Solutions. Occur in all phases u The solvent does the dissolving. u The solute is dissolved. u There...
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Transcript of Solutions. Occur in all phases u The solvent does the dissolving. u The solute is dissolved. u There...
Solutions
Occur in all phases The solvent does the dissolving. The solute is dissolved. There are examples of all types of
solvents dissolving all types of solvent.
We will focus on aqueous solutions.
Ways of Measuring
Molarity = moles of solute Liters of solvent
% mass = Mass of solute x 100 Mass of solution
Mole fraction of component A
A = NA
NA + NB
Molality = moles of solute Kilograms of solvent
Molality is abbreviated m Normality - read but don’t focus on
it (unless you plan on being an “Old School Biologist”)
Ways of Measuring
Energy of Making Solutions Heat of solution ( Hsoln ) is the energy
change for making a solution. Most easily understood if broken into
steps. 1.Break apart solvent 2.Break apart solute 3. Mixing solvent and solute
1. Break apart Solvent Have to overcome attractive forces.
H1 >0
2. Break apart Solute. Have to overcome attractive forces.
H2 >0
3. Mixing solvent and solute H3 depends on what you are mixing.
Molecules can attract each other H3 is large and negative.
Molecules can’t attract- H3 is small and negative.Solutions which form are typically exothermic
overall!
This explains the rule “Like dissolves Like”
Energy
Reactants
Solution
H1
H2
H3
Solvent
Solute and Solvent
Size of H3 determines whether a solution will form
H3
Solution
Types of Solvent and solutes
If Hsoln is small and positive, a solution will still form because of entropy.
Typically, greater forms of disorder are favored over ordered matter.
Structure and Solubility Water soluble molecules must have
dipole moments -polar bonds. To be soluble in non polar solvents
the molecules must be non polar. I2 is very slightly soluble in water, but
becomes more soluble in a concentrated KI solution…Explain.
Soap
P O-
CH3
CH2CH2
CH2CH2
CH2
CH2
CH2
O-
O-
Soap
Hydrophobic non-polar end
P O-
CH3
CH2CH2
CH2CH2
CH2
CH2
CH2
O-
O-
Soap
Hydrophilic polar end
P O-
CH3
CH2CH2
CH2CH2
CH2
CH2
CH2
O-
O-
P O-
CH3
CH2CH2
CH2CH2
CH2
CH2
CH2
O-
O-
_
A drop of grease in water Grease is non-polar Water is polar Soap lets you dissolve the non-polar
in the polar.
Hydrophobic ends dissolve in
grease
Hydrophilic ends dissolve in water
Water molecules can surround and dissolve grease.
“Helps get grease out of your way.”
Pressure effects Changing the pressure doesn’t effect
the amount of solid or liquid that dissolves
They are incompressible. It does effect gases.
Dissolving Gases Pressure effects the
amount of gas that can dissolve in a liquid.
The dissolved gas is at equilibrium with the gas above the liquid.
The gas is at equilibrium with the dissolved gas in this solution.
The equilibrium is dynamic.
If you increase the pressure the gas molecules dissolve faster.
The equilibrium is disturbed.
The system reaches a new equilibrium with more gas dissolved.
Henry’s Law.
P= kCPressure =
constant x Concentration of gas
Examples:
1. Soda Can
2. Lake Nyos Tragedy
Lake Nyos Tragedy
•Suffocated 1,700 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock.•300 ft fountain of water created an 82 ft wall of water which scoured the lake shore.
Degassing Lake Nyos
Temperature Effects In general: Increased temperature
increases the rate at which a solid dissolves.
However, we can’t always predict whether it will increase the amount of solid that dissolves.
Consult a graph of experimental data for the best results!
20 40 60 80 100
Gases are predictable
As temperature increases, solubility decreases.
Gas molecules can move fast enough to escape the solution.
Thermal pollution of lakes.
Vapor Pressure of Solutions A nonvolatile solvent lowers the
vapor pressure of the solution. The molecules of the solvent
must overcome the force of both the other solvent molecules and the solute molecules.
