1_Solution Chemistry.ppt

1
Chemical Mixtures
• Many materials exist as homogeneous mixtures
– air (g), lake water (l), gasoline (l), stainless steel (s)
• Mixtures include suspensions, colloids, and
solutions
• Solution - homogeneous mixture of of two or more
substances; often comprised of solvent and solute, e.g., when
gases or solids are dissolved in a liquid
• Solvent - component with same phase as solution; substance
present in excess in liquid-liquid mixtures
• Solute - minor component mixed with solvent
• Degree of mixing of fluids
– Miscible - mix or dissolve in all proportions
– Immiscible - form layers upon mixing
• Solubility - maximal amount of solute accommodated
by solvent at specified temperature

2
Molecular Forces
• All condensed matter has intermolecular attractive
forces (IAFs) between particles (formula units, ions,
molecules)
• Predominant attractive forces
• Coulombic - attraction of oppositely charged
particles
• van der Waals forces: a. dispersion (London);
b. dipole-dipole; c. ion-dipole
• Hydrogen-bonding
• Solutions form when it is energetically favorable
• Solutions form when substances with similar IAFs are
mixed: “like dissolves like,” compounds of like polarity
are most compatible

3
Lattice and Hydration Energies
• Lattice Energy – energy of attraction between
counter ions in an ionic solid (Coulombic forces)
– Lattice bonds must be broken to separate ions in
solid
– IAF must be broken to separate solvent (water)
molecules; this involves breaking hydrogen bond
network between water molecules
• Hydration Energy – New IAF formed between
ions and water molecules; involves forming ion-
dipole bonds
– Water orients around released ions according to
polarity

4
Energy Changes
• Heat of Solution (Hsoln); Heat flow in dissolution
solute-solute interactions (+H1 to break IFAs)
solvent-solvent interactions (+H2 to break IFAs)
solute-solvent interactions (-H3 to form IFAs)
Hsoln = H1 + H2 + H3
The sign of Hsoln (+ or -) depends on the magnitudes
of H’s 1-3
• Entropy of Solution (Ssoln); Randomness in dissolution
When substances are mixed, Ssoln is + because disorder
is increased (or order is decreased)

5
Solution Process
• Factors which promote dissolution are:
– increase in randomness of system (entropy)
– release of heat upon dissolution (enthalpy), IAFs
• this occurs when solvent-solute IAF bonds are
very strong
• Dissolving an ionic solid in water
– water molecules collide with exterior ions of the solid
lattice
– ions break free from lattice and are hydrated by
water molecules, stabilizing the ion in solution
– extent of dissolution depends on lattice vs.
hydration energies

6
Predicting Solubility
• Substances that are compatible have
similar IAFs (i.e., like dissolves like)
• Nonpolar substances (e.g., hydrocarbons)
have dispersion force IAFs
• Polar substances have dipole driven IAFs
(e.g., methanol)
• Substances with similar IAFs mix to the
greatest extent
– Ionic compounds are most soluble in polar
solvents

7
Solution Nomenclature
• Solution Formation: Dissolving a solid in a liquid
• undersaturated solution (below solubility limit)
NaCl(s) Na+(aq) + Cl-(aq) ionic
C6H12O6(s)  C6H12O6(aq) molecular
• saturated solution (at solubility limit)
NaCl(s)  Na+(aq) + Cl-(aq) Dynamic Equilibrium
C6H12O6(s)  C6H12O6(aq)  =
reversible
rate of solution = rate of crystallization; heterogeneous mixture
forms
s = constant @ T with introduction of more solute
• supersaturated solution - solution which contains
more than saturated concentration of solute

8
Effect of P on Solutions
• P changes do not greatly effect solubility of liquids
and solids significantly aside from extreme P changes
(vacuum or very high pressure)
• Gases are compressible fluids and therefore
solubilities in liquids are P dependent
• Solubility of gas in liquid  partial pressure of same
gas (only) above solution
• Quantitatively defined in terms of Henry’s Law:
Cg = kPg
Cg = solubility of gas (mol/L); Pg = partial pressure of
gas (atm); k = Henry’s Law Constant (mol/L-atm)

9
Effects of T on Solutions
• Solubility of the solute is temperature dependent
• Gaseous solutes in liquid solvents
 T leads to solubility (& vice versa)
A(g) A(l)  A(aq) H < 0
• Solid solutes in liquid solvents
– most salts:  T leads to solubility (& vice versa)
A(s) A(l)  A(aq) H > 0
– some ionic compounds:  T leads to solubility (&
vice versa)
A(s) A(l)  A(aq) H <0
• Liquid solute behavior in liquid solvents is unpredictable

10
Concentration Terms
solvent
of
kg
solute
mol
(m)
Molality =
0.5 mol of solute in 1 kg of solvent = 0.5 molal solution
molality is not temperature dependent
solution
of
mol
solute
of
mol
(X)
fraction
Mole =
1 mol of solute (A) in 3 mol of solvent (B) = 0.25
Mole Fraction Convention:
A = solute and B = solvent

11
Concentration Terms
solution
L
solute
mol
(M)
Molarity =
0.5 mol of solute in 0.5 L of sol’n = 1 molar solution
molarity is temperature dependent
100
solution
of
mass
solute
of
mass
(%)
percent
Mass 
=
1 g of solute in 100 g of solution = 1% solution
Remember that solution mass or volume =
mass/volume of solute + mass/volume of solvent

12
Colligative Properties of Solutions
• Important solution properties which show colligative
behavior
– vapor pressure, Pv
– freezing point, Tf
– boiling point, Tb
– osmotic pressure, 
• Colligative properties: solution properties which
depend only on the relative number of solute to solvent
particles (solute concentration) and not on the chemical
properties of the solute
• Differences between solvent and solution illustrated in
phase diagram (P versus T plot)

13
Colligative Mathematical
Property Relation
1. vapor pressure
(Raoult’s Law)
2. freezing point depression
3. boiling point elevation
4. osmosis
o
A
A
A P
X
P =
m
K
T f
f =

m
K
T b
b =

MRT
=


14
Vapor Pressure, Pv, and Vapor
Pressure Lowering, Pv
• Vapor pressure: partial pressure of vapor above
liquid
• Solution vapor pressure
– Examples include volatile solvent with non-volatile
solute and volatile solute and volatile solvent
– In example of non-volatile solute, vapor pressure
of solution proportional to concentration of solvent
Pv,A  [solvent A] = XAPo
A
As solvent concentration decreases, Pv decreases
– In example of volatile solute (B) and solvent (A)
PT = PA + PB = XAPo
A + XBPo
B

15
14
Semipermeable membrane
  
 
  
 
  
 
  
 
= solute molecule
 = solvent molecule
1 2
•Osmosis = solvent flow across membrane
•Net movement of solvent is from 2 to 1; pressure increases
in 1
•PA,1 < P A,2, osmosis follows solvent vapor pressure
•Osmotic Pressure = pressure applied to just stop osmosis
2 Compartment Chamber

1_Solution Chemistry.ppt

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