Lecture 30: Solutions 4

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1 Lecture 30: Solutions 4 Read: BLB HW: BLB 13:7,15,21,23,33 Sup13:1 7 Know: how solutions form solubility & saturation factors affecting solubility Need help?? Get help!! TAs in CRC (211 Whitmore) and SI hours on Chem 110 website; my office hours (Mon 12:30-2 & Tues 10:30-12 in 324 Chem Bldg [or 326 Chem]) Bonus deadline for BST #9: Solutions & dilutions, April 2 Review chemical nomenclature (e.g., BST #5, Lecture 8 ); & memorize those strong acids & bases (BLB Table 4.2) Check out the Exam #3: Monday, April 6:30 pm; Sign up for the conflict exam, if needed. Last day to sign up is Wed, April 1. No textprogrammable calculators (PDAs, ipods, etc). Bring PSU ID and several pencils; Late drop deadline: Friday, April 11:59 pm via elion Sheets Page 1 Lecture 30

The solution process solvation: solutions form when the attractive IM forces between solute & solvent are comparable in magnitude (& nature) to those that exist between solute solute or solvent

2 The solution process solvation: solutions form when the attractive IM forces between solute & solvent are comparable in magnitude (& nature) to those that exist between solute solute or solvent solvent alone hydration: solvation in water for solvation to occur: 1. break solute solute interactions 2. break solvent solvent interactions 3. solute solvent interactions form Sheets Page 2 Lecture 30

3 !H 1 ΔH 2 ΔH 3 Sheets Page 3 Lecture 30

4 The solution process (cont.) processes occur spontaneously when: energy is released (exothermic), ΔH ; & disorder increases (entropy), ΔS these processes are linked: a process can be endothermic when the increase in entropy (disorder) is large enough; this is true for any type of process (dissolution, reaction, etc.) forming a solution always increases disorder because of mixing, thus spontaneity depends on ΔH ΔH soln = ΔH 1 + ΔH 2 + ΔH 3 ΔH soln : overall enthalpy change in forming a solution a solution will form in most cases unless solute solute or solvent solvent interactions are too strong, relative to the solute solvent interactions Sheets Page 4 Lecture 30

5 Dynamic equilibrium dynamic equilibrium: no change, BUT change is occurring on a molecular level; this equilibrium can be a physical change (e.g., vapor pressure or solvation) or a chemical change (more in Chap 15) forward rate = backward rate solute + solvent dissolve crystallize solution NOTE: vs. Sheets Page 5 Lecture 30

6 Solubility solubility: amount of a substance that can be dissolved in solvent under given conditions (e.g., T) saturated solution: solution that contains the concentration of solute under the given conditions; a solution in dynamic equilibrium with undissolved solute; additional solute will NOT dissolve if added to the solution unsaturated solution: solution that contains than maximum concentration of solute; more solute can dissolve if added to the solution supersaturated solution: solution that contains than maximum concentration of solute; more than the equilibrium amount; solution is unstable (that is, NOT at equilibrium) Sheets Page 6 Lecture 30

7 Solubility generalization in general: like dissolves like polar solvents dissolve & solutes nonpolar solvents dissolve solutes consider IM forces that are broken & formed; if they are the ~same, then dissolution is likely solubility in water alcohol at 25 C (g/100g of H 2 O) CH 3 OH CH 3 CH 2 OH CH 3 CH 2 CH 2 OH CH 3 CH 2 CH 2 CH 2 OH 8.06 CH 3 CH 2 CH 2 CH 2 CH 2 OH 2.82 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 OH 0.62 as length of hydrocarbon chain, solubility of alcohol in water ; why is this the case??? Sheets Page 7 Lecture 30

8 To (phase) separate or not to (phase) separate, that is the question a mol solute/100 g H 2 O at 20 C miscible: 2 substances dissolve in all proportions immiscible: 2 substances that do NOT dissolve in each other to a significant extent examples NaCl in water? KCl in hexane? sugar (polar) in water? Ni metal in water? NH 3 in water or hexane? in hexane; in water Sheets Page 8 Lecture 30

9 Example: Which one of the following will be most soluble in benzene [C 6 H 6 (l)]? A. H 2 O(l) B. CH 3 OH(l) C. HCl(l) D. CH 3 CH 2 OH(l) E. heptane (C 7 H 16 ) (l) Sheets Page 9 Lecture 30

10 Factors that affect solubility other factors: T, P temperature can either increase or decrease solubility, depending on ΔH soln for ionic solids, solubility for gases, solubility Henryʼs law: relationship between solubility of gases with its partial pressure; solids & liquids not affected; why pop (umm, I mean soda) fizzes C g = K H P g C g [C g on data sheet; S g in BLB!!] = solubility of gas in solution phase K H = Henryʼs law constant; different for each solute/solvent pair; varies with temperature P g = partial pressure of gas (the solute) over solution Sheets Page 10 Lecture 30

11 Le Chatelierʼs principle a dynamic equilibrium tends to oppose any change in conditions (more about this in Chap 15!) so how do T & P changes affect solubility, anyway? 1. pressure: as P increases, the system tries to reduce P, so more gas dissolves 2. temperature: as T increases, the system tries to reduce T, using the solution process to take up heat if ΔH soln < 0 (heat released, exothermic), e.g., solute comes out of solution (gases) if ΔH soln > 0 (heat taken up, endothermic), solute dissolves more (ionic solids)* *ΔH soln > 0 at saturation for most ionic solids Sheets Page 11 Lecture 30

12 More examples To increase the solubility of O 2 in water, need to do which of the following A. increase T B. decrease T C. increase P O2 D. decrease P O2 How will the solubility of KClO 3 be affected by an increase in pressure? Sheets Page 12 Lecture 30

13 Before next class: Read: BLB HW: BLB 13:9,58,61,67,69,75 Sup13:12 18 Know: colligative properties vapor pressure lowering boiling point elevation freezing point depression osmotic pressure colloids Answers: p. 9: E p. 12: top, B,C; bottom, KClO 3 solubility is not affected by pressure Sheets Page 13 Lecture 30

Lecture 31: Solutions 5

Lecture 31: Solutions 5 Read: BLB 13.5 13.6 HW: BLB 13:9,58,61,67,69,75 Sup13:12 18 Know: colligative properties vapor pressure lowering boiling point elevation freezing point depression osmotic pressure