Week 10: Lectures 28 30 Lecture 28: W 10/26 Lecture 29: F 10/28 Lecture 30: M 10/31 (Halloween Demo Show) Reading: BLB Ch 4.1, 4.5, 13.1 13.4 Homework: BLB 4: 3, 37, 72; Supp 4: 1 5; BLB 13: 7, 21, 23; Supp 13: 1-12 Reminder: Angel Quiz 9 due on Thur 10/27 ALEKS Objective 10 due on Tue 11/1 Jensen Office Hour: 501 Chemistry Building Tuesdays & Thursdays, 10:30 11:30 am Exam 3: Mon, Nov. 7, 6:30 7:45 pm Solutions Solutions: Homogeneous mixtures of 2 or more substances; uniformly mixed on a molecular level Solute: what dissolves (a change in state [or phase]); typically the minor component Solvent: what the solute is dissolved in (unchanged state); the major component Aqueous Solution: solvent is water Critical aspects with regard to solutions how does the solute exist in the solution free ions? = electrolyte (weak or strong) no ions? = nonelectrolyte what goes into the solution concentration solubility (BLB Table 4.1) Jensen Chem 110 Chap 13 Page: 1 Jensen Chem 110 Chap 13 Page: 2
Solute: what type of compound? Ionic compounds: compounds consisting of positive and negative ions Metal + nonmetal (usually) e.g. NaCl Mg(NO 3 ) 2 Na 2 SO 4 NH 4 Cl Molecular compounds: compounds consisting of individual molecules All nonmetals or nonmetals and metalloids e.g. H 2 O HCl CH 3 COOH The Solution Process Solutions form when the solute solvent IM forces are comparable in magnitude and nature to the combined solute solute and solvent solvent IM forces 1. Break solute solute interactions (!H 1 ) 2. Break solvent solvent interactions (!H 2 ) " Energy is 3. Form solute solvent interactions (!H 3 ) " Energy is Processes occur spontaneously when:! energy is released (exothermic)! disorder increases NH 3 Jensen Chem 110 Chap 13 Page: 3 Jensen Chem 110 Chap 13 Page: 4
Enthalpy Changes for the Solution Process Dissolution of NaCl in water The enthalpy change of the overall process depends on!h for each of these steps Left: Net exothermic process (!Hsoln<0) Example: NaCl Ion-dipole attractions between the ions and water molecules are sufficiently strong to pull the ions from their positions in the crystal. NaCl(s) + H2O # Na+(aq) + Cl (aq) Right: Net endothermic process (!Hsoln>0) Example: NH4NO3 A process can be endothermic and spontaneous when the increase in disorder is large enough. Jensen Chem 110 Chap 13 Page: 5 Jensen Chem 110 Chap 13 Page: 6
Solution Process: Dynamic equilibrium Solute + Solvent dissolve crystallize forward rate = backward rate Solubility: maximum amount of a substance that can be dissolved in solvent under given conditions Three different types of solutions: Solution Unsaturated solution contains less than maximum concentration of solute; more solute can dissolve if added to the solution Saturated solution contains maximum concentration of solute under the given conditions; In dynamic equilibrium with undissolved solute; additional solute will NOT dissolve if added to the solution Supersaturated solution contains more than maximum concentration of solute; Solution is unstable (that is NOT at equilibrium) Factors that affect solubility! Type of intermolecular interactions and their strengths (LIKE DISSOLVES LIKE)! Temperature " Effect on solubility of ionic solids " Effect on solubility of gases! Pressure I. Effect of IM Forces (Like dissolves like) Polar solvents dissolve polar & ionic solutes Nonpolar solvents dissolve nonpolar solutes Molecule solubility in water at 25 C (g/100g of H 2 O) CH 3 OH total CH 3 CH 2 OH total CH 3 CH 2 CH 2 OH total 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 Jensen Chem 110 Chap 13 Page: 7 Jensen Chem 110 Chap 13 Page: 8
