Solutions

1. Solutions A homogeneous mixture of two or more substances.

2. The Solution Process We will focus on solid or liquid solutes dissolved in a liquid solvent. Since all particles are in contact with each other, the solute-solute and solvent-solvent forces of attraction are disrupted, and new, solute-solvent forces of attraction are created.

3. The Solution Process The disruption of solute-solute and solvent-solvent forces of attraction requires energy, and is endothermic. The interaction of solvent and solute usually releases energy. The sum of the energy of all three steps is called the enthalpy of solution, ΔHosoln. Note that solutions may form whether the net process is endothermic or exothermic.

4. The Solution Process

5. The Solution Process In addition to the enthalpy of solution, we must also consider the entropy of mixing. Entropy is a measure of randomness or disorder. An increase in entropy makes a process more likely to occur. Since mixing pure substances increases entropy, this factor makes processes that are slightly endothermic favorable.

6. Entropy of Mixing

7. The Solution Process The general rule on solution formation is: Like dissolves like. Polar and ionic compounds dissolve in polar solvents. Non-polar compounds dissolve in non-polar solvents.

8. Forces in Mixtures

9. Like Dissolves Like Vitamin A consists almost entirely of carbon and hydrogen, and is non-polar. As a result, vitamin A is fat-soluble, and can be stored in the body.

10. Like Dissolves Like Vitamin C contains polar C-O and O-H bonds. It is water soluble, and must be consumed often, as it is excreted easily. C-O bond O-H bonds

11. Like Dissolves Like Ionic Compounds

12. Like Dissolves Like

13. The Solution Process Disrupt-ion of solute Disrupt-ion of solvent Solute/Solvent interact-ion

14. Ionic Aqueous Solutions When an ionic compound is dissolved in water, the energy required to separate the ions of the solute is equal to –(lattice energy), or -ΔHlattice. The energy released as the gaseous ions dissolve in water is called the hydration energy, ΔHhydration. The net energy change is ΔHsoln.

15. Heat of Hydration

16. Factors Affecting Solubility • Molecular Structure • Pressure (for gaseous solutes) • Temperature

17. Pressure Effects Gases dissolved in a liquid solute obey Henry’s Law: C = kP where C is the concentration, k is a constant specific to solute and solvent, and P is the pressure of the gas above the solution

18. Pressure Effects Gases dissolved in a liquid solute obey Henry’s Law: C = kP The amount of a gas dissolved in a solution is directly proportional to the pressure of the gas above the solution.

19. Henry’s Law

20. Pressure Effects

21. Temperature Effects For gases dissolved in liquids, the solubility decreases as temperature increases. That is, gases dissolve better in cold liquids than in warmer liquids.

22. Temperature Effects For solid solutes dissolved in water, the effect of temperature on solubility is difficult to predict, although many solids dissolve more as temperature increases.

23. Solution Concentration Although molarity, M, (moles of solute per liter of solution) is used for stoichiometry calculations, there are many other ways to express the concentration of a solution. Molarity will vary slightly with changes in temperature as the volume of the solution expands or contracts. Units such as mass percent, mole fraction, or molality remain constant as temperature changes.

24. Solution Concentration Mass percent = (mass of solute) (100%) (mass of solution) Mole fraction (XA) = (moles of A) total # of moles Molality (m) = moles of solute kg of solvent

25. Very Dilute Solutions The concentration of very dilute solutions are expressed in parts per million (ppm) or parts per billion (ppb). ppm = [(mass solute) x 106 ] ÷(mass soln) ppb = [(mass solute) x 109 ] ÷(mass soln)

26. Liquid-Liquid Solutions When two volatile liquids mix, they form a solution. An ideal solution, similar to an ideal gas, will exert a vapor pressure which is related to the vapor pressures of the pure liquids and their relative abundance in the mixture.