Raoult’s Law:Psoln = solvent x Psolvent Vapor pressure of the solution =
mole fraction of solvent times the vapor pressure of the pure solvent
Applies only to an ideal solution where the solute doesn’t contribute to the vapor pressure.
Raoult’s Law Example Determine the vapor pressure of a
solution made by dissolving 125 g of sucrose into 455 mL of water at 25.0ºC.
The vapor pressure of water at this temp is 23.76 torr. (Density of water at this temp is 0.9971 g/cm3)
Solution (get it?) 125g sucrose = .365 mol 155 mL of water = 8.58 mol of water = .959 Pressure = 22.8 torr (down from
23.76 torr)
Ionic vs Covalent Solute? What if we used the same moles of
salt rather than sucrose? Same result? I don’t think so…you get twice the
number of solute particles = twice the effective mole fraction of solute!
of water drops to .921 from .959…so the vapor pressure drops as well!
Aqueous Solution
Pure water
Water has a higher vapor pressure than a solution
Aqueous Solution
Pure water
Water evaporates faster from the water than the solution…
The water condenses faster in the solution so it should all end up there.
Aqueous Solution
Pure water
What is the composition of a pentane-hexane solution that has a vapor pressure of 350 torr at 25ºC ?
The vapor pressures at 25ºC are
• pentane 511 torr
• hexane 150 torr.
• Nonideal solution: both can evaporate What are the component vapor
pressures?
Review Question
Review Question Nonideal Solutions: Ptotal=PA + PB
Psoln = sol A x Psol A + sol B x Psol B
350 torr = A511 torr + B 150 torr(A + B = 1…so B = 1-A now you may substitute…)
350 torr = 511 A+ 150 (1-A)
A = .554 (so B = 1- .554…) B = .446
Check it in the original equation!
Colligative Properties Because dissolved particles affect
vapor pressure - they affect phase changes.
Colligative properties depend only on the number - not the kind of solute particles present
Useful for determining molar mass
Boiling point Elevation
Because a non-volatile solute lowers the vapor pressure it raises the boiling point.
The equation is: T = Kbmsolute
T is the change in the boiling point Kb is a constant determined by the
solvent. msolute is the molality of the solute
Freezing point Depression
Because a non-volatile solute lowers the vapor pressure of the solution it lowers the freezing point.
The equation is: T = Kfmsolute
T is the change in the freezing point Kf is a constant determined by the solvent
msolute is the molality of the solute
Freezing/Boiling Point Addition of a nonvolatile solute
extends the liquid phase! Decreasing the freezing point of the
solution… Increasing the boiling point of the
solution!
1 atm
Vapor Pressure of solution
Vapor Pressure of pure water
1 atm
Freezing and boiling points of water
1 atm
Freezing and boiling points of solution
1 atm
TfTb
Electrolytes in solution Since colligative properties only
depend on the number of molecules. Ionic compounds should have a
greater effect. When they dissolve they dissociate. Individual Na and Cl ions fall apart. 1 mole of NaCl makes 2 moles of ions. 1mole Al(NO3)3 makes 4 moles ions.
Electrolytes have a more significant impact on on melting and freezing points per mole because they generate more particles.
Relationship is expressed using the van’t Hoff factor: i
i = Moles of particles in solution
Moles of solute dissolved The expected value can be determined
from the formula.
The actual value is usually less because…
…at any given instant some of the ions in solution will be paired.
…ion pairing increases with concentration.
…i decreases with increasing concentrations.
We can change our formulas to
T = imK
Liquid-liquid solutions where both are volatile.
Modify Raoult’s Law to Ptotal = PA + PB = APA
0 + APB0
Ptotal = vapor pressure of mixture PA
0= vapor pressure of pure A If this equation works then the
solution is ideal. Solvent and solute are alike.
Ideal solutions
Deviations If Solvent has a strong affinity for
solute (H bonding). Lowers solvents ability to escape. Lower vapor pressure than expected. Negative deviation from Raoult’s law. Hsoln is large and negative
exothermic. Endothermic Hsoln indicates
positive deviation.
Osmotic Pressure ∏ = MRT ∏ = osmotic pressure M = molarity (not molality) R = gas law constant T = Kelvin temp
For electrolyte solutions:
∏ = iMRT