01:160:162 Lecture Notes - Lecture 14: Intermolecular Force, Pantothenic Acid, Gas Constant
19 Dec 2016
School
Department
Course
Professor
Chapter 14: Solutions
I. 14.1 Antifreeze in Frogs
A. Most cold blooded animals cannot survive freezing temperatures because the
water within their cells freezes and expands, damaging the cells
B. To prevent this wood frogs secrete an amount of glucose into its bloodstream
that incorporates into the cells resulting in cells filled with concentrated glucose
solution
1. Causes a lower freezing point than the corresponding pure liquid
2. Extracellular bodily fluids will freeze solid but fluid within the cells will
remain liquid, high glucose concentration
a) Antifreeze
C. Solution is a homogenous mixture of two or more substances or components
1. Solvent is the majority component
2. Solute is the minority component
3. Formed by intermolecular forces, particles of solute interact with particles
of the solvent
II. 14.2 Types of Solutions and Solubility
A. Aqueous solutions- water is the solvent, solid/liquid/gas is the solute
B. Solubility of a substance is the amount of the substance that will dissolve in a
given amount of solvent
1. Depends on nature’s tendency to mix and types of intermolecular forces
C. Nature’s tendency towards mixing: entropy
1. Recall: many physical systems tend toward lower potential energy (such
as two particles with opposite charges move toward each other because
their potential energy decreases as their separation decreases according
to coulomb’s law)
a) Formation of solution does not necessarily lower the potential
energy of its constituent particles
(1) Example: formation of a solution of neon and argon in a
container with a removable barrier. Remove barrier, they
mix together to form a solution but their potential energy is
unchanged
(2) Cannot think of mixing of two ideal gases as lowering their
potential energy. Tendency to mix- entropy
find more resources at oneclass.com
find more resources at oneclass.com
2. Entropy- measure of energy randomization or energy dispersal in a
system
a) Gas at any temperature above 0K has a kinetic energy due to the
motions of its atoms and will be confined to compartments.
b) When barrier is removed, gas and kinetic energy becomes spread
out/dispersed over a larger volume
c) Mixture of two gases has a greater energy dispersal, or greater
entropy, than the separated components
3. Pervasive tendency for energy to spread out, or disperse, when not
restrained from doing so is the real reason ideal gases mix
D. The effects of intermolecular forces
1. In the absence of intermolecular forces, two substances spontaneously
mix to form a homogenous solution
2. Forces that may contribute to or oppose formation of a solution
a) Dispersion
b) Dipole-dipole
c) Hydrogen bond
d) Ion-dipole
3. Exists between
a) Solvent-solute, solute-solute, solvent solvent
4. Solvent-solute interacts greater than or equal to solvent-solvent and
solute-solute interactions will form solutions
5. Miscible- two substances are soluble in each other in all proportions
a) ex/ C5H12, C7H16. Intermolecular forces within both are
dispersion forces, between is also dispersion forces.
(1) Formation of solution is driven by tendency toward mixing
or greater entropy
6. Solution may still form even if solute-solute and solvent-solvent is greater
than solute-solvent. Small disparity-energetically uphill but big, solution
does not form.
7. LIKE DISSOLVES LIKES
E. Solubility examples
1. Vitamins are either fat soluble or water soluble
a) Vitamin C has four OH- bonds, making it highly polar and
hydrogen bond with water. Water soluble!
b) Vitamin K3 has C-C bonds which are nonpolar and C-H bonds are
nearly so. C=O is polar but the bond dipoles oppose and largely
cancel each other out, dominated by nonpolar bonds. Fat soluble
c) Vitamin A has C-C bonds that are nonpolar and C-H bonds are
nearly so. The one polar -OH bond may increase vitamin A’s
water solubility slightly, but overall it is nonpolar and therefore fat
soluble.
find more resources at oneclass.com
find more resources at oneclass.com
d) Vitamin B5 has three -OH bonds and one -NH bond, making it
highly polar and allow it to hydrogen bond with water. It is water
soluble
III. 14.3 Energetics of Solution Formation
A. Energy changes can occur when a solution forms, depending on the magnitude
of the interactions between solute and solvent particles
B. Energy changes during solution formation
1. Steps
a) Separating the solute into its constituent particles
(1) Always endothermic (positive △H) because energy
required to overcome the forces that hold the solute
particles together
b) Separating the solvent particles from each other, to make room for
solute particles
(1) Endothermic (positive △H), because energy is required to
overcome the intermolecular forces among the solvent
particles
c) Mixing the solute particles with the solvent particles
(1) Exothermic, negative △H, because energy is released as
the solute particles interact, through intermolecular forces,
with the solvent particles
2. According to Hess’s law the overall enthalpy change upon solution
formation (enthalpy of solution (△Hsoln) is the sum of the changes in
enthalpy for each step
a) △Hsoln = △H solute + △H solvent + △Hmix
b) _ = endothermic (+) + endothermic (+) + exothermic (-)
c) Overall sign of △Hsoln depends on the magnitude of the individual
terms
3. So,
a) If sums of the endothermic terms is approximately equal in
magnitude to the exothermic term, then △Hsoln is about zero.
Increasing entropy upon mixing drives the formation of a solution,
while overall energy of the system remains nearly constant
b) If sum of the endothermic terms is smaller than exothermic term,
then △Hsoln is negative and solution process is exothermic. Both
the tendency toward lower energy and toward greater entropy
drive the formation of a solution
c) If the sum of the endothermic terms is greater than exothermic
term, then △H soln is positive and solution is endothermic. As long
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Remove barrier, they mix together to form a solution but their potential energy is unchanged (2) cannot think of mixing of two ideal gases as lowering their potential energy. Intermolecular forces within both are dispersion forces, between is also dispersion forces. (1) formation of solution is driven by tendency toward mixing or greater entropy: solution may still form even if solute-solute and solvent-solvent is greater than solute-solvent. Small disparity-energetically uphill but big, solution does not form: like dissolves likes, solubility examples, vitamins are either fat soluble or water soluble, vitamin c has four oh- bonds, making it highly polar and hydrogen bond with water. Water soluble: vitamin k3 has c-c bonds which are nonpolar and c-h bonds are nearly so. C=o is polar but the bond dipoles oppose and largely cancel each other out, dominated by nonpolar bonds. Fat soluble: vitamin a has c-c bonds that are nonpolar and c-h bonds are nearly so.
