Solutions
http://www.wsd1.org/grantpark/staff/patenaude/powerpoi
nt/Solutions_30SE.ppt
http://www.wsd1.org/grantpark/staff/patenaude/powerpoint/Solutions
Definitions
A solution is a homogeneous mixture of a
solute dissolved in a solvent.
The solvent is generally in excess an aqueous
solution has water as solvent.
Example
The solution NaCl(aq) is sodium chloride
NaCl(s) dissolved in water H2O(l)
The solute is NaCl(s) and the solvent is H2O(l).
Air is an example of a solution with one
“solvent” (nitrogen) and many “solutes”
(oxygen, helium, argon, carbon dioxide, etc.)
Definitions
http://www.authorstream.com/Presentation/Mar
got-
Solutions can be classified as
saturated or unsaturated.
A saturated solution contains the
maximum quantity of solute that
dissolves at that temperature.
A saturated solution represents
equilibrium: rate of dissolving
equals to rate of crystallization
An unsaturated solution contains
less than the maximum amount
of solute that can dissolve at a
particular temperature
A SUPERSATURATED
SOLUTIONS contain more solute
than is possible to be dissolved
http://www.authorstream.com/Presentation/Margot-
Supersaturated solutions are
unstable. The supersaturation is
only temporary, and usually
accomplished in one of two ways:
1. Warm the solvent so that it will
dissolve more, then cool the
solution
2. Evaporate some of the solvent
carefully so that the solute does
not solidify and come out of
solution.
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Electrolyte and Non-electrolyte

Electrolyte: a substance that conducts
electricity when dissolved in water.
– Acids, bases and soluble ionic solutions are
electrolytes.

Non-electrolyte: a substance that does
not conduct electricity when dissolved in
water.
– Molecular compounds and insoluble ionic
compounds are non-electrolytes.
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Electrolytes
Some solutes can
dissociate into
ions.
Electric charge can
be carried.
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high conductivity
Types of
solutes
Strong
Electrolyte 100%
dissociation,
all ions in
solution
Na+
Cl-
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Types of
solutes
slight conductivity
Weak
Electrolyte partial
dissociation,
molecules and
ions in solution
CH3COOH
H+
CH3COO-
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no conductivity
Types of
solutes
Nonelectrolyte No
dissociation,
all molecules
in solution
sugar
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Types of Electrolytes
• Strong electrolyte dissociates
completely.
– Good electrical conduction.
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Weak electrolyte partially
dissociates.
– Fair conductor of electricity.
Non-electrolyte does not dissociate.
– Poor conductor of electricity.
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Representation of Electrolytes using Chemical
Equations
A strong electrolyte:
MgCl2(s)+H2O → Mg2+(aq) + 2 Cl- (aq)
A weak electrolyte:
→ CH3COO -(aq) +H+(aq)
CH3COOH(aq) ←
A non-electrolyte:
CH3OH(aq)
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Strong Electrolytes
Strong acids: HNO3, H2SO4, HCl, HClO4
Strong bases: MOH (M = Na, K, Cs, Rb etc)
Salts: All salts dissolving in water are completely ionized.
Stoichiometry & concentration relationship
NaCl (s) +H2O  Na+ (aq) + Cl– (aq)
Ca(OH)2 (s) +H2O  Ca2+(aq) + 2 OH– (aq)
AlCl3 (s) +H2O  Al3+ (aq) + 3 Cl– (aq)
(NH4)2SO4 (s) +H2O  2 NH4 + (aq) + SO42– (aq)
http://www.wsd1.org/grantpark/staff/patenaude/powerpoint/Solutions
3 Stages of Solution Process

Separation of Solute
– must overcome IMF(Intermolecular forces) or ion-ion attractions
in solute
– requires energy, ENDOTHERMIC ( + DH)

Separation of Solvent
– must overcome IMF of solvent particles
– requires energy, ENDOTHERMIC (+ DH)

Interaction of Solute & Solvent
– attractive bonds form between solute particles and solvent
particles
– “Solvation” or “Hydration” (where water = solvent)
– releases energy, EXOTHERMIC (- DH)
http://www.wsd1.org/grantpark/staff/patenaude/powerpoint/Solutions_30SE.ppt
Dissolution at the molecular level?

