IM Forces
Forces Between Particles in Solids
and Liquids
• Ionic compounds
– Attractive forces between oppositely charged
ions hold ionic compounds together.
– Ionic bonds are the strongest interparticle
force.
– Smaller the ion and the larger the charge on
the ion the stronger the attractive forces
among the ions
Ionic
Bonding
Forces Between Particles in Solids
and Liquids
• Forces between molecular compounds
– Intermolecular (IM) forces between molecules
attract molecules to each other in the liquid
and solid state.
• IM forces are very weak as compared to ionic or
covalent bonds
States of Matter
IM Forces
Three types of IM Forces
1. Dipole-dipole force
2. Hydrogen “bonding”
3. London dispersion forces
Interparticle Forces and
Physical Properties
• The stronger the attractive forces between
particles in a liquid or solid, the
– Higher the:
•
•
•
•
Melting point
Boiling point
Surface tension
Viscosity
– Lower the:
• Vapor pressure
IM Forces
• Dipole-dipole forces
– Attractive forces between oppositely charged
dipoles.
– Dipole-dipole forces are found between polar
compounds.
• The more polar the compound the stronger the
dipole-dipole force.
IM Forces
• Hydrogen “bonds”
– Attractive force between a d+ H bonded to an
O, N, or F and a d- O, N, or F generally on
another molecule.
• Really a relatively strong dipole-dipole force
– Hydrogen bonding is the strongest of the IM
forces.
– H bonding is very important in water and in
many biological molecules.
• Hydrogen “bond” is a weak attractive
force between a d + hydrogen and a
d- O, N, or F in a second polar bond
London Dispersion Forces
• London Dispersion force
– Very weak and short-lasting attractive forces
between temporary dipoles
• See figure 10.5
– Weakest of the IM forces
London Dispersion Forces
• London Dispersion forces
– Found between all molecules in liquid/solid
state.
• Of greatest significance in nonpolar molecules as
it’s the only IM force between nonpolar molecules
– The larger the molecule the stronger the
dipersion forces.
Dispersion Forces
Occur between every compound and arise from the net attractive forces
amount molecules which is produced from induced charge imbalances
The magnitude of the Dispersion Forces
is dependent upon how easily it
is to distort the electron cloud.
The larger the molecule the greater
it’s Dispersion Forces are.
Dispersion Forces and
Molecular Shape
• Elongated ['i:lɔŋɡeitid, i'lɔŋ-] molecules
have higher dispersion forces分散力than
compact [kəm'pækt, 'kɔmpækt]简洁
molecules
• Ringed structures have higher dispersion
forces than straight chain molecules.
– Consider:
• Hexane
• Cyclohexane
• 2,2 – dimethyl butane
Interparticle Forces
• Weakest to Strongest:
Intermolecular forces – all relatively weak
London dispersion forces
Dipole-dipole force
Hydrogen Bonding
Ionic bond - BY FAR THE Strongest:
- not an IM Force
Properties of Liquids
• Freezing and boiling point
• Surface tension
• Capillary action
• Viscosity [vi'skɔsəti]粘性，[
Which are directly related to the strength
of the IM forces present between
molecules?
Change of State
• Normal Freezing/Melting point
– temperature at which the liquid and solid state
co-exist at 1 atm pressure
• Normal boiling point
– temperature at which the liquid and gaseous
state co-exist at 1 atm pressure
• Predict the relative BP of:
– Methane, acetone, methanol, ethanol, NaCl
Surface Tension
• Surface tension
– Resistance of a liquid to increase its surface
area
– Measure of the energy needed to break the
IM forces at the surface
Capillary Action
• Capillary action
– Spontaneous rising of a liquid in a narrow
tube
• Related terms:
– Cohesive forces – attractive forces among like
molecules
– Adhesive forces – attractive forces among
dislike molecules
See Figure 10.7, page 444
Concave meniscus
Adhesion > Cohesion
Convex meniscus
Cohesion > adhesion
Viscosity
• Viscosity – resistance of a liquid to flow
– Highly viscous liquids are thick (syrupy)
– Consider relative viscosity of:
• Propanol
• Glycerol
Graphite
• Layers of ringed carbon structures
– Each C is bonded to 3 other C
– Each C is sp2 hybridized
Diamond
• A diamond is a gigantic molecule, each C
atom is bonded to 4 other C atoms
• Each C is sp3 hybridized
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
11.9
11.9
CH 11: Properties of Solutions
1. Describing Solutions – concentration
units
2. Energetics of solution formation
3. Colligative Properties of solutions
•
•
•
•
BP elevation
FP depression
Osmotic pressure
Vapor Pressure
Terms
• Solution – homogeneous mixture
• Solvent – generally the larger component
of the solution
– Determines the physical state of the solution
• Solute – generally the smaller component
of the solution
– Solute is dispersed in the solvent
Solution Composition
• Concentrated solution – relatively large
amount of solute
• Dilute solution – relatively small amount of
solute
Solution Composition
• Unsaturated solution –solution with less
than the maximum amount of solute that
will normally dissolve at a given
temperature
• Saturated solution - solution with
maximum amount of solute that will
normally dissolve at a given temperature
Solution Composition
• Super-saturated solution - solution with
more than the maximum amount of solute
that will normally dissolve at a given
temperature
Concentration Units
Molarity (M) = moles solute/Liters solution
Molality (m) = moles solute/kg solvent
Mass % = Mass solute/mass solution x100%
Mole fraction (cA) = moles A/total moles
Practice!
