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Chapter 11 – Colloids – Larger Particles but Still Uniform (Milk, Fog)

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Chapter 11 – Colloids – Larger Particles but Still Uniform (Milk, Fog) Solutions • Homogeneous mixtures: – Solutions – ions or molecules (small particles) Chapter 11 – Colloids – larger particles but still uniform (milk, fog) Solutions and Their Properties 1 The Solution Process The Solution Process • During dissolution, some forces are • Solvent-solvent interactions broken and new forces are created • Solute-solute interactions • Solvent-solute interactions ∆∆∆ ∆∆∆ ∆∆∆ ∆∆∆ Hsoln = H1 + H2 + H3 Figure 12.2 3 4 The Solution Process Hot and Cold Packs • Generally, • Hot packs – dissolving solid releases • if the solute is more strongly attracted heat ∆∆∆ to the solvent attraction (than to itself – – CaCl 2 (s) Hsoln = -81.3 kJ/mol ∆∆∆ weak IMF), then dissolution is – MgSO 4 (s) Hsoln = -91.2 kJ/mol ∆∆∆ favorable; exothermic ( Hsoln < 0) • Cold packs – dissolving solid absorbs • if the solute is more strongly attracted heat ∆∆∆ to itself (than to the solvent – strong – NH 4NO 3 (s) Hsoln = +25.7 kJ/mol IMF), then dissolution is not favorable; ∆∆∆ endothermic ( Hsoln > 0) Ways of Expressing Concentration Concentration Units • Variety of units • Molarity – Most commonly used is M (molarity) – Molarity = moles solute / liter solution = mol/L – Also ppm, mole fraction, molality, and Normality – Depends on temperature; density of liquids o changes with temperature (d H2O at 20 C = 3 • Qualitative terms relating to solubility 0.9982 g/cm ) – insoluble, slightly soluble, soluble, very soluble – Molarity: – <0.1 g/100g >2 g/100 g – Ex: 5.0 g NaCl in water that gives a volume of 251mL • Other comparative terms: – Ans: 0.34 M NaCl – dilute, concentrated (solids in liquids) – miscible, immiscible, partially miscible (liquids in • Mole fraction (X): liquids) – Mole Fraction = mole A / (mole A + mole B) – Ex: 5.0 g NaCl in 251 mL water – Ans: 0.0061 NaCl; 0.994 water 7 8 Concentration Units Types of Solutions • Mass Percent • Unsaturated solution : contains less solute than it – Mass Percent = (mass of solute / mass of solution) has the capacity to dissolve x 100% – Ex: 5.0 g of NaCl in 251 mL water • Saturated solution : contains the maximum amount – Ans: 2.0 % NaCl of solute that will dissolve in a given solvent at a specific temperature • Molality – molality = moles solute / kilograms solvent = • Supersaturated solution : contains more solute than mol/kg is present in a saturated solution – Independent of temperature because masses do not change with temperature • Crystallization : process in which a dissolved – Ex: 5.0 g NaCl in 251 mL water solute comes out of solution and forms crystals – Ans: 0.34 m NaCl Supersaturated Solution Supersaturated Solution Acetate Solid Solubility and Temperature Effect of Temperature on Solubility • Solubility of ionic compounds in water generally • The solubility of gases in water decreases with increases with higher water temps increasing temperature. Why? • Solubility goes to 0 at boiling point of water. Figure 11.6 14 Effect of Pressure on Solubility Effect of Pressure on Solubility • Strong effect only for gases dissolved in • Henry’s Law : concentration (solubility) of a gas in a liquids. liquid is proportional to the pressure of the gas over the solution • What happens to the amount of dissolved • Why does a soft drink fizz when the nd solute when pressure is increased (2 container is opened? picture)? Figure 11.8 More dilute solution More concentrated solution 15 16 Molarity to molality Colligative Properties Remember the distinction between molarity (M) and molality (m) • Colligative properties: depend only on the number of solute particles in solution and not on the nature of the solute particles. The particles may be covalent Moles of solute Moles of solute molecules or ionic compounds. (We’ll only deal with M = L of solution m = kg of solvent covalent molecules.) To do these calculations, you must have one extra piece of information: density • Colligative means “depending on the collection” – Examples: vapor pressure What is the molality of a 0.123 M HCl( aq ) solution? boiling point The density of the solution is 1.030 g/mL. freezing point osmosis / osmotic pressure What is the molarity of a 3.21 m KOH( aq ) solution? (density of solution = 1.163 g/mL) 19 Vapor Pressure Lowering Vapor Pressure Lowering • Adding a solute to a solvent lowers the •Pvap is lowered because fewer surface positions are Pvap occupied by solvent molecules and because • P of a solution < P of a pure solvent intermolecular forces in solutions are usually greater vap vap than those in the separated substances. • Raoult’s Law : partial pressure of a solvent vapor over a solution is the product of the mole fraction of the solvent (X 1) and the vapor pressure of the solvent o (P 1). o –P1 = X 1P 1 20 B. P. Elevation/F. P. Depression B. P. Elevation/F. P. Depression • Phase diagrams can be used to see how • Lower vapor pressure (red lines) vapor pressure changes cause changes in boiling and freezing points. Freezing pure water Freezing solution Figure 11.12 22 Freezing Point Depression Boiling Point Elevation • Changes in freezing point are directly proportional to • Changes in boiling point are directly proportional to the the number of solute particles number of solute particles • Equations that govern these effects: • Equations that govern these effects: ∆ ∆ Tb = K b m Tf = K f m ∆ ∆ Tb is the change in normal boiling point Tf is the change in normal freezing point –Kb is based on the identity of the solvent –Kf is based on the identity of the solvent – m is molality = moles solute/kg solvent – m is molality = moles solute/kg solvent • Calculate the freezing point of a solution when 62.345 • Calculate the boiling point of a solution when 62.345 g of vinyl chloride (CH 2CHCl) is added to 1264 g g of vinyl chloride (CH 2CHCl) is added to 1264 g cyclohexane. cyclohexane. 25 B.P. Elevation/F.P. Depression Group Work • Calculate the molar mass of an unknown solute when the freezing point of water is lowered by 7.77°C after 651 g of the solute is dissolved in 2505 g of water. • Kb and K f depend only on the solvent being used Osmosis – pickling of cucumbers Osmotic Pressure • Osmosis : selective passage of solvent molecules through a porous membranePos anim from a dilute solution to a more concentrated one. – Semipermeable membrane : allows the passage of solvent molecules but blocks the passage of solute molecules. ΠΠΠ = MRT 30 Reverse Osmosis • Uses high pressure to force water from a more concentrated solution to a less concentrated one through a semipermeable membrane. The membrane allows water molecules to pass through but not dissolved ions..
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