Chapter 10: CHM 2045 (Dr. Capps)

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Phase Diagram Phase diagrams for CO 2 and H 2 O Chapter 13. Solutions and Their Physical Properties Shows pressures and temperatures at which gaseous, liquid, and solid phases can exist.

1 Phase Diagram Phase diagrams for CO 2 and H 2 O Chapter 13. Solutions and Their Physical Properties Shows pressures and temperatures at which gaseous, liquid, and solid phases can exist. Allows us to predict phase of substance stable at any given T and P Plus melting, boiling pt, etc. Triple point: all 3 phases are in equilibrium What phase is water at 600 torr and 10 C? At 600 torr and 50 C? CO 2 solid gas H 2 O solid liquid gas Solutions Solution Concentrations Solution: homogeneous mixture of a solute in a solvent Pure Substance Constant Composition Homogeneous Matter Solute: dissolved substance - Often changes phase (if different from the solvent) - is present in lesser amount (if the same phase as the solvent) Mixture Variable Composition Concentration: amount of solute present in a specified amount of solution or solvent Expressed as molarity (M) molality (m) mole fraction (x) Solvent: substance the solute dissolves in retains its phase (if different from the solute) 1

2 Molality (m) Molality (m) = kilograms of solvent Molality is useful for colligative properties : What is the molality of a solution that contains g NH 3 (MM = g mol -1 ) dissolved in 125 g of water? Mole Fraction (x) mole fraction = number of moles of individual component divided by total number of moles Mole fraction (x A ) = molesa total moles in solution : What is the mole fraction of CH 3 OH in a solution that contains 42.0 g CH 3 OH (MW = 32.1 g mol -1 ) 35.0 g C 2 H 5 OH (MW = 46.1 g mol -1 ) 50.0 g C 3 H 7 OH (MW = 60.1 g mol -1 ) 1. Molarity (M) = 2. Molality (m) = Solution Composition 3. Mole fraction (x A ) = liters of solution kilograms of solvent molesa total moles in solution Colligative Properties Colligative Properties Vapour Pressure Colligative Properties: Physical properties of solutions that depend primarily on the number of particles present and not on their nature, e.g. 1) vapour pressure lowering 2) boiling point elevation 3) freezing point depression 4) osmosis Independent of nature (kind, size, or charge) and depend only on concentration of solute particles. 1 mol sugar, C 12 H 22 O 11 1 mol C 2 H 4 (OH) 2 1molNO 2 1 mol of ions each dissolved in 1 kg H 2 O freeze at C. Freezing points are the same despite chemical differences in solutes (same concentration of total solute particles). A liquid in a closed container will establish an equilibrium with its vapour Pressure exerted by vapour in equilibrium is vapour pressure Substance with a VP is volatile. Substance with no measurable VP is nonvolatile. 2

3 1. Vapour Pressure and Raoult s Law (1887) 2. Boiling Point Elevation Nonvolatile solute lowers vapour pressure of solvent. P soln = vapour pressure of the solution x solvent = mole fraction of the solvent P solvent = vapour pressure of the pure solvent Vapour-pressure lowering depends only on mole fraction of solute particles and not on their identity. A solution contains 5.00 g of urea [CO(NH 2 ) 2, MW = 60.1 g mol -1 ], a nonvolatile solute, per kg of water. If the vapour pressure of pure water at 25 C is 23.7 Torr, what is the vapour pressure of the solution? As we heat a liquid, its VP increases. Boils when VP = ext P Nonvolatile solute added: VP lowered Higher temp needed to increase VP to point where solution boils. 1 mol of nonvolatile non-electrolyte (sugar or urea) dissolved in 1 kg of water raises BP by C. Boiling Point Elevation Boiling Point Elevation Nonvolatile solute elevates boiling point of solvent. ΔT = K b m solute ΔT = change in temperature, i.e. BP solution BP solvent K b = molal boiling point elevation constant m = molality of the solute What is the boiling point of a 1.93 m solution of a nonvolatile solute in nitrobenzene? K b = 5.24 C/m, BP = C See Table 13.2 (p. 552) for K b, BP, K f and MP for several solvents. Be careful when comparing molecular and ionic compounds! Extent to which vapour pressure of solvent is lowered and boiling point elevated depends on number of solute particles present in a given amount of solvent. 1 mole of NaCl causes nearly twice as great a rise in boiling point as 1 mole of sugar (C 12 H 22 O 11 ) and CaCl 2 causes a change about three times as great. Copyright Houghton Mifflin Company.All rights reserved

