Chapter 12 Physical Properties of Solutions

Chapter 12 Physical Properties of Solutions
General Chemistry: An Integrated Approach Hill, Petrucci, 4th Edition Chapter 12 Physical Properties of Solutions Mark P. Heitz State University of New York at Brockport © 2005, Prentice Hall, Inc.

Some Types of Solutions
EOS Chapter 12: Physical Properties of Solutions

Solution Concentration
Molarity is the amount of solute dissolved into a volume of solvent – the ratio is moles solute to liters solution = mol L–1 Percent by mass is the mass ratio of solute to solution multiplied by 100 Percent by volume is the volume ratio of solute to solution multiplied by 100 EOS Mass/volume percent is the mass of solute divided by the volume of solution multiplied by 100 Chapter 12: Physical Properties of Solutions

Chapter 12: Physical Properties of Solutions
Solutions by “Parts” Parts per million is the number of particles of solute per one million particles of solution units are 1 ppm = 1 mg/L Parts per billion is the number of particles of solute per one billion particles of solution. 1 ppb = 1 mg/L EOS Parts per trillion is the number of particles of solute per one trillion particles of solution. 1 ppt = 1 ng/L Chapter 12: Physical Properties of Solutions

Molality Molality (m) is the number of moles of solute per one kilogram of solvent (not solution!) Molarity (M) varies with temperature due to the expansion or contraction in the volume of the solution EOS To be independent of temperature, a concentration unit must be based on mass only, not volume Chapter 12: Physical Properties of Solutions

Mole Fraction and Mole Percent
The mole fraction (xi) of a solution component i is the fraction of all the molecules in the solution that are molecules of i Sum of xi must equal 1 EOS The mole percent of a solution component is its mole fraction multiplied by 100 Chapter 12: Physical Properties of Solutions

Enthalpy of Solution Solution formation can be considered to take place in three steps Move the molecules of solvent apart to make room for the solute molecules. DH1 > 0 Separate the molecules of solute to the distances found between them in the solution. DH2 > 0 Allow the separated solute and solvent molecules to mix randomly. DH3 > 0 EOS DHsoln = DH1 + DH2 + DH3 Chapter 12: Physical Properties of Solutions

Visualizing Enthalpy of Solution

Intermolecular Forces in Solution Formation
If all intermolecular forces are of comparable strength, this type of solution is called an ideal solution and DHsoln = 0 If the intermolecular forces between solute and solvent molecules are stronger than other intermolecular forces, DHsoln < 0 EOS For solute/solvent forces that are weaker than other intermolecular forces, DHsoln > 0 Chapter 12: Physical Properties of Solutions

Non-ideal Solutions Ethanol and water have strong attractions – when mixed, the volume of 50 mL H2O + 50 mL EtOH is less than 100 mL EOS Chapter 12: Physical Properties of Solutions

Aqueous Solutions of Ionic Compounds
The forces causing an ionic solid to dissolve in water are ion–dipole forces, the attraction of water dipoles for cations and anions. EOS The attractions of water dipoles for ions pulls the ions out of the crystalline lattice and into aqueous solution Chapter 12: Physical Properties of Solutions

Some Solubility Terms Liquids that mix in all proportions are called miscible When there is a dynamic equilibrium between an undissolved solute and a solution, the solution is saturated EOS A solution which contains less solute than can be held at equilibrium is unsaturated Chapter 12: Physical Properties of Solutions

Solubility The concentration of the solute in a saturated solution is the solubility of the solute EOS About 95% of all ionic compounds have aqueous solubilities that increase with increasing temperature Chapter 12: Physical Properties of Solutions

Supersaturation A supersaturated solution is created when a warm, saturated solution is allowed to cool without the precipitation of the excess solute EOS Chapter 12: Physical Properties of Solutions

Selective Crystallization
When KNO3(s) is crystallized from an aqueous solution of KNO3 containing CuSO4 as an impurity, CuSO4 (blue) remains in the solution EOS Chapter 12: Physical Properties of Solutions

The Solubilities of Gases
Most gases become less soluble in liquids as the temperature increases a common example is carbonated beverages At a constant temperature, the solubility (S) of a gas is directly proportional to the pressure of the gas (Pgas) in equilibrium with the solution. S = k Pgas EOS The effect of pressure on the solubility of a gas is known as Henry’s law Chapter 12: Physical Properties of Solutions

