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Solutions

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  1. Solutions • Review of Solutions • Intermolecular Forces and Solutions (or Like Dissolves Like) • Factors in Solubility • Types of Solutions • Factors in Rate of Dissolution

  2. What are Solutions? • Homogeneous mixtures • Substances are soluble or miscible • They can be in all phases • Solid Solutions • Liquid Solutions • Gas Solutions

  3. Why do Solutions Form?Can we predict when a solution will form?What will happen if…?

  4. Solutions • You know that oil and water don’t mix together to form a solution. • The burning question is WHY??? • They are both liquids, why shouldn’t they mix together easily to form a solution? • And why does NaCl dissolve in water but NOT in oil?

  5. Rehash of Intermolecular Forces • Dipole-Dipole (as in HCl) • Hydrogen Bonding (as in HF or H2O) • Induced Dipole-Induced Dipole or London Dispersion Forces (as in H2 or He)

  6. Intermolecular Forces • Ion-Dipole • Soluble salt dissolves into ions • Ions are attracted to partial charges

  7. Intermolecular Forces

  8. Intermolecular Forces and Solutions • You’ve learned to predict which type of force predominates based on the polarity of the molecule. • You learned that the intermolecular forces are responsible for the boiling point (and melting point) of a substance. • But that’s not all that intermolecular forces do!

  9. Intermolecular Forces and Solutions • Intermolecular forces drive the solution process. • If we understand how intermolecular forces work, we can predict whether two substances will mix or not, AND explain WHY!

  10. The Solution Process • What happens when we mix two substances together? • We can visualize the mixing as occurring in three steps:

  11. Solvation Or Hydration

  12. The Solution Process • The overallΔH is the sum of three steps. ΔHsln=ΔH1 +ΔH2 +ΔH3 • But onlyΔH3 is exothermic!

  13. The Solution Process • So the solution process can be exothermic or endothermic. • Exothermic processes are favored by nature, but endothermic processes also occur naturally. • How can we tell whether they will occur naturally? • The magnitude ofΔHslnis crucial!

  14. The Solution Process • If the energy released forming the solute-solvent intermolecular forces is small compared to the energy required to break the solute-solute and solvent-solvent forces, then the solution process is highly endothermic. • The two substances won’t mix!

  15. The Solution Process • If the energy released forming the solute-solvent intermolecular forces is comparable to the energy required to break the solute-solute and solvent-solvent forces, then the solution process is exothermic or slightly endothermic. • The two substances will mix!

  16. The Solution Process

  17. The Solution Process • What this also means is the strength and thus the “type” of intermolecular forces must be similar between the solute and solvent.

  18. The Solution Process • If the solute and solvent have similar kinds of intermolecular forces, then -ΔH3is similar toΔH1 +ΔH2, and they will mix. • If the solute and solvent have different types of intermolecular forces, then -ΔH3is much less than ΔH1 +ΔH2, and they will NOT mix.

  19. Like Dissolves Like • This is summed up as the rule “Like dissolves like.” • What types of forces are similar?

  20. Like Dissolves Like • Ionic compounds are held together by ion-ion attractions. Ions have full charges. • Polar molecules are held together by dipole-dipole or hydrogen bonding. The molecules have partial charges. • Nonpolar molecules are held together by London forces (induced dipoles), the weakest intermolecular force.

  21. Like Dissolves Like • So polar molecules tend to mix with other polar molecules. • Ionic compounds tend to mix with polar compounds. • Nonpolar molecules (or atoms) tend to mix with other nonpolar substances.

  22. Like Dissolves Like • It’s actually even more complicated. • Here’s an example: • Chloroform, CHCl3, is a polar molecule, with a dipole moment of 1.04 D. • But it is not very water soluble. (1 mL dissolves in 200 mL water)

  23. Like Dissolves Like • Methyl chloride, CH3Cl, is polar with a dipole moment of 1.9 D. • It is considered to be slightly water soluble (100x more than chloroform). • Ethyl methyl ether, CH2CH3OCH3, is polar with a dipole moment of 1.12 D. • It is water soluble.