4 Determination of Molecular Mass 3. Freezing Point Depression Freezing Point Depression A solution of 35.7 g of a non-electrolyte in g of chloroform has a boiling point of 64.5 C. Whatisthemolarmassofthis the mass of this compound? K b = 3.63 C/m, BP = 61.7 C Solutions freeze at lower temperatures than pure liquids. 1 mol sugar, C 12H 22O 11 1 mol C 2 H 4 (OH) 2 1 mol N 2 O 1 mol of ions each dissolved in 1 kg H 2 O freeze at C. We can use salt, calcium chloride, or urea to melt ice because solutions of these substances freeze at lower temperatures than pure water. Antifreezes such as ethylene glycol are used in automobile radiators to reduce the freezing point of water in the winter (and raise its boiling point in summer). also used to de-ice planes Freezing Point Depression Osmosis Nonvolatile solute depresses freezing point of solvent. ΔT = K f m solute ΔT = change in temperature, i.e. FP solvent FP solution K f = molal freezing point depression constant m = molality of the solute Calculate the freezing point of a solution of g of CaCl 2 (MM = g mol -1 ) in 175 g of water. See Table 13.2 (p. 552) for K b, BP, K f and MP for several solvents Osmosis: tendency of solvent molecules to pass through a semipermeable membrane from a more dilute to a more concentrated solution Osmotic pressure: The excess hydrostatic pressure on the solution compared to pure solvent. Important biologically Copyright Houghton Mifflin Company.All rights reserved Copyright Houghton Mifflin Company.All rights reserved

5 Reverse Osmosis Osmotic Pressure Determination of molar masses Reverse Osmosis: If a pressure greater than the osmotic pressure is exerted on the solution, the solvent passes back through the membrane to the dilute side. This is important in the desalination of seawater. When a solution containing n in a volume V m 3 is in contact with the pure solvent at a temperature T K, ΠV = nrt where Π is the osmotic pressure in Pa and R is the gas constant ( J K -1 mol -1 ). Measurements of osmotic pressure are used to determine hgh molar masses (e.g. for proteins, polymers, etc.). A 1 mol dm -3 solution of sucrose causes ΔT b = 0.5 K, ΔT f = 1.9 K but exerts an osmotic pressure sufficient to sustain a column of water 250 m high. Copyright Houghton Mifflin Company.All rights reserved Copyright Houghton Mifflin Company.All rights reserved Copyright Houghton Mifflin Company.All rights reserved Colligative Properties of Non-electrolytes Phase Diagram for an Aqueous Solution of a Non-electrolyte Freezing point for solution is lower than pure water ΔT = K b m solute ΔT = K f m solute ΠV = nrt (or Π = crt) Higher BP for solution is shown by displacement of line separating liquid and gas states to the right Decrease in vapour pressure of solution is indicated by vertical distance between two lines 1 m non-electrolyte (aq) vs. pure water 5

PHASE CHEMISTRY AND COLLIGATIVE PROPERTIES

PHASE CHEMISTRY AND COLLIGATIVE PROPERTIES Phase Diagrams Solutions Solution Concentrations Colligative Properties Brown et al., Chapter 10, 385 394, Chapter 11, 423-437 CHEM120 Lecture Series Two : 2013/01