The Solution Process CHEMISTRY. Properties of Solutions. The Central Science. Prof. Demi Levendis Room GH807 Gate House

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1 CHEMISTRY The Central Science Properties of Solutions The Solution Process Solutions: Air; brass; body fluids; sea water When a solution forms some questions we can ask are: What happens on a molecular level? Is energy absorbed or released? (ΔH) Why do some substances not dissolve? Prof. Demi Levendis Room GH807 Gate House 21 What factors affect dissolution? How do we quantify solubility? How are freezing and melting points affected? 31 Solutions can be gases, liquids or solids: NaCl (solute) dissolving in water (solvent): the H-bonds of water NaCl dissociates ion-dipole forces form the ions are solvated by water (hydrated). What about NaCl in benzene? Hydrated Na + and Cl - ions. 1

2 Energy Changes and Solution Formation Endothermic and exothermic processes Examples: NaOH + H 2 O, ΔH soln = kj/mol MgSO 4 + H 2 O ΔH soln = kj/mol (solution heats up) NH 4 NO 3 + H 2 O (solution cools down) ΔH soln = kj/mol. Solution Formation and Chemical Reactions physical vs. chemical processes. Ni(s) + 2HCl(aq) NiCl 2 (aq) + H 2 (g). When all the water is removed from the solution, no Ni is found only NiCl 2 6H 2 O (green). Therefore, Ni dissolution in HCl is a chemical process. Applications in heat or ice packs CRYSTAL GROWTH (GABA and GABAPENTIN) In a recent project by MSc students (Hayley Reece and Ahmed Shaikjee) ) six different crystalline forms of important pharmaceuticals GABA and gabapentin could be isolated from an ethanol solution (1:1 GABA/gabapentin gabapentin; vapour diffusion, hexane into EtOH) Solution Formation and Chemical Reactions Scanning Tunneling Microscopy: Hydrogen in Palladium GABA gabapentin Solution Formation and Chemical Reactions sodium chloride in water: NaCl(s) + H 2 O (l) Na + (aq) + Cl - (aq). When the water is removed from the solution, NaCl is found. Therefore, NaCl dissolution is a physical process. Dissolve Crystallize Saturation Solubility Supersaturated Saturated Solutions and Solubility Definitions to learn

3 Saturated solution: Supersaturated solution: (crystallization and dissolution are in equilibrium) Factors Affecting Solubility Miscible liquids: mix in any proportions. Immiscible liquids: do not mix. The number of carbon atoms in an alcohol chain can affect its solubility in water or n-hexane Polar liquids tend to dissolve in polar solvents. Nonpolar liquids ~insoluble in polar solvents. water and ethanol have similar hydrogen bonds (draw) The number of hydroxyl (-OH) groups within a molecule increases its solubility in water. (e.g. see vitamin A,C) fat soluble water soluble Generalization: like dissolves like

4 Pressure Effects Network solids do not dissolve (in non-polar liquids) because the strong intermolecular forces in the solid are not re-established in any solution. NaCl + benzene? 201 Solubility of a gas in a liquid is a function of the pressure of the gas. 211 Pressure Effects higher the pressure, more molecules of gas are close to the solvent greater the chance of a gas molecule striking the surface and entering the solution. Therefore, the higher the pressure, the greater the solubility. 221 C g = kp g Pressure Effects If Cg is the solubility of a gas, k is a constant, and Pg is the partial pressure of a gas, then Henry s Law gives: Cg Pg 231 Factors Affecting Solubility Pressure Effects Carbonated beverages are bottled with a p CO2 ~ 4 atm. As the bottle is opened, the partial pressure of CO 2 decreases and the solubility of CO 2 decreases. Therefore, bubbles of CO 2 escape from solution. Pressure Effects: Example Calculate the concentration of CO 2 in a carbonated beverage that was bottled with a partial pressure of CO 2 = 4.0 atm at 25 C; Henry s law constant for CO 2 in water (25 C) is 3.1x10-2 mol/l-atm. 241 Note, at atmospheric pressure (after opening) C g < 10-5 M ( M) since the partial pressure drops Note: Henry s law can be expressed in other ways

5 Pressure Effects: EXERCISE At 0 C and at an O 2 pressure of 1.00 atm, the aqueous solubility of O 2 (g) is 48.9 ml per liter. What is the molarity of O 2 in a saturated water solution when the O 2 is under its normal pressure in air, atm? 261 Temperature Effects Temperature Effects see the CRC Handbook for Physics and Chemistry for many other compounds where solubility (in water) decreases as temperature increases sodium selenate lithium carbonate ytterbium sulfate... Also: virtually any substance that's a gas at ordinary temperatures, including nitrogen, oxygen, hydrogen, helium, carbon dioxide, and ammonia. Gases are less soluble as temperature increases. Thermal pollution or global warming: if lakes get too warm, CO 2 and O 2 become less soluble and are not available for plants or animals. 291 Ways of Expressing Concentration methods: amount of solute per amount of solvent (or solution). amounts are masses, moles or liters. Qualitatively solutions are dilute or concentrated. Quantitatively we typically measure: percent composition (by mass); parts per million; molarity; molality 311 mole fraction 5