Consider the dissolution of NaOH in H2O
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Factors Affecting Solubility
1. Nature of Solute / Solvent- Like dissolves like (IMF)
2. Temperature i) Solids/Liquids- Solubility increases with Temperature
Increase K.E. increases motion and collision between solute / solvent.
ii) Gas - Solubility decreases with Temperature
Increase K.E. result in gas escaping to atmosphere.
3. Pressure Factor i) Solids/Liquids - Very little effect
Solids and Liquids are already close together, extra pressure will not
increase solubility.
ii) gas - Solubility increases with Pressure.
Increase pressure squeezes gas solute into solvent.
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Solubility curve
Saturated
Supersaturated
Unsaturated
http://www.authorstream.com/Presentation/Margot-
Solubility curve
Any solution can be made saturated, unsaturated, or
supersaturated by changing the temperature.
http://www.wsd1.org/grantpark/staff/patenaude/powerpoint/Solutions_30S
E.ppt
Solubilities of Solids vs Temperature
Solubilities of several
ionic solid as a function
of temperature. MOST
salts have greater
solubility in hot water.
A few salts have
negative heat of solution,
(exothermic process)
and they become less
soluble with increasing
temperature.
http://www.authorstream.com/Presentation/Margot-
The rate of solution
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The rate of solution is a measure of how fast a substance
dissolves. Some of the factors determining the rate of
solution are:
size of the particles -- When a solute dissolves, the
action takes place only at the surface of each particle.
When the total surface area of the solute particles is
increased, the solute dissolves more rapidly. Breaking a
solute into smaller pieces increases its surface area and
hence its rate of solution.
(Sample problem: a cube with sides 1.0 cm long is cut in
half, producing two pieces with dimensions of 1.0 cm x
1.0 cm x 0.50 cm. How much greater than the surface
area of the original cube is the combined surface areas of
the two pieces?
2.0 cm2
http://www.authorstream.com/Presentation/Margot-
The rate of dissolution
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stirring -- With liquid and solid solutes, stirring
brings fresh portions of the solvent in contact with
the solute, thereby increasing the rate of solution.
amount of solute already dissolved -- When there
is little solute already in solution, dissolving takes
place relatively rapidly. As the solution
approaches the point where no solute can be
dissolved, dissolving takes place more slowly.
temperature -- For solid, liquid and gaseous
solutes, changing the temperature not only
changes the amount of solute that will dissolve
but also changes the rate at which the solute will
dissolve.
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Temperature & the Solubility of Gases
The solubility of gases decreases at higher
temperatures WHY???
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The effect of partial pressure on solubility of gases
Henry’s Law
At pressure of few atmosphere or less, solubility of gas solute
follows Henry Law which states that the amount of solute gas
dissolved in solution is directly proportional to the amount of
pressure above the solution.
c=kP
c = solubility of the gas (M)
k = Henry’s Law Constant
P = partial pressure of gas
Henry’s Law Constants (25°C), k
N2
8.42 •10-7 M/mmHg
O2
1.66 •10-6 M/mmHg
CO2
4.48•10-5 M/mmHg
How does Henry’s Law apply??
& Soft Drinks
Henry’s Law & Soft Drinks
 Soft drinks contain “carbonated
water” – water with dissolved
carbon dioxide gas.
 The drinks are bottled with a CO2
pressure greater than 1 atm.
 When the bottle is opened, the
pressure of CO2 decreases and
the solubility of CO2 decreases,
according to Henry’s Law.
 Therefore, bubbles of CO2
escape from solution.
Colligative Properties
On adding a solute to a solvent, the properties of
the solvent are modified.
 Vapor pressure
decreases
 Melting point
decreases
 Boiling point
increases
 Osmosis is possible (osmotic pressure)
These changes are called COLLIGATIVE
PROPERTIES.
They depend only on the NUMBER of solute
particles relative to solvent particles, not on the
KIND of solute particles.
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Vapor Pressure Lowering for a Solution

The diagram below shows how a phase diagram is affected by
dissolving a solute in a solvent. Notice the changes in the freezing
& boiling points.
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Vapor Pressure Lowering

The presence of a non-volatile solute means that
fewer solvent particles are at the solution’s surface,
so less solvent evaporates!
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Raoult’s Law
Describes vapor pressure lowering mathematically

The lowering of the vapour pressure when a
non-volatile solute is dissolved in a volatile
solvent (A) can be described by Raoult’s Law:
PA = cAP°A
only the solvent (A) contributes to
the vapour pressure of the solution
PA = vapour pressure of solvent A above solution
XA = mole fraction of the solvent A in solution.
P°A = vapour pressure of pure solvent A .
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Mixtures of Volatile Liquids
Both liquids evaporate & contribute to the vapor pressure
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Raoult’s Law: Mixing Two Volatile Liquids

