Transcript Solutions Part III

Solutions Part III:
IMF, Energetics of
Solutions, Henry’s Law
Much of this is basic concepts that you should
review on your own.
Study Slides 1 thru 14 on your own.
Slides 15 thru 26 are summarized in a handout.
You should review these slides on your own.
Lecture begins on Slide 27.
(Jespersen Chap. 13 Sec 1 – 4)
Dr. C. Yau
Fall 2014
1
What exactly does "solution" mean?
solute + solvent = solution
They can be in any physical state.
Most common is solid in liquid, such as
NaCl in water to give salt water.
The only requirement is they form a
HOMOGENEOUS MIXTURE, which
means at the "particulate" level,
particles are uniformly mixed.
2
You can have a “solution” of gas in gas:
e.g. Air is a homogeneous mixture of mainly
N2 and O2.
You can have a “solution of gas in liquid:
e.g. O2 dissolved in water. This is how fish
“breathes.”
You can have a solution of liquid in liquid:
e.g. Rubbing alcohol is a homogeneous
mixture of isopropyl alcohol (liquid) in
water.
3
Like Dissolves Like
KNOW THESE RULES WELL!
Nonpolar solutes prefer to dissolve in
nonpolar solvents.
Polar solutes prefer to dissolve in polar
solvents.
Ionic compounds are more like polar
compounds than nonpolar.
They prefer to dissolve in polar solvents.
4
Solution at the Particulate Level
solid + solid
Cu
+ Zn
solid solution
brass (an alloy)
uniformly mixed at the
particulate level
attracted by London forces
5
Solution at the Particulate Level
gas + gas
N2 + O2
gas solution
air
For ideal gases, there is no IMF.
6
Solution at the Particulate Level
solid + liquid
solution
NaCl + water
salt water
Na+ClNa+ClNa+Cl
H
ClNa+ClNa+ClNa+
Na+ClNa+ClNa+Cl
ionic
bonding
H
H
O
H
H
H
H
H
O
H
O
H
O
H
+
O
O
H
O
O
O
O
Na
H
H
H
H
H
O
H
hydrogen bonding
H
H
ion-dipole
force
O
H
H
H
H
H
H
O
Cl
_
H
H
H
O
O
H
7
8
Solution at the Particulate Level
liquid + liquid
solution
alcohol + water
alcohol soln
C H3C H2
C H3C H2
C H3C H2
O
H
C H3C H2
H
O
H
O
H
H
H
O
H
O
H
C H3C H2
H
H
H
O
O
H
O
H
O
H
H
O
H
O
H
H-bonding
O
H
O
H
O
H
C H3C H2
H
H
H
H
H-bonding
O
O
H
H
H-bonding
9
Dissolution Of A Polar Compound In Water
Dipole of the water interacts with the
oppositely charged dipoles of the solid,
extracting them from the crystal
10
Why is oil and water immiscible?
oil + water
two separate layers
as a heterogeneous mixture
London forces
very strong Hbonding,
stronger than oilwater attraction
11
Miscibility of Liquids
• Liquids that can
dissolve in one
another are
miscible, while
insoluble liquids are
immiscible
• Ethanol and water
are miscible, while
benzene and water
are not. WHY NOT?
12
Which of the following are miscible in water?
water
acetic acid
carbon
disulfide
ammonia
Acetic acid & ammonia can H-bond with water.
CS2 is non polar and is too “unlike” water.
13
Which of the following are likely to be
miscible with water?
A. CH3CH2CH2CH3
B. C6H6
C. CH3CO2H
D. All are expected to be miscible
A and B are hydrocarbons (nonpolar). C. is acetic
acid and can H-bond with water.
14
Solvation is the process of solute
particles becoming surrounded by
solvent molecules.
Hydration is the process of solute
particles becoming surrounded by
water molecules
Hydration is solvation when the solvent
is water, and the resultant solution...
is called an "aqueous solution."
15
Enthalpy (Heat) Of Solution
• Heat of solution (Ηsoln ) is the energy
exchanged when a solute dissolves in a
solvent at constant pressure.
• Enthalpy is a state function: the energy
change depends only on the initial and
final states of the system and not on the
pathway one takes to go from the initial to
final state.
• When Ηsoln=0, solution is called an ideal
solution. (It is neither endothermic nor exothermic)
16
What is molar ΔHsoln ?
Molar ΔHsoln is the molar enthalpy of solution.
the heat of solution of one mole of solute.
(Note: 1 mol solute not 1 mol solution. We cannot talk
about a “mole of solution” because a solution is not a
compound. It is a homogeneous mixture, with a
variable ratio of solute to solvent.)
It may be positive OR negative.
Heat may be absorbed OR evolved (released).
Can we predict which will happen?
17
Enthalpy of Solution
There are 3 steps in the process of solution:
1) Solute separates from each other
(endothermic)
2) Solvent separates from each other to
make room for solute (endothermic)
3) Solute fits into the space between solvent
molecules (exothermic)
If the net change in heat is negative, the
solution process will give off heat.