Temperature and Solubility of Gases

Pressure and Solubility of Gases
Gas compressed into a smaller volume increases the number of molecules per unit volume EOS The concentration of the solution increases Chapter 12: Physical Properties of Solutions

Solubility and Gas Pressure

Vapor Pressures of Solutions
Raoult’s law states that the vapor pressure of the solvent above a solution (Psolv) is the product of the vapor pressure of the pure solvent (Posolv) and the mole fraction of the solvent in the solution (xsolv) Psolv = xsolv . Posolv EOS Chapter 12: Physical Properties of Solutions

Solution Separation The vapor in equilibrium with an ideal solution of two volatile components has a higher mole fraction of the more volatile component than is found in the liquid EOS Separation can be achieved by fractional distillation Chapter 12: Physical Properties of Solutions

Colligative Properties
Freezing Point Depression (FPD) Boiling Point Elevation (BPE) Consider situations with a volatile solvent and a solute that is nonvolatile, nonelectrolytic, and soluble in the liquid solvent, but not the solid solvent EOS The vapor pressure of the solution is that of the solvent in the solution, and at all temperatures this vapor pressure is lower than that of the pure solvent Chapter 12: Physical Properties of Solutions

FPD and BPE The presence of the solute lowers (depresses) the freezing point of the solvent (DTf) and increases (elevates) the boiling point of the solvent (DTb) EOS Example: adding salt to water allows the water temperature to exceed 100 oC, thereby cooking food faster Chapter 12: Physical Properties of Solutions

Vapor Pressure Lowering
In a solution, the solvent vapor pressure is lowered and the fusion curve is displaced to lower temperatures EOS Chapter 12: Physical Properties of Solutions

FPD and BPE Constants EOS Chapter 12: Physical Properties of Solutions

Osmosis Semipermeable membranes are usually films of a material containing a network of microscopic pores through which small solvent molecules can pass, but larger solute molecules cannot EOS Osmosis is the net flow of solvent molecules from pure solvent through a semipermeable membrane into a solution Chapter 12: Physical Properties of Solutions

Osmotic Pressure The pressure required to stop osmosis is called the osmotic pressure of the solution p = (n/V)RT = M RT A solution (green) is separated from pure water by a membrane permeable to H2O molecules but not to solute particles EOS When the flow of water is at equilibrium, the hydrostatic pressure is now called the osmotic pressure, p. Chapter 12: Physical Properties of Solutions

Osmosis Applications Water purification: Reverse osmosis is the process of reversing the normal net flow of solvent molecules through a semipermeable membrane by applying to the solution a pressure exceeding the osmotic pressure EOS Chapter 12: Physical Properties of Solutions

Solutions of Electrolytes
The van’t Hoff factor (i) is used to modify the equations for colligative properties FPD: DTf = –i Kfm BPE: DTb = i Kbm OP: p = i M RT EOS i is dependent on solution molality Chapter 12: Physical Properties of Solutions

Colloids A colloid is a dispersion in an appropriate medium for particles ranging in size from about 1 to 1000 nm EOS The scattering of a light beam through a colloidal material is known as the Tyndall effect Chapter 12: Physical Properties of Solutions

Common Colloids EOS Chapter 12: Physical Properties of Solutions

Formation and Coagulation of a Colloid
A high concentration of an electrolyte can cause a colloid to coagulate, or precipitate EOS Chapter 12: Physical Properties of Solutions

A Suspension and a Colloid

Summary of Concepts Molarity (M) is expressed as moles per liter and molality (m) is expressed as moles of solute per kilogram of solvent Units used for very dilute concentrations of solutes include parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt) EOS The type and magnitude of intermolecular forces are important in solution composition Chapter 12: Physical Properties of Solutions

Summary of Concepts The solubility of a solute is its concentration in a saturated solution The solubility of gases in solutions decreases with an increase in temperature but increases with an increase in pressure EOS The presence of solutes lowers the vapor pressure of the solution and causes both a freezing point depression and a boiling point elevation of the solvent Chapter 12: Physical Properties of Solutions

Summary of Concepts Colligative properties depend on the particular solvent and the number of solute particles present Colloids are dispersions in an appropriate medium of particles ranging in size from 1 nm to 1000 nm EOS Chapter 12: Physical Properties of Solutions

Chapter 12 Physical Properties of Solutions

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