  24. Like Dissolves Like • What’s going on? • Why aren’t the polar molecules methyl chloride and chloroform water soluble? • Because “like dissolves like” can be stated more specifically:

  25. Like Dissolves Like • Substances which dissolve in each other usually have similar types of intermolecular forces. • This can explain why chloroform and methyl chloride aren’t very water soluble.

  26. Like Dissolves Like • Chloroform, methyl chloride, and ethyl methyl ether have dipole-dipole intermolecular forces. • Water has H-bonding. • So they don’t seem very “like” water. • Why is ethyl methyl ether water soluble?

  27. Like Dissolves Like • Ethyl methyl ether can H-bond to water as it contains an O. • Chloroform and methyl chloride can’t. • So ethyl methyl ether is more “like” water and is water soluble.

  28. Like Dissolves Like • It also gets more complicated for larger molecules with polar and nonpolar regions. • If the molecule has lots of polar regions (particularly H-bonding), it is more likely to be water soluble.

  29. Like Dissolves Like

  30. Like Dissolves Like • Some molecules like soap or ethanol are soluble in both polar and nonpolar solvents. • This is because they have both polar and nonpolar regions and so can dissolve in both types of solvents.

  31. Like Dissolves Like

  32. Like Dissolves Like • Now you can answer the questions: • Why doesn’t water (a polar molecule) mix with oil (a nonpolar molecule)? • Why does NaCl (an ionic compound) dissolve in water but NOT in oil?

  33. Other Solubility Factors • Now you can understand and predict whether 2 substances will mix. • But are there any other factors in solubility? • Temperature • Pressure (for gas solubility)

  34. Temperature & Solubility • For most substances, the solubility increases with increasing temperature. • This is not true for ALL compounds. • For gases, the reverse is true: the solubility decreases with increasing T. • This is why sodas go “flat” when open at room temperature.

  35. Temperature & Solubility

  36. Temperature & Solubility

  37. Pressure & Solubility of Gases • As gases are compressible, we can affect the solubility by changing the pressure. • The higher the vapor pressure of a gas, the more it dissolves.

  38. Pressure & Solubility of Gases

  39. Pressure & Solubility of Gases • This is Henry’s Law: s = kHP Where s is the solubility in M, P is the partial pressure of the gas, and kH is Henry’s Constant. • Soda and sparkling wine manufacturers rely on Henry’s Law! • Deep sea divers HATE this law! Why?

  40. Solution Types • Unsaturated • Saturated • Supersaturated

  41. Solution Types • Unsaturated • Solutions where LESS solute is dissolved than is possible at that temperature. • So the solute is not at its solubility limit (usually in M or g/mL or g/L) • Solution is CLEAR!

  42. Solution Types • Saturated • Solutions where as much solute is dissolved as is possible at that temperature. • Solute is at the solubility limit • Solution may be clear or there may be solute at the bottom.

  43. Solution Types • Supersaturated • Solutions where MORE solute is dissolved as is possible at that temperature. • Solute is over the solubility limit! • Solution is clear. • Unstable sln; may crystallize or “crash” easily.

  44. Solution Types • How do you tell them apart? • If there is undissolved solute, it IS saturated. • If there is no undissolved solute, then it could be any of the 3.

  45. Solution Types • How do you tell them apart? • Add a small amount of the solute • If it “crashes”, then it was supersaturated. • If the solute doesn’t dissolve, then it was saturated. • If the solute dissolves, then it was unsaturated.

  46. Solution Types

  47. Factors in Rate of Dissolution • Now you know the factors which determine whether substances will mix. • But what factors help determine how FAST they will mix? • You actually know all of these already! • How do YOU get sugar to dissolve in water?

  48. Factors in Rate of Dissolution • Stirring • Temperature • Surface Area

  49. Solution Concentration Units • Mass % • Volume % • Mass/Volume % • Molarity (you know) • Mole Fraction (you know for gas) • Molality (abbrev. m)