Chapter 14 : Solutions

1. Chapter 14 : Solutions

2. Definitions Solution An intimate, homogeneous mixture that has no visible boundaries among its components, particles (atoms, molecules or ions) are uniformly distributed throughout. Solute The substance that dissolves in a solvent (usually in lesser quantity). The part of a solution which is generally in lesser quantity. Solvent The substance in which a solute dissolved (usually in greater quantity). The part of a solution in greater quantity.

3. Types of Solutions Many solvents are liquids like water. When water is a solvent, the solution is aqueous. Other liquids may be solvents: gasoline, charcoal lighter fluid, ethanol, hexane, toluene. Solutes may be liquids, solids or gases: sucrose (sugar), salt (NaCl), ethanol, carbon dioxide.

4. Solutions Solvents may be gases: air is a solution of nitrogen, oxygen, argon, and other gases. Steel is a solution of one solid dissolved in another. Other alloys are solutions of various metals.

5. Solubility Solubility The maximum amount of solute that dissolves in a fixed quantity of a particular solvent at a specified temperature when excess solute is present Soluble Capable of being dissolved. Having a solubility greater than a certain amount (e.g., 0.1 mol/L). A solute is said to be more soluble if it has a solubility greater than a substance which is said to be less soluble.

6. Solubility Terms Miscible Two liquids that are soluble in each other. If they are soluble in any proportion, they are said to be completely miscible. Liquids that are partially soluble are said to be partially miscible Immiscible Liquids that are not soluble when mixed.

7. Solubility Terms When a substance does not appear to dissolve in a solvent, it is said to be insoluble. A concentrated solution has a relatively large amount of solute for given quantity of solution. A dilute solution has a smaller amount of solute for a given quantity of solution.

8. Ionic Solubility Water will dissolve many ionic compounds (salts). When salts dissolve in water: 1. The ionic forces holding ions together are broken. 2. The attractive forces holding some of the water molecules together are broken. 3. Ions must interact with the solvent molecules to form attractive forces.

9. Ionic Solubility (continued) The process in which water molecules surround the ions is called hydration. For other solute/solvent interactions, this process is called solvation. In order for hydration to occur, the energy released by the interaction of the solvent molecules with the ions must be greater than the energy needed to break the ions apart and separate solvent molecules.

11. Ionic Solubility Some ionic compounds with high charges are insoluble in water. The solvent-ion interaction energy is not strong enough to overcome the lattice energy holding the ions together. For example: CaCO3, AlPO4, BaSO4 Solubility charts serve as a guide to ionic solubilities.

12. Solubility of Covalent Compounds Like dissolves like. Non-polar solutes dissolve in non-polar solvents. Polar solutes dissolve in polar solvents. Fats, oils and greases, which are non-polar, do not dissolve in water, a polar solvent, but do dissolve in hexane, a non-polar solvent. Hydrogen bonding between solute and solvent molecules plays an important role.

14. Solubility Equilibria Most solutes have a limited solubility in a given solvent. When more solute is present than the solubility limit, a dynamic equilibrium is established: solute + solvent solution

15. Solubility Calculation Calculate the number of grams of NaCl that can be dissolved in 50 g of water at 20oC. (The solubility of NaCl is 36g NaCl/100g H2O) 50g H2O x 36g NaCl/100g H2O =

16. Effects of Temperature on Solubility Most solids become more soluble as temperature increases. (There are some exceptions.) If a saturated solution is cooled, with solid present, more solid will precipitate until equilibrium is re-established. If a saturated solution is cooled with no solid present, sometimes precipitation does not occur immediately. The solution is said to be supersaturated.

18. Supersaturated Solutions Solute may be caused to precipitate from supersaturated solutions by: Stirring the solution Scratching the inside of the container to create a seed crystal. Adding a seed crystal of the solute. Following precipitation, equilibrium is re-established.

