Properties of Solutions

Properties of Solutions
Chapter 11 Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved.
Overview Introduce student to solution composition and energy of solution formation. Factor affecting solubilities will be discussed: structure, pressure and temperature effects. Vapor pressure of solutions, boiling point, freezing point affected by solute addition. Colligative properties of electrolyte solutions and colloids. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A solution is a homogenous mixture of 2 or more substances The solute is(are) the substance(s) present in the smaller amount(s) The solvent is the substance present in the larger amount Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature. An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature. A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature. Sodium acetate crystals rapidly form when a seed crystal is added to a supersaturated solution of sodium acetate. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Concentration Units The concentration of a solution is the amount of solute present in a given quantity of solvent or solution. Percent by Mass x 100% mass of solute mass of solute + mass of solvent % by mass = = x 100% mass of solute mass of solution Mole Fraction (X) XA = moles of A sum of moles of all components Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Concentration Units Continued
Molarity (M) M = moles of solute liters of solution Molality (m) m = moles of solute mass of solvent (kg) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Normality (N) mass N = Number of equivalent liters of solution 1 = x Equivalent mass V Number of equivalent Acid-Base reaction: is the amount needed to accept one mole of H+ MM Equivalent mass of H2SO4 = 2 MM Equivalent mass of Ca(OH)2 = 2 Copyright©2000 by Houghton Mifflin Company. All rights reserved.

What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is g/mL? m = moles of solute mass of solvent (kg) M = moles of solute liters of solution Assume 1 L of solution: 5.86 moles ethanol = 270 g ethanol 927 g of solution (1000 mL x g/mL) mass of solvent = mass of solution – mass of solute = 927 g – 270 g = 657 g = kg m = moles of solute mass of solvent (kg) = 5.86 moles C2H5OH 0.657 kg solvent = 8.92 m Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Steps in Solution Formation
Step 1 - Expanding the solute (endothermic) Step 2 - Expanding the solvent (endothermic) Step 3 - Allowing the solute and solvent to interact to form a solution (exothermic) Hsoln = Hstep 1 + Hstep 2 + Hstep 3 Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The formation of a liquid solution can be divided into three steps: (1) expanding the solute, (2) expanding the solvent, and (3) combining the expanded solute and solvent to form the solution. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Factor Affecting Solubility
Structure Pressure Temperature Copyright©2000 by Houghton Mifflin Company. All rights reserved.

“like dissolves like” Two substances with similar intermolecular forces are likely to be soluble in each other. non-polar molecules are soluble in non-polar solvents CCl4 in C6H6 polar molecules are soluble in polar solvents C2H5OH in H2O ionic compounds are more soluble in polar solvents NaCl in H2O or NH3 (l) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The molecular structures of (a) vitamin A (nonpolar, fat-soluble) and (b) vitamin C (polar, water-soluble). The circles in the structural formulas indicate polar bonds. Note that vitamin C contains far more polar bonds than vitamin A. Water Soluble Fat Soluble Hydrophobic Hydrophilic Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) A gaseous solute in equilibrium with a solution. (b) The piston is pushed in, increasing the pressure of the gas and number of gas molecules per unit volume. This causes an increase in the rate at which the gas enters the solution, so the concentration of dissolved gas increases. (c) The greater gas concentration in the solution causes an increase in the rate of escape. A new equilibrium is reached. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Henry’s Law The amount of a gas dissolved in a solution is directly proportional to the pressure of the gas above the solution. P = kC P = partial pressure of gaseous solute above the solution C = concentration of dissolved gas k = a constant Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Henry’s Law Applied for
Dilute solutions Gases that do not dissociate or react with solvent: O2/water Applied HCl/water Is not applied Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Temperature Effect on Gases
The solubilities of several gases in water as a function of temperature at a constant pressure of 1 atm of gas above the solution. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Temperature Effect on Gases
The solubilities of several solids as a function of temperature. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The Vapor Pressure of Solutions
Solutions have different physical properties from pure solvent. Solutions of nonvolatile solutes differ from solutions of volatile solvents. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