• Start by writing definitions for the
concentration units
M=
m=
Mass % =
Mole fraction =
Starting with Molarity
Solution:
– 3.75 M H2SO4 solution with a density of 1.23
g/mL
Calculate:
– Mass %
– Molality
– mole fraction of H2SO4
Starting with Masses
Solution:
– A solution is made by combining 66.0 grams of
acetone (C3H6 O) with 146.0 grams of water.
– Solution has a density of 0.926 g/mL
Calculate:
–
–
–
–
Molarity – need volume of solution
Mass %
Molality
Mole fraction of acetone
Starting with Mass %
Solution:
– 35.4 % H3PO4
– Density of 1.20 g/mL
Calculate:
– Molarity
– Molality
– Mole fraction of H3PO4
Starting with Molality
Solution:
– 2.50 m HCl solution
– Density of 1.15 g/mL
Calculate:
– Molarity – need _______
– Mass %
– Mole fraction of HCl
Solution Formation
Formation of a solution involves 3 steps
1. Separate the solute particles
•
expand the solute
2. Separate the solvent particles
•
Expand the solvent
3. Form the solution
– Solute and solvent interact
Solution Formation
• Each step of solution formation involves
energy and has a DH.
DH1 = energy needed to separate the
solute
DH2 = energy needed to separate the
solvent
DH3 = energy released when solution
forms
Solution Formation
DHsolution =
DH1 + DH2 + DH3
Solutions form when the DHsolution is a small
value – see page 492
Factors Impacting Solubility
• Structure – like dissolves like
– #38 on page 520
Factors Impacting Solubility
• Pressure
– Pressure has little impact on the solubility of
liquids and solids
– Pressure has a significant impact on the
solubility of gases in a liquid
• The higher the pressure of gaseous solute above a
liquid the higher the concentration of the gas in the
solution
Henry’s Law
• Henry’s Law: C = kP
C = Concentration of dissolved gas
k = solution specific constant
P = partial P of the solute gas above
the solution
• No calculations required.
Temperature and Solubility
• Temperature has variable effects on the
amount of solid that will dissolve in an
aqueous solution!
– See figure 11.6 page 496
• Solutes do dissolve more rapidly at higher
temperatures
Temperature and Solubility
• The solubility of a gas in water decreases
as temperature increases.
– See figure 11.7 on page 496
– Thermal pollution – read the story on page
497 when you get a chance
Vapor Pressure of Solutions
• See Raoult’s Law on page 498
• Psolution = csolvent P0 solvent
Colligative Properties
• Colligative properties
– properties of a solution that depend upon the
amount of dissolved solute, not the identity of
the solute.
• Freezing point depression
• Boiling point elevation
• Osmotic Pressure
• Note: I will be weaving section 11.7 and the van’t
Hoff factor (i) into my consideration of these
properties and not consider it separately.
Colligative Properties
D FP = Kf m i
D BP = Kb m i
See page 505 for needed constants
1. Calculating the bp or fp of a solution
2. Calculating the molar mass of a solute
from fp or bp data
Osmotic Pressure
• Osmotic Pressure (P) is often used to
determine the molar mass of large
biological molecules
– See figure 11.17 on page 508
P = MRTi