5.111 Lecture Summary #20 Monday October 27, 2014
Reading for Today: Sections 9.8 - 9.13 (8.8-8.13 same in 4th ed.) on solubility; Sections 11.1
– 11.2, 11.4-11.6 (10.1-10.2, 10.4-10.6 in 4th ed.) on acids and bases.
Reading for Lecture #21: Sections 11.7-11.9, 11.11-11.13 (10.7 -10.9, 10.11 – 10.13 in 4th ed).
Topics: I. Solutions and Solubility
II. Classification of Acids and Bases
I. SOLUTIONS AND SOLUBILITY
So far, we’ve been discussing pure compounds. However, most substances are .
Solutions are homogeneous mixtures.
Solvent: the substance that does the dissolving (i.e. water)
Solute: any dissolved substance in a solution
MOLAR SOLUBILITY
Ionic solids. Consider NaCl dissolving in water.
Polar water molecules ions at the surface of the
salt’s crystal lattice, prying some of the Na+ and Cl- ions away.
Stirring can the process by bringing more free
water molecules to the surface of the solid, and taking more
hydrated ions away.
Solubility equilibrium: NaCl (s) Na+ (aq) + Cl- (aq)
Ksp = [Na+][Cl-] where sp stands for “solubility product”
is a measure of the dissolution of an ionic solid in water.
Note NaCl does not appear in the expression since it is a solid.
Organic solids. Consider glucose dissolving in water.
Water molecules form hydrogen bonds to the glucose molecules near the surface of a
glucose crystal. Some glucose molecules are pulled away by the surrounding water (are
solubilized), other molecules are not.
Glucose is a hydrogen bond .
A solution is when the solvent has dissolved all the solute that it can
and some undissolved solute remains.
The dissolved and undissolved solute are in
with each other.
1
, The amount that dissolves depends on the molar solubility (s) of the substance.
Molar solubility (s) of a substance is its molar concentration in a saturated solution, and
represents the limit of its ability to dissolve in a given solvent. (units: )
Note: molar solubility (s) and Ksp are not the same, but they can be calculated from each
other. [Na+] = [Cl-] = s (is the molar solubility of either ion at equilibrium), thus
Ksp = [Na+][Cl-] = s2 for this particular ionic compound.
Like-Dissolves-Like Rule
A polar liquid like water is generally the best solvent for ionic and
compounds.
Conversely, nonpolar liquids, including hexane and tetrachloroethane (used in dry
cleaning), are better for nonpolar (hydrophobic) compounds.
Applications of this rule:
pharmaceutical drug design -- solubility of nonpolar enzyme inhibitors in aqueous
solutions.
cleaning products -- want to dissolve polar and nonpolar stains
IMPACT OF PRESSURE AND TEMPERATURE ON
SOLUBILITY
Pressure and Gas Solubility
Henry’s Law: the solubility of a gas (s) is directly proportional to
its partial pressure (P).
s = kHP where kH is Henry’s constant, and depends on
the gas, the solvent, and the temperature.
© W. H. Freeman & Co Ltd. All rights reserved. This content
is excluded from our Creative Commons license. For more
information, see http://ocw.mit.edu/help/faq-fair-use/.
The solubility of a gas is to its partial pressure, because an increase
in pressure corresponds to an in the rate at which gas molecules
strike the surface of the solvent.
In Their Own Words
Former MIT postdoctoral scholar Dr. Hector Hernandez discusses
how gas solubility and Le Chatelier’s principle relate to his research
on CO2 capture and storage in the lab of MIT Professor Janelle
Thompson.
Hector’s s video can be found at:
http://chemvideos.mit.edu/all-videos/.
Courtesy of Hector Hernandez. Used with permission.
2
Reading for Today: Sections 9.8 - 9.13 (8.8-8.13 same in 4th ed.) on solubility; Sections 11.1
– 11.2, 11.4-11.6 (10.1-10.2, 10.4-10.6 in 4th ed.) on acids and bases.
Reading for Lecture #21: Sections 11.7-11.9, 11.11-11.13 (10.7 -10.9, 10.11 – 10.13 in 4th ed).
Topics: I. Solutions and Solubility
II. Classification of Acids and Bases
I. SOLUTIONS AND SOLUBILITY
So far, we’ve been discussing pure compounds. However, most substances are .
Solutions are homogeneous mixtures.
Solvent: the substance that does the dissolving (i.e. water)
Solute: any dissolved substance in a solution
MOLAR SOLUBILITY
Ionic solids. Consider NaCl dissolving in water.
Polar water molecules ions at the surface of the
salt’s crystal lattice, prying some of the Na+ and Cl- ions away.
Stirring can the process by bringing more free
water molecules to the surface of the solid, and taking more
hydrated ions away.
Solubility equilibrium: NaCl (s) Na+ (aq) + Cl- (aq)
Ksp = [Na+][Cl-] where sp stands for “solubility product”
is a measure of the dissolution of an ionic solid in water.
Note NaCl does not appear in the expression since it is a solid.
Organic solids. Consider glucose dissolving in water.
Water molecules form hydrogen bonds to the glucose molecules near the surface of a
glucose crystal. Some glucose molecules are pulled away by the surrounding water (are
solubilized), other molecules are not.
Glucose is a hydrogen bond .
A solution is when the solvent has dissolved all the solute that it can
and some undissolved solute remains.
The dissolved and undissolved solute are in
with each other.
1
, The amount that dissolves depends on the molar solubility (s) of the substance.
Molar solubility (s) of a substance is its molar concentration in a saturated solution, and
represents the limit of its ability to dissolve in a given solvent. (units: )
Note: molar solubility (s) and Ksp are not the same, but they can be calculated from each
other. [Na+] = [Cl-] = s (is the molar solubility of either ion at equilibrium), thus
Ksp = [Na+][Cl-] = s2 for this particular ionic compound.
Like-Dissolves-Like Rule
A polar liquid like water is generally the best solvent for ionic and
compounds.
Conversely, nonpolar liquids, including hexane and tetrachloroethane (used in dry
cleaning), are better for nonpolar (hydrophobic) compounds.
Applications of this rule:
pharmaceutical drug design -- solubility of nonpolar enzyme inhibitors in aqueous
solutions.
cleaning products -- want to dissolve polar and nonpolar stains
IMPACT OF PRESSURE AND TEMPERATURE ON
SOLUBILITY
Pressure and Gas Solubility
Henry’s Law: the solubility of a gas (s) is directly proportional to
its partial pressure (P).
s = kHP where kH is Henry’s constant, and depends on
the gas, the solvent, and the temperature.
© W. H. Freeman & Co Ltd. All rights reserved. This content
is excluded from our Creative Commons license. For more
information, see http://ocw.mit.edu/help/faq-fair-use/.
The solubility of a gas is to its partial pressure, because an increase
in pressure corresponds to an in the rate at which gas molecules
strike the surface of the solvent.
In Their Own Words
Former MIT postdoctoral scholar Dr. Hector Hernandez discusses
how gas solubility and Le Chatelier’s principle relate to his research
on CO2 capture and storage in the lab of MIT Professor Janelle
Thompson.
Hector’s s video can be found at:
http://chemvideos.mit.edu/all-videos/.
Courtesy of Hector Hernandez. Used with permission.
2