27. Liquid-Liquid Solutions The solution obeys Raoult’s law: PA = χAPoA PB = χBPoB where χA is the mole fraction of component A and PoA is the vapor pressure of pure A

28. Liquid-Liquid Solutions Raoult’s law is best seen graphically. The vapor pressure of the mixture is the sum of the vapor pressures of each component.

29. Liquid-Liquid Solutions Ideal solutions typically involve non-polar molecules with similar structures. Mixtures of liquid hydrocarbons often form ideal solutions.

30. Liquid-Liquid Solutions If the two components of the mixture are strongly attracted to each other, such as two polar molecules, the vapor pressure of the mixture is often lessthan that predicted by Raoult’s law.

31. Liquid-Liquid Solutions This is known as a negative deviation from Raoult’s law. It occurs with mixtures of liquid acids and water. As the acid ionizes, the forces of attraction in the mixture increase.

32. Liquid-Liquid Solutions If a mixture contains liquids that have stronger attractive forces when pure than when mixed, the mixture will exert a vapor pressure that is greater than that predicted by Raoult’s law. This is called a positive deviation from Raoult’s law.

33. Liquid-Liquid Solutions A mixture of ethanol and water exhibits a positive deviation from Raoult’s law. The hydrogen bonding of each pure liquid is disrupted when the two liquids are mixed.

34. The Colligative Properties The colligative properties are properties that depend upon the concentration of particles (molecules or ions) dissolved in a volatile solvent, and not on the nature of the particles. They include: • vapor pressure • freezing point • boiling point • osmotic pressure

35. The Colligative Properties Relatively simple mathematical relationships can be used to predict the changes in vapor pressure, freezing and boiling point, etc. The properties can be predicted for dilute solutions (<0.1M) of non-volatile solute (usually solids) dissolved in a volatile solvent (usually a liquid).

36. Vapor Pressure The addition of a non-volatile solute to a volatile solvent lowers the vapor pressure of the solvent.

38. Vapor Pressure The decrease in vapor pressure can be understood by looking at the evaporation process. We need to compare the enthalpy change (ΔHvap) and entropy change of evaporation.

39. Vapor Pressure The vapor pressure of the pure solvent or the solution is the result of solvent molecules escaping the liquid surface and becoming gaseous. Since the solute is non-volatile, it does not evaporate. Since only solvent molecules evaporate, the enthalpy change for pure solvent or the solution is the same.

40. Vapor Pressure The decrease in vapor pressure of the solution is the result of changes in entropy. The vapor in either container is disordered, due to the random motion of gaseous solvent.

41. Vapor Pressure The liquid phases differ in entropy. The pure solvent is relatively ordered since all of the molecules are the same (solvent).

42. Vapor Pressure The liquid phase of the solution is much more random, since it is a mixture.

43. Vapor Pressure Upon evaporation, the pure solvent undergoes a greater increase in entropy than the solution.

44. Vapor Pressure Systems tend to maximize entropy. The pure solvent evaporates more readily, because it undergoes a greater increase in entropy.

45. Vapor Pressure Lowering The change in vapor pressure can be calculated as follows: ∆vp = -Xsolute Psolvent where X is the mole fraction of solute particles Posolvent is the vapor pressure of the pure solvent o

46. Vapor Pressure Lowering ∆vp = -Xsolute Posolvent The sign is negative because the vapor pressure decreases.

47. Vapor Pressure Lowering Psoln = Xsolvent Posolvent The mole fraction of solvent, Xsolvent , = moles of solvent/total moles of particles and solvent.

48. Problem – Vapor Pressure Water has a vapor pressure of 92.6 mmHg at 50oC. Compare the vapor pressure of two aqueous solutions at 50oC. One contains .100 mole of sucrose dissolved in 1.00 mol of water. The other contains .100 moles of CaCl2 dissolved in 1.00 mol of water. Will the two solutions have the same vapor pressure? If not, why?

49. Solution Phase Diagrams The lowering of the vapor pressure due to the presence of a non-volatile solute affects several properties. The phase diagram for the solution will be shifted, due to the lower vapor pressure of the solution.

50. Solution Phase Diagrams