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Chapter 14.3 Properties of Solutions. Adapted by SA Green from: John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice Hall, Inc. Solutions. Solutions are homogeneous mixtures of two or more pure substances. - PowerPoint PPT Presentation
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Solutions
Chapter 14.3Properties of Solutions
Adapted by SA Green from:
John D. Bookstaver
St. Charles Community College
St. Peters, MO
2006, Prentice Hall, Inc.
Solutions
Solutions
• Solutions are homogeneous mixtures of two or more pure substances.
• In a solution, the solute is dispersed uniformly throughout the solvent.
Solutions
Solutions
How does a solid dissolve into a liquid?
What ‘drives’ the dissolution process?
What are the energetics of dissolution?
Solutions
How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (H) changes with each interaction broken or
formed.
Ionic solid dissolving in water
Solutions
How Does a Solution Form?1. Solvent molecules attracted to surface ions.2. Each ion is surrounded by solvent molecules.3. Enthalpy (H) changes with each interaction broken or
formed.
Solutions
How Does a Solution Form
The ions are solvated (surrounded by solvent).
If the solvent is water, the ions are hydrated.
The intermolecular force here is ion-dipole.
Solutions
Energy Changes in SolutionTo determine the enthalpy
change, we divide the process into 3 steps. 1. Separation of solute
particles.
2. Separation of solvent particles to make ‘holes’.
3. Formation of new interactions between solute and solvent.
Solutions
Dissolution vs reaction
• Dissolution is a physical change—you can get back the original solute by evaporating the solvent.
• If you can’t, the substance didn’t dissolve, it reacted.
Ni(s) + HCl(aq) NiCl2(aq) + H2(g) NiCl2(s)dry
Solutions
Degree of saturation
• Saturated solutionSolvent holds as much
solute as is possible at that temperature.
Undissolved solid remains in flask.
Dissolved solute is in dynamic equilibrium with solid solute particles.
Solutions
Degree of saturation
• Unsaturated SolutionLess than the
maximum amount of solute for that temperature is dissolved in the solvent.
No solid remains in flask.
Solutions
Degree of saturation
• SupersaturatedSolvent holds more solute than is normally
possible at that temperature.These solutions are unstable; crystallization can
often be stimulated by adding a “seed crystal” or scratching the side of the flask.
Solutions
Degree of saturation
Unsaturated, Saturated or Supersaturated?
How much solute can be dissolved in a solution?
More on this in Chap 17(solubility products, p 739)
Solutions
Factors Affecting Solubility
• Chemists use the axiom “like dissolves like”:Polar substances tend to
dissolve in polar solvents.Nonpolar substances tend
to dissolve in nonpolar solvents.
Solutions
Factors Affecting Solubility
The stronger the intermolecular attractions between solute and solvent, the more likely the solute will dissolve.Example: ethanol in water
Ethanol = CH3CH2OH
Intermolecular forces = H-bonds; dipole-dipole; dispersion
Ions in water also have ion-dipole forces.
Solutions
Factors Affecting Solubility
Glucose (which has hydrogen bonding) is very soluble in water.
Cyclohexane (which only has dispersion forces) is not water-soluble.
Solutions
Factors Affecting Solubility• Vitamin A is soluble in nonpolar compounds
(like fats).• Vitamin C is soluble in water.
Solutions
Which vitamin is water-soluble and which is fat-soluble?
Solutions
Gases in Solution
• In general, the solubility of gases in water increases with increasing mass.
Why?• Larger molecules
have stronger dispersion forces.
Solutions
Gases in Solution
Solutions
Gases in Solution
• The solubility of liquids and solids does not change appreciably with pressure.
• But, the solubility of a gas in a liquid is directly proportional to its pressure.
Increasing pressure above solution forces more gas to dissolve.
Solutions
Henry’s Law
Sg = kPg
where
• Sg is the solubility of the gas;
• k is the Henry’s law constant for that gas in that solvent;
• Pg is the partial pressure of the gas above the liquid. So
• S1/P1 = S2/ P2
Solutions
Temperature
Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.
Solutions
Temperature• The opposite is true of
gases. Higher temperature drives gases out of solution.
Carbonated soft drinks are more “bubbly” if stored in the refrigerator.
Warm lakes have less O2 dissolved in them than cool lakes.