19. Gas Solubility Gases become less soluble in liquids as temperature is increased. Gas solubility increases with pressure. The relationship between gas solubility and pressure is called Henry�s Law. Gas solubility is directly proportional to the gas pressure: S = kP

21. Concentration The concentration of a solution is the amount of solute dissolved in a certain quantity of solute or solution. One measure of concentration is molarity, M. M = (mol solute)/Liters solution mol solute = (M)(L) L = (mol solute)/M

22. Molarity Problem Tell how to prepare 250 mL of a 3.00 M solution of NaCl.

23. Preparing solutions of a given molarity (continued) The plan for solving the above problem was:

24. Another Molarity Problem How many mL of 12.00 M HCl solution are required to deliver 0.500 mol? L = moles/M = (0.500 mol)/(12.00 mol/L)

25. Percent by Volume [%(v/v)] Percent by Volume = [(volume solute)/(volume solution)]x100% Calculate the percent by volume alcohol prepared by mixing 15.0 mL with enough water to make a total volume of 50.0 mL. Percent by volume = [(15 mL)/(50 mL)] x 100 = 30%(v/v)

26. Percent by mass [%(w/w)] Percent by mass = [(mass solute)/(total mass of solution)]x100% =[(g solute)/(g solute + g solvent)] x 100% How many g of NaCl are required to prepare 100 g of a 5.0% solution? Mass solute = (percent/100)x(total mass) = (5/100)x100 g = 5.0 g NaCl

27. Concentrations of Very Dilute Solutions Parts per million, ppm (one g solute in a million grams solution) (1 mg/L) parts per billion, ppb (1 ?g/L) parts per trillion, ppt (1 ng/L) Calculate the quantity of lead present in 100 L or water that has a concentration of 3.0 ppm. 3.0 ppm = 3.9 mg/L x 100 L

28. Preparation of Solutions by Dilution V1C1 = V2C2 Calculate the volume of 10.0%(v/v) ethanol require to prepare 500 mL of 5.00% ethanol. C1 = 10.0, V2 = 500 mL, C2 = 5.00 V1 = V2C2/C1 = (500mL)(5.00%)/(10.0%)

29. Colligative Properties Properties of liquids change when solutes are dissolved in them: boiling point is raised melting point is decreased The higher the concentration of solute particles, the greater the effect. Colligative Properties are properties of solutions that depend on the number of dissolved solute particles.

30. Counting Particles A solution of 1.00 L of a 1.00 M solution of glucose contains 6.022 x 1023 particles of glucose molecules. But a solution of 1.00 L of a 1.00 M solution of NaCl contains 2x6.022x1023 particles because of ionization. The freezing point depression of 1.0 M NaCl is almost twice that of 1.0 M glucose.

31. Colloids The temporary dispersion of one substance in another is a mixture called a suspension. Part of the mixture will settle out from the other part or can be separated by filtration. Colloids or colloidal dispersions are defined as mixtures having a particle size of 1.0 nm - 100 nm. Colloids appear milky or cloudy. They scatter light by the Tyndall effect.

32. Colloids There are 8 different types of colloids based on the particles and the phase of the dispersal agent. Some examples are foams and emulsions. Colloids are stabilized by emulsifying agents such as soap or bile salts.

33. Osmosis and Dialysis Certain materials are: Permeable: allow water and solvents to pass through. Impermeable: do not allow solvents to pass. Semipermeable: material which allows solvent molecules to pass, but not solute molecules. The process of solute molecules passing through a semipermeable membrane is called osmosis. If small ions and molecules pass through the membrane with the solvent, but colloidal particles are retained the process is called dialysis.

35. Osmosis The pressure required to prevent the flow of solvent from the less concentrated side to the more concentrated side of a membrane is called the osmotic pressure. Isotonic solutions is one that has the same osmotic pressure as the fluid in a living cell. Hypotonic solutions have a lower concentration than inside the cell. Water flows from a solution into a cell.

36. Osmosis in Cells The rupture of a cell by hypotonic solutions is called plasmolysis (or hemolysis in the case of red blood cells) Hypertonic solutions contain a greater concentration of particles than the fluid in cells. Water will flow from the cells to the solution. This process is called crenation.