Since BOTH liquids are volatile and contribute to the
vapour, the total vapor pressure can be represented
using Dalton’s Law:
PT = PA + PB
The vapor pressure from each component follows
Raoult’s Law:
PT = cAP°A + cBP°B
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Also, cA + cB = 1 (since there are 2 components)
Ideal solutions obtained if solute-solute, solute-solvent,
and solvent-solvent interactions are similar,
i.e. ΔH = 0.
soln
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Deviations from ideality occur if, , there are strong solutesolvent
interactions as may be in H-bonding between solute and
solvent.
Such solutions are called nonideal solutions.
― Deviations from Raoult’s law
ΔHsoln << 0 ⇒ negative deviation
ΔHsoln >> 0 ⇒ positive deviation
Benzene - Toluene mixture:
– The vapor pressure from each component follows
Raoult's Law.
Recall that with only two components, c Bz + c Tol = 1
Benzene: when cBz = 1, PBz = P°Bz = 384 torr &
when cBz = 0 , PBz = 0
Toluene: when cTol = 1, PTol = P°Tol = 133 torr &
when cTol
= 0, PBz = 0
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384 torr
384 torr
P (Total)
P (Benzene)
133 torr
133 torr
P (Toluene)
0
X Benzene
1
1
X Toluene
0
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Normal Boiling Process
Normal Boiling Point: BP of Substance @ 1atm
When solute is added , BP > Normal BP
Boiling point is elevated when solute inhibits solvent from escaping.
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Boiling Point Elevation
ΔTb = (Tb -Tb°) = i ·m ·kb
Where, ΔTb = BP. Elevation
Tb = BP of solvent in solution
Tb° = BP of pure solvent
m = molality , kb = BP Constant
Some Boiling Point Elevation and Freezing Point Depression Constants
Solvent
Normal bp (°C)
pure solvent
Kb
(°C/m)
Normal fp (°C)
pure solvent
Kf
(°C/m)
Water
Benzene
Camphor
Chloroform
100.00
80.10
207
61.70
+0.5121
+2.53
+5.611
+3.63
0.0
5.50
179.75
- 63.5
1.86
4.90
39.7
4.70
(CH3Cl)
Boiling Point Elevation and
Freezing Point Depression
∆T = i K m
i = van’t Hoff factor = number of particles per
molecule/formula unit.
For covalent compounds, i = 1.
For ionic compounds, i = the number of ions
Compound
glycol
NaCl
CaCl2
Ca3(PO4)2
Theoretical Value of i
1
2
3
5
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Freezing Point Depression
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Normal Freezing Point: FP of Substance @ 1atm
When solute is added, FP < Normal FP
FP is depressed when solute inhibits solvent from crystallizing.
When solution freezes the solid form is almost
always pure.
Solute particles does not fit into the crystal lattice
of the solvent because of the differences in size.
The solute essentially remains in solution and
blocks other solvent from fitting into the crystal
lattice during the freezing process.
http://www.wsd1.org/grantpark/staff/patenaude/powerpoint/Solutions_30SE.ppt
Freezing Point Depression
DTf = i ·m ·kf
Phase Diagram and the lowering of the
freezing point.
Where, DTf = FP depression
i = van’t Hoff Factor
m = molality , kf = FP Constant
Generally freezing point
depression is used to determine
the molar mass of an unknown
substance.
Derive an equation to find
molar mass from the
equation above.
http://www.wsd1.org/grantpark/staff/patenaude/powerpoint/Solutions_30SE.ppt
Osmotic pressure
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Osmosis is the spontaneous movement of water
across a semi-permeable membrane from an area of
low solute concentration to an area of high solute
concentration
Osmotic Pressure - The Pressure that must be
applied to stop osmosis
P = i CRT
where P = osmotic pressure
i = van’t Hoff factor
C = molarity
R = ideal gas constant
T = Kelvin temperature
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Osmosis and Blood Cells
(a) A cell placed in an isotonic solution. The net movement of
water in and out of the cell is zero because the concentration of
solutes inside and outside the cell is the same.
(b) In a hypertonic solution, the concentration of solutes outside
the cell is greater than that inside. There is a net flow of water out
of the cell, causing the cell to dehydrate, shrink, and perhaps die.
(c) In a hypotonic solution, the concentration of solutes outside of
the cell is less than that inside. There is a net flow of water into the
cell, causing the cell to swell and perhaps to burst.