An aqueous solution and pure water in a closed environment. (a) Initial stage. (b) After a period of time, the water is transferred to the solution. Vapor pressure of pure water is higher VP of solution is lower Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Raoult’s Law The presence of a nonvolatile solute lowers the vapor pressure of a solvent. Psoln = solvent Psolvent Psoln = vapor pressure of the solution solvent = mole fraction of the solvent Psolvent = vapor pressure of the pure solvent solvent = Moles of solvents Moles of solvent + Moles of solute Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Note Solutions that obey Raoult’s Law are called Ideal Solutions. Can be used to determine the molar mass of unknown. For ionic compounds you should multiply by the total number of ions per molecule e.g. Na2SO4 n = 3 x Na2SO4 Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) ideal liquid-liquid solution by Raoult's law. (b) This solution shows a positive deviation from Raoult's law. (c) This solution shows a negative deviation from Raoult's law. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

< & > & PT is greater than predicted by Raoults’s law
PT is less than predicted by Raoults’s law Force A-B A-A B-B < & Force A-B A-A B-B > & Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Colligative Properties of Non-Electrolyte Solutions
Depend only on the number, not on the identity, of the solute particles in an ideal solution. Boiling point elevation Freezing point depression Osmotic pressure Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Boiling Point Elevation
A nonvolatile solute elevates the boiling point of the solvent. T = Kbmsolute Kb = molal boiling point elevation constant m = molality of the solute Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Boiling-Point Elevation DTb = Tb – T b T b is the boiling point of the pure solvent T b is the boiling point of the solution Tb > T b DTb > 0 DTb = Kb m m is the molality of the solution Kb is the molal boiling-point elevation constant (0C/m) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Freezing Point Depression
A nonvolatile solute depresses the freezing point of the solvent. T = Kfmsolute Kf = molal freezing point depression constant m = molality of the solute Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Freezing-Point Depression DTf = T f – Tf T f is the freezing point of the pure solvent T f is the freezing point of the solution T f > Tf DTf > 0 DTf = Kf m m is the molality of the solution Kf is the molal freezing-point depression constant (0C/m) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Osmotic Pressure Osmosis: The flow of solvent into the solution through the semipermeable membrane. Osmotic Pressure: The excess hydrostatic pressure on the solution compared to the pure solvent. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Osmotic Pressure (p) Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one. A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules. Osmotic pressure (p) is the pressure required to stop osmosis. p = MRT M is the molarity of the solution R is the gas constant T is the temperature (in K) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The normal flow of solvent into the solution (osmosis) can be prevented by applying an external pressure to the solution. The minimum pressure required to stop the osmosis is equal to the osmotic pressure of the solution. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

(a) A pure solvent and its solution (containing a nonvolatile solute) are separated by a semipermeable membrane through which solvent molecules (blue) can pass but solute molecules (green) cannot. The rate of solvent transfer is greater from solvent to solution than from solution to solvent. (b) The system at equilibrium, where the rate of solvent transfer is the same in both directions. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Colligative Properties of Electrolyte Solutions
van’t Hoff factor, “i”, relates to the number of ions per formula unit. NaCl = 2, K2SO4 = 3 (Theoretically) T = imK  = iMRT Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Colligative Properties of Electrolyte Solutions
Boiling-Point Elevation DTb = i Kb m Freezing-Point Depression DTf = i Kf m Osmotic Pressure (p) p = iMRT Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Colloids Colloidal Dispersion (colloid): A suspension of tiny particles in some medium. aerosols, foams, emulsions, sols Coagulation: The addition of an electrolyte, causing destruction of a colloid. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A representation of two colloidal particles.
The opposing charge repel and prevent precipitation The destruction of colloids is called coagulation, achieved by Increasing temperature where colliding at higher veloicities to penetrate the ion barriers and aggregate. Adding electrolytes to neutralize ion layers, this is why clay deposits where rivers reach the oceans. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

A colloid is a dispersion of particles of one substance throughout a dispersing medium of another substance. Colloid versus solution colloidal particles are much larger than solute molecules colloidal suspension is not as homogeneous as a solution Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The Cottrell precipitator installed in a smokestack. The charged plates attract the colloidal particles because of their ion layers and thus remove them from the smoke. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Properties of Solutions

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