Colligative Properties of
Solutions
Reaction Rates…
How can I make a reaction
happen faster?
What is a colligative property?
• A property that depends on:
1. the concentration of solute particles in
solution
2. the solvent used
A colligative property does not depend on:
1. the identity of the solute
Dilute Versus Concentrated
Solutions
• Dilute--> a low amount of solute is dissolved
in a solvent
• Concentrated--> a high amount of solute is
dissolved in a solvent
• These terms will not tell us whether a solution
is saturated, unsaturated, or supersaturated
• The concentration of a solution can be found
using the Molarity (M) and Molality (m)
equations
Freezing Point Depression
• When a solute is dissolved in a solvent,
the freezing point of the solvent will
depress (become lower) than the
normal freezing point of the pure solute
What does the FP depression
depend on?
•
The amount the FP will be lowered depends
on 3 factors:
1. The concentration of the solution
Higher molality (m) = higher depression
2. Whether or not the solution is an electrolyte
Electrolytes = higher depression
3. The identity of the solvent used
H2O vs. C2H5OH will exhibit different
depressions
What is an electrolyte?
• A substance that MUST contain ions
and conducts electricity (metal &
nonmetal)
• Electrolytes break apart into + and charged particles which would have a
greater effect on the FP depression
• Which of the following are examples of
electrolytes?
NaCl, C12H22O11, CaCl2, C2H5OH
How many ions do each of the
electrolytes have?
How do you find the FP
depression of the solvent?
∆Tf = kf n m
∆Tf = FP depression (how many °C
lower than the FP of the pure solvent)
Kf = FP constant which is dependant on
the solvent used (pg. 438 in text)
n = number of ions (if applicable)
m = molality of the solution
Sample Problem
• How much will the freezing point be
lowered if enough sucrose (C12H22O11)
is dissolved in H2O to make a .50 m
solution? Kf for H2O = 1.86ºC/m
• What if the solute was barium bromide?
Calculate the FP depression.
• What is the freezing point of a solution
containing 16.5 g of sodium chloride in
300. g of acetic acid?
Practical Application of FP
Depression
• Why does putting salt on the roads
when it is icy prevent the roads from
being slick?
The addition of salt as a solute will
lower the FP of H2O and help to prevent
ice from forming
Boiling Point Elevation
• When a solute is dissolved in a solvent,
the boiling point of the solvent will
elevate (become higher) than the
normal boiling point of the pure solute
• BP elevation depends on the same
factors that FP depression does
How do you find the BP
elevation of the solvent?
∆Tb = kb n m
∆Tb = BP elevation (how many °C higher
than the BP of the pure solvent)
Kb = BP constant which is dependant on
the solvent used (pg. 438 in text)
n = number of ions (if applicable)
m = molality of the solution
Sample Problem
• A nonelectrolyte solute is dissolved in
50 kg of benzene (C6H6). The new BP
of this solution is 80.61ºC. What is the
amount in moles of solute if pure
benzene boils at 80.10ºC?
• What is the expected boiling point of for
a solution that contains 136 g of
potassium nitrate in 1.00 kg
Practical Application of BP
Elevation
• Why do cooks add salt to their water
when boiling water?
• The addition of salt as a solute will force
the H2O to boil at a higher temperature
which will in turn, cook the food placed
in the H2O faster
Homework Problems
•
•
•
•
Pg. 440 #1-4
Pg. 441 #1-4
Pg. 445 #1-3
All of the listed problems above have
answers provided
• Pg. 449 #19c, 20a, 21a, 21b, 22, 25c,
26a, 30, 31
What Factors Affect the Rate
of a Reaction?
• 1. Increase in surface area
Ex. Increasing the surface area of a
substance allows for more particle
contact and collisions, therefore
increasing the rate of a reaction
Granulated sugar should dissolve more
quickly in a solvent because of its greater
surface area
• 2. Increase in temperature
Ex. Increasing the temperature of a
reaction allows for more frequent
particle contact and collisions due to an
increase in the kinetic energy of the
system, therefore increasing the rate of
a reaction
• 3. Increase in concentration of
reactants
Ex. Increasing the concentration of
reactants allows for more particle
collisions because there are more
particles present, therefore
increasing the rate of a reaction
• 4. Addition of a catalyst
Ex. The addition of a catalyst will force
the particles to come in contact at a
quicker rate, therefore increasing the
rate of the reaction
• 5. Agitation/Stirring
The Collision Theory
• In order for a reaction to occur, 2 particles
must have an effective collision. A successful
collision needs:
• 1. Proper orientation (particles must hit in
the right spots)
• 2. Suitable kinetic energy (particles must hit
with the right amount of speed --> too much
or too little may result in an ineffective
collision)
Reaction Rate Diagrams
Activation Energy (Ea) = the
energy needed to get a reaction
started
This diagram is representing an
exothermic reaction because energy
is being released and the products
contain less energy than the
reactants
Lowers Ea
Inhibitors increase Ea by
slowing down a reaction