m solute Phase Diagram for an Aqueous Solution In effect, a dissolved solute acts to extend the liquid range of the solvent

23. Freezing Point Depression An Argument Based on Vapor Pressures For pure liquid water and ice, the vapor pressures of the solid and liquid are equal at 0 o C. With solute present, the vapor pressure of the liquid decreases, causing the solid to melt. Decreasing the temperature lowers the vapor pressure of the liquid faster than that of ice, and a lower freezing point is obtained. ice in pure water ice in a solution

24. Boiling-Point Elevation and Freezing-Point Depression

25. Colligative Properties of Electrolyte Solutions  T = imK  = iMRT van’t Hoff factor, “ i ”, relates to the number of ions per formula unit. NaCl = 2, K 2 SO 4 = 3 Data for 0.05 m Solutions Electrolyte i (expected) i (observed) NaCl 2.0 1.9 MgCl 2 3.0 2.7 FeCl 3 4.0 3.4

26. Osmosis Some substances form semipermeable membranes , allowing some smaller particles to pass through, but blocking other larger particles. In biological systems, most semipermeable membranes allow water to pass through, but solutes are not free to do so.

27. In osmosis, there is net movement of solvent from the area of higher solvent concentration ( lower solute concentration ) to the are of lower solvent concentration ( higher solute concentration ). Osmosis The pressure required to stop osmosis, known as osmotic pressure ,  , is where M is the molarity of the solution If the osmotic pressure is the same on both sides of a membrane (i.e., the concentrations are the same), the solutions are isotonic n V  = ( ) RT = MRT Osmotic Pressure

28. Colloids Colloids are particles that are large on the molecular scale but still small enough to remain suspended indefinitely in a solvent system. (aka colloidal dispersions .) They are intermediate between solutions and heterogeneous mixtures. They have the ability to scatter light; known as the Tyndall effect. Hydrophilic and Hydrophobic Colloids The most important colloids are those in which water is the solvent, or dispersing agent. Hydrophilic colloids are water loving and are found in the human body and help keep molecules suspended in water. (enzymes, antibodies) Hydrophobic colloids are water fearing and must be stabilized before they can be mixed into water. (droplets of oil.)