Demo Solutes: I 2 CuSO 4 IM forces LDF ion-ion Solvents: pentane (C 5 H 12 ) LDF water LDF, dipole-dipole, H-bonding Predictions? Will I 2 dissolve in pentane? Will I 2 dissolve in water? Will CuSO 4 dissolve in pentane? Will CuSO 4 dissolve in water? Will pentane and water mix? Practice Examples: 1. 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) 2. Which one of the following substances is most likely to dissolve in water? A. HOCH 2 CH 2 OH B. CHCl 3 C. CH 3 (CH 2 ) 9 OCH 3 D. CH 3 (CH 2 ) 8 CH 2 OH E. CCl 4 Jensen Chem 110 Chap 13 Page: 9 Jensen Chem 110 Chap 13 Page: 10
II. Effect of Temperature on Solubility For ionic solids, solubility usually increases as temperature increase For gases, solubility decreases as temperature increases Example: A 40 g sample of potassium nitrate is dissolved in 100 g of water at 50 C, with precautions taken to avoid water evaporation. The solution is cooled to 20 C and no precipitate is observed. This solution is. A. Unsaturated B. Saturated C. Supersaturated III. Effect of Pressure on Solubility Pressure does not have any effect on the solubility of solid or liquid; However, the solubility of gas (C g, note: BLB uses S g ) is proportional to the partial pressure of that gas (P g ) above the solution. Henry s Law: C g = k P g Jensen Chem 110 Chap 13 Page: 11 Jensen Chem 110 Chap 13 Page: 12
Example: What is solubility for He in water at 30 C, if its partial pressure above the solution is 350 torr? The solubility constant for He in water at this temperature is 3.7 x 10-4 M/atm. Demo pure H 2 O Which Bulbs Light Up? Bulb Wattage 7.5 25 40 NaCl(aq) Practice Example: Which of the following changes increase the solubility of N 2 gas in water? 1. increasing the temperature 2. decreasing the temperature 3. increasing the partial pressure of the solute 4. decreasing the partial pressure of the solute 1M HCl(aq) 1M CH 3 COOH sugar(aq) CH 3 OH A. 2 only B. 4 only C. 1 and 3 D. 2 and 4 E. 2 and 3 Jensen Chem 110 Chap 13 Page: 13 Jensen Chem 110 Chap 13 Page: 14
Electrolytes: A substance that conducts electricity (dissociates into ions) when dissolved in water Strong electrolytes: good conductor completely ionized (in solution) NaCl (s) + H 2 O # Na + (aq) + Cl (aq) + H 2 O HCl (aq) + H 2 O # H 3 O + (aq) + Cl (aq) Weak electrolytes poor conductor partially ionized (in solution) NH 3 (aq) + H 2 O! NH 4 + (aq) + OH (aq) HF (aq)+ H 2 O! H 3 O + (aq) + F (aq) Identifying an Electrolyte A) Is the compound ionic? Yes: All ionic compounds are strong electrolytes Note: it could be insoluble B) If the compound is molecular, is it an acid (donate H + ) or a base (accept H + ) in solution? No: it is a non electrolyte c) If a compound is an acid or a base, is it one of the seven strong acids or eight strong bases? Yes: it is a strong electrolyte No: it MUST be a weak electrolyte Non-electrolytes non-conductors not ionized (in solution) C 6 H 12 O 6 (s) + H 2 O # C 6 H 12 O 6 (aq)+ H 2 O (glucose) Note: You MUST memorize these strong acids and bases; all are strong electrolytes! Jensen Chem 110 Chap 13 Page: 15 Jensen Chem 110 Chap 13 Page: 16
