chapter 12 intermolecular forces and liquids · chapter 12 intermolecular forces and liquids ......
TRANSCRIPT
Jeffrey Mack
California State University,
Sacramento
Chapter 12 Intermolecular Forces and Liquids
• Why is water usually a liquid and not a gas?
• Why does liquid water boil at such a high
temperature for such a small molecule?
• Why does ice float on water?
• Why do snowflakes have 6 sides?
• Why is I2 a solid whereas Cl2 is a gas?
• Why are NaCl crystals little cubes?
All of these questions may be answered by
“Intermolecular Forces”
Why?
• The forces holding solids and liquids together are
called intermolecular forces.
• Intermolecular Forces are the attractions and
repulsions between molecules.
• They are NOT chemical bonds.
• The intermolecular forces of a substance may
exhibit are a function of:
1. charge (ions vs. neutrals)
2. polarity (molecular shape, dipoles)
3. molar mass
Intermolecular Forces
Intermolecular forces influence chemistry in many
ways:
• They are directly related to properties such as
melting point, boiling point, and the energy
needed to convert a solid to a liquid or a liquid to
a vapor.
• They are important in determining the solubility
of gases, liquids, and solids in various solvents.
• They are crucial in determining the structures of
biologically important molecules such as DNA
and proteins.
Intermolecular Forces
C–H (413 kJ/mol)
C=C (610 kJ/mol)
C–C (346 kJ/mol)
CN (887 kJ/mol)
Covalent Bonding Forces for Comparison of Magnitude
Intermolecular forces are much weaker than the bonds that
make up compounds.
20 to 30 kJ/mol
D(H-Cl) = 432 kJ/mol
Covalent Bonding Forces for Comparison of Magnitude
The forces that govern charged particles are defined by
Coulomb’s law.
These are the strong forces that lead to salts with high melting
temperatures.
2
Q QF k
r
NaCl, mp = 800
°C
Q = the charges on the cation and
anion
r = the distance between
k = a constant
MgO, mp = 2800 °C H2O, mp = 0 °C
Greater charge = stronger attraction
Greater distance = weaker attraction
Ion-Ion Forces: Formal Charges
The polar nature of water provides for attractive forces between ions and water.
Attractions Between Ions & Permanent Dipoles
When a cation exists in solution, it is surrounded by the
negative dipole ends of water molecules.
When as anion exists in solution, it is surrounded by
the positive dipole ends of water molecules.
+
Solvation of Ions
As the size of the ion increases, the
exothermicity of the process decreases.
This is due to the weaker ion-dipole forces.
Enthalpies of Hydration: A Measure of Ion-Dipole Forces
As the size of the ion increases, the
exothermicity of the process decreases.
This is due to the weaker ion-dipole forces.
Enthalpies of Hydration: A Measure of Ion-Dipole Forces
Water is highly polar and can interact with positive ions to give hydrated ions in water.
Attraction Between Ions & Permanent Dipoles
Molecular Geometry Non-Polar Molecule
Linear Both atoms the same (outer the same for linear tri-atomic)
Trigonal Planar All bonding groups the same
Tetrahedral All bonding groups the same
Trigonal bipyramidal All bonding groups the same or both axial groups the same and all three equatorial groups the same,
Octahedral All bonding groups the same or all groups trans to one another the same.
Any deviations of symmetry yield a polar molecule.
Molecular Polarity
Dipole-dipole forces bind molecules having permanent dipoles to one another.
Dipole-Dipole Forces
Compound Molar Mass
(amu)
Dipole
Moment
(D)
BP
(K)
CH3CH2CH3 44.1 0.1 231
CH3OCH3 46.07 1.3 248
CH3Cl 40.49 1.9 249
CH3CN 41.05 3.9 355
molar Mass
Dipole
BP
0
50
100
150
200
250
300
350
400
1
2
3
4
Dipole Moment
Boiling point
Molar Mass
As the polarity for a
given set of molecules
with similar molar
masses increases, the
boiling point increases.
Dipole-Dipole Forces
A special form of dipole-dipole attraction, which
enhances dipole-dipole attractions.
H-bonding is strongest when X and Y are N, O, or F
Hydrogen Bonding
capillary action surface tension why ice floats
The water molecules
network with one
another.
H–bonding in water
brings about a
network of
interactions which
explain phenomena
such as:
Hydrogen Bonding
Molecules at surface behave differently than those in the
interior.
• Molecules at surface experience net INWARD force of attraction