6 Definitions: percent composition (by mass); mass of component in solution mass % of component = 100 Example. 13.5g of glucose, C 6 H 12 O 6, in 0.100kg of water has a mass percentage (solute) of? mass of component in solution 6 ppm of component = 10 Parts per million (ppm) can be expressed as 1 mg of solute per kilogram of solution. (if the density of the solution is 1g/mL, then 1 ppm = 1 mg solute per liter of solution) Widely used in environmental reports, purity of water etc Example. A 2.5g sample of groundwater was found to contain 5.4 μg of Zn 2+. What is the concentration of Zn 2+ in ppm? EXERCISE what would the concentration of Zn 2+ have been (in ppm) if we had found 5.4 μg ZnSO 4 in 2.5ml H 2 O? (Or alternatively 2.2mg/L (since the density of the aqueous solution is approximately 1g/mL) (what would this concentration be if we had 5.4 μg ZnSO 4 in 2.5ml H 2 O?) Mole Fraction, Molarity, and Molality mass of component in solution 9 ppb of component = 10 Parts per billion (ppb) are 1 μg of solute per kilogram of solution. Used in environment: e.g. allowed limits in one water supply: 45ppm NO 3 - (mass can be converted to moles using the molar mass) moles of component in solution Mole fraction of component = total moles of solution Example: calculate the mole fraction of HCl (X HCl ) in a solution containing 36.5g of HCl and 144mL of water. Question: 0.5ppb CCl 4 Do concentrations expressed as ppm or ppb change with temperature?

7 Mole Fraction, Molarity, and Molality (mass can be converted to moles using the molar mass) Mole fraction of component = moles of component in solution total moles of solution Example: A 16.00% aqueous solution of glycerol, C 3 H 5 (OH) 3, by mass, has a density of g/ml. Calculate the mole fraction of C 3 H 5 (OH) 3 (X glycerol ) in this solution. Mole Fraction with more than two components The mole fraction of A, X A, in a solution consisting of A, B, C,... is calculated using the equation: To calculate the mole fraction of B, X B, use: Mole Fraction, Molarity, and Molality moles solute Molarity = liters of solution Example: calculate the molarity of HCl in a solution containing 36.5g of HCl in 144mL of water. Mole Fraction, Molarity, and Molality We define molality: Molality, m = moles solute kg of solvent Example: what is the molality, (and molarity), of a solution made by dissolving 4.35g glucose, C 6 H 12 O 6, in 25.0mL water? Question: Does molarity change with temperature? Example: what is the molality, (and molarity), of a solution made by dissolving 4.35g glucose, C 6 H 12 O 6, in 25.0mL water? Conversion of concentration units Example: A bleach solution contains 3.62 mass percent NaOCl in water. Calculate (a) the molality, (b) the mole fraction of NaOCl in the solution?

8 Conversion of concentration units EXERCISE: An ethanol-water solution is prepared by dissolving ml of ethanol, C 5 H 5 OH (d = g/ml), in a sufficient volume of water to produce ml of a solution with a density of g/ml. What is the concentration of ethanol in this solution, expressed as: (a) volume percent; (b) mass percent; (c) mass/volume percent; (d) mole fraction; (e) mole percent; (f) molarity; (g) molality? Conversion of concentration units Converting between molarity (M) and molality (m) requires the density of the solvent. Example: The density of a solution of 5.0g toluene and 225g benzene is 0.876g/mL. Calculate the molarity and molality of the solution and the mass percent of toluene in the solution Conversion of concentration units Example: The density of a solution of 5.0g toluene and 225g benzene is 0.876g/mL. Calculate the molarity and molality of the solution and the mass percent of toluene in the solution. 461 Temperature Dependence of Several Concentration Expressions concentration expression measurements required temperature dependent? percent composition mass of solute no (mass does not change with T) (by mass) mass of solution parts per million (ppm) mass of solute no (mass does not change with T) Or ppb mass of solution moles of solute yes molarity (volume changes with T) volume of solution molality moles of solute mass of solvent no (neither mass nor moles changes with T) mole fraction moles of solute no (moles does not change with T) moles of solvent example 471 Colligative Properties Summary mass of component in solution mass % of component = 100 Mass/volume % mass of component in solution 6 ppm of component = 10 Mole fraction of component = moles solute Molarity = liters of solution Molality, m = moles of component in solution total moles of solution moles solute kg of solvent 481 Colligative properties depend on quantity of solute molecules/particles but not on their identity. (e.g. freezing point depression and melting point elevation.) Colligative properties that we will consider are: 1. vapor pressure lowering, 2. boiling point elevation, 3. freezing point lowering, and 4. osmotic pressure