Summary of Electrolytes Strong electrolytes ionic compounds, strong acids, and strong bases that ionize completely in solution. Weak electrolytes weak acids and weak bases that are partially ionized in solution. Nonelectrolytes molecular compounds that are not acids or bases and do not ionize in solution Examples of Organic Non-electrolytes: Alcohols Aldehydes ketones Carbohydrates (sugar) Examples of Organic Weak electrolytes: Weak Acids (including Carboxylic Acids) Weak Bases (including Amines) These are all water soluble compounds. Are they strong, weak or non-electrolytes? A. HNO 3 B. CuCl 2 C. CH 3 OH D. CH 3 COOH (acetic acid) E. NH 3 F. Ca(OH) 2 G. (NH 4 ) 2 SO 4 I. Glucose (C 6 H 12 O 6 ) Jensen Chem 110 Chap 13 Page: 17 Jensen Chem 110 Chap 13 Page: 18
Concentration Expressions Molarity (M = ) mass percentage = mole fraction = X i = parts per million (ppm): molarity (M) = Example: What is the molarity of a solution that contains 2.76 g NH 4 NO 3 in 400 ml of solution? A. 11.6 M B. 0.0862 M C. 8.62 x 10-5 M D. 1.16 x 10 +4 M E. 8.62 x 10-3 M molality (m) = Jensen Chem 110 Chap 13 Page: 19 Jensen Chem 110 Chap 13 Page: 20
Molality (m = ) Example: What is the molality of a solution prepared by dissolving 75.6 g of C 6 H 8 O 6, in 150 g of water? A. 0.286 m B. 28.6 m C. 1.90 m D. 2.86 m E. 1.43 m Conversion Between Concentration Units Example: What is the molality of an aqueous solution that is 27.0% H 2 SO 4 by mass and has a density of 1.198 g/ml? A. 3.77 m B. 0.849 m C. 3.30 m D. 32.3 m E. 0.275 m Jensen Chem 110 Chap 13 Page: 21 Jensen Chem 110 Chap 13 Page: 22
Practice Example: What is the molarity of an aqueous solution that is 13.0% NaCl by mass and has a density of 1.10 g/ml? A. 143 M B. 2.45 M C. 2.56 M D. 2.23 M E. 1.43 x 10-2 M Concentration of Strong Electrolytes Example: Which solution contains the largest concentration of chloride ions? A. 100 ml of 1.0 M BaCl 2 B. 40 ml of 1.0 M NaCl C. 60 ml of 1.0 M FeCl 3 Which solution contains the largest moles of total ions? Jensen Chem 110 Chap 13 Page: 23 Jensen Chem 110 Chap 13 Page: 24
Concentration and Dilution Dilution: making a weaker solution by adding solvent Before and after dilution: mass (moles) of solute remains the same moles of solute = M V M = concentration (molarity) of solution V = volume of solution (L) SO... Example: A solution of sodium sulfide is prepared by diluting 50.0 ml of 0.874 M sodium sulfide solution to a total volume of 250.0 ml. What is the concentration of the final solution? A. 0.0875 M B. 4.37 M C. 0.525 M D. 0.350 M E. 0.175 M M i * V i = mol solute = M f * V f Concentration Dilution Problem Solving Steps: 1. Organize information 2. Draw pictures if applicable 3. Solve for unknows 4. Manipulate info to get required answer Jensen Chem 110 Chap 13 Page: 25 Jensen Chem 110 Chap 13 Page: 26
Practice Example: A stock solution is prepared by adding 25 ml of 1.3 M NaNO 3 to enough water to make 47 ml. How many moles of NaNO 3 are present in 35 ml of the stock solution? A. 0.024 mol B. 0.032 mol C. 0.044 mol D. 0.048 mol E. 0.061 mol Practice Example: When 30 ml of an 80 ml stock solution containing 1.1 M AlCl 3 is diluted to 51 ml, what is the concentration of Cl - in the new solution? A. 1.1 M B. 3.3 M C. 0.65 M D. 1.72 M E. 1.94 M Jensen Chem 110 Chap 13 Page: 27 Jensen Chem 110 Chap 13 Page: 28