If the net change is positive, the soln will
absorb heat (the solution feels cooler).
18
Hess’ Law
REMEMBER! Just like balancing a bank
account....
Need to ADD heat to separate particles of
solute and solvent.
Get heat back from solvation. Balance sheet? 19
Dissolution: Liquid in Liquid (Ideal)
Shown here is an ideal solution where H = 0.
H  0 if H1 + H2  H3 . (Examples on next slide.)
20
Liquid into Another Liquid
e.g. Acetone in water is exothermic
but ethanol in hexane is endothermic.
Hydration energy is large
WHY?
O
H
C
H3C
C H3C H2
O
H
C H3
because the acetone can
form H-bonding with water,
making up for breaking Hbonding in water.
O
H
CH3CH2CH2CH2CH2CH2CH2CH3
Solvation energy is small because there is no H-bonding
between ethanol and hexane; whereas, there is very
strong H-bonding between the ethanol molecules.
21
Heat of Solution
of Ionic Compound in Water
Step 1) Ionic compound breaks up into gaseous
cations and gaseous anions
This is called "lattice energy."
It is a positive value.
Step 2) Gaseous ions become surrounded by
water molecules.
This is called "hydration energy."
It is a negative value.
Overall energy change: sum of the two.
22
KI in Water
KI (s)
K+ (g)
Overall:
KI (s)
K+ (g)
+ I- (g)
+ I- (g)
ΔH = + 632 KJ
K+ (aq) + I- (aq)
ΔH = - 619 KJ
K+ (aq) + I- (aq)
ΔH = + 13 KJ
What does this mean?
Heat is absorbed.
Solution will feel cooler during the process
because it is absorbing heat from the
surroundings.
23
Water already has
“pockets” for ions to fit in
and therefore does not
need to “expand.”
24
Dissolution: Gas In Liquid: 2 scenarios
• Step1: Expansion of solvent
• Step2: Mixing
Why missing one step?
• Ηsoln = Η1 + Η2
Solute does not
have to separate.
Dissolution is endothermic.
Dissolution is exothermic.
25
Gas Solubility
Heats of solution for gases in organic
solvents are often endothermic.
This is because it takes more energy to
open a "pocket" of space in the organic
solvents than the energy released when
the gases go into the "pocket."
Heats of solution for gases in water are
often exothermic.
This is because water already has pockets
to hold the molecules so it does not take
any energy, but energy is released in the
hydration step.
26
Effect of Temp on Solubility
Solubility refers to the maximum amount of
solute that will dissolve in a given volume
of solvent.
It is usually given as #g solute /100g solvent.
This is the "saturation point" when no more
solute can go into solution.
The solution is then referred to as the
"saturated solution," which is an
equilibrium state.
27
Saturated Solution
solute (undissolved)
solute (dissolved)
Dissolution of solids in water is usually
endothermic.
heat + solute(undiss)
solute (dissolved)
Thus, adding heat would shift the equilibrium
to the right, making the substance more
soluble (as is the case with sugar in
water).
28
Gas in Solution
In the case of a gas in an aqueous solution, it is
usually exothermic.
solute (undissolved)
solute (dissolved) + heat
If heat is added, the equilibrium will shift to the
LEFT, making the gas LESS soluble.
This is what you observe when you open a soda
bottle in hot weather. The gas will come out
much faster than in cold weather.
Sodas go flat faster at rm temp than in the fridge.
29
Fig.13.10 p.594
Solubility in water vs.
temp for several
substances.
What does this
graph tell
you?
Examine the
curve for
table salt!
What is
unusual?
...and
Ce2(SO4)3?
30
Effect of Pressure on Solubility
gas + solvent
solution
If pressure is increased, the equilibrium will shift to
the right to relieve the pressure. This is because
a solution occupies less space than a gas.
Henry's Law
Concentration of a gas (C) in a liquid at any
given temperature is directly proportional to
the partial pressure (P) of the gas over the
solution.
C(gas)  P(gas)
C(gas) = kH P(gas)
You can increase the concentration of a gas in
a solution by increasing the pressure above it.
That is how sodas are carbonated.
31
Application of Henry's Law
C(gas) = kH P(gas)
Ex. 13.1 (p.596)
At 20oC the solubility of N2 in water is
0.0152 g L-1 when the partial pressure of
nitrogen is 585. torr. What will be the
solubility of N2 in water at 20oC when its
partial pressure is 823 torr?
Ans. 0.0214 g/L
32
Solubility of Gases in Water
Some gases are more soluble than others:
• Polar gases are more soluble because they
form stronger attraction to the solvent. (e.g.
SO2, NH3)
• Gases that react with the solvent are more
soluble because the rxn removes the product
& thus the equilibrium will shift to replenish the
lost product.
H 2O
H 2O
e.g. SO2 (g)
SO2 (aq)
H2SO3 (aq)
The product SO2 (aq) is funneled off as H2SO3, so the
equilibrium shifts to the right to replenish the lost SO2 (aq).
33