Lecture 1 Chapter 12 Sections 1-2. Solutions Solubility

Transcription

1 Lecture 1 Chapter 12 Sections 1-2 Solutions Solubility

2 Chapt 12: Nature of Solutions Solution a homogeneous mixture of two or more substances, in which one is called the solvent and the other is the solute Solvent main medium, what is there in the largest quantity Solute dissolved in the solvent Without bubbles! X X??

3 Solution Concentration Concentration units we have already mentioned: Mole Fraction (see Section 5.5) ppm and ppb (see Section 5.5) Really the same thing as mole fraction (remember pph = percent) Molarity common measure for solutions (see Section 3.6) A new concentration unit: Molality n M = V n X A = n solute solution A total Moles per liter Mole per mole unitless

4 Molality (m) Useful for applications where the temperature of a solution changes. Defined as the number of moles of solute divided by the mass of the solvent in kg Units are mol/kg c = m n m solute solvent

5 Example: Aqueous ammonia Concentrated aqueous ammonia is 14.8 M and has a density of g/ml. What is the molality of ammonia in this solution? Assume 17.0 g/mol as the molecular weight of NH 3

6 Solubility Solubilities vary tremendously Chemists often use qualitative descriptions, for example: Miscible: when two liquids mix in all proportions (Acetone and Water) Immiscible: when two liquids DO NOT mix at all (Oil and water) Insoluble: when a solid does not dissolve in a solvent (NaCl in gasoline) Saturated: a solution that has dissolved the maximum possible solute.

7 Solubility A more quantitative measure of solubility is the highest concentration solution that can be created. A saturated solution of material X in water is 2.5 M A saturated solution of material Y in water is 0.3 M So, X is more soluble than Y

8 Supersaturation A solution holding the maximum amount of solute is saturated A solution that could hold more solute is unsaturated It is possible in some cases to add more solute to a solution that is already saturated. This solution is then called supersaturated This is possible because solubility of solids in liquids increases with heat. (Gasses become less soluble with heat.) So, we make a saturated solution at high temp then carefully cool it If everything is just right no excess solute will crystallize out of solution.

9 Supersaturation deaux Crystallization is a complicated process Some kind of nucleation point is needed. Imperfection in glass wall Piece of dust that is correct shape Already existing crystal Sodium acetate works well for this because it forms a complicated crystal shape. So, really the only thing that makes a good nucleation point is another crystal of sodium acetate.

10 Supersaturated Sodium Acetate Five pieces must come together in just the right pattern Na + CH 3 COO - 3 H 2 O NaCH 3 COO 3H 2 O(s) Sodium acetate trihydrate

11 Supersaturated Sodium Acetate Note that in going from a liquid state to a solid state, heat is released. The supersaturated solution was not the most stable state. Energy is released as the system becomes more stable.

12 Like Dissolves Like What does this mean? Vinegar (acetic acid) dissolves in water, oil does not We are talking about polarity here Compounds with similar polarity will be soluble in each other. In other words, substances that dissolve in each other usually have similar types of intermolecular interactions.

13 Which is the most soluble in hexane? 1. Hexanol 2. NaCl 3. Octane 4. Water

14 Example Observe the following trend of alcohol solubilities in water: n-propanol CH 3 OH Completely Miscible n-butanol CH 3 OH 1.1 M n-pentanol CH 3 OH 0.30 M n-hexanol CH 3 OH M

15 Solubility of Solids Remember there are 4 basic solids? 1. Network solids (diamond, graphite) cannot dissolve without breaking covalent bonds. 2. Molecular solids like dissolve like 3. Metals do not dissolve in water (some will react, but not dissolve) 4. Ionic Solids remember the solubility rules?

16 Which of the following Example vitamins are fat-soluble (dissolve fatty hydrocarbons) and which are water soluble? Vitamins are organic molecules that are required for proper function but are not synthesized by the human body. Thus, vitamins must be present in the foods people eat. Vitamins fall into two categories: fatsoluble, which dissolve in fatty hydrocarbon-like tissues and water-soluble.

17 Alloys Alloy: a mixture of substances with metallic properties. (a solid solution) Some are true solutions homogeneous (brass) Others are heterogeneous (solder)

18 Solubility of Salts (Remember Chapt 4)

19 Today Read syllabus Start CAPA 1 (due Monday) Friday Finish reading Chapt 12 Plan to attend seminar Remember: You are done with the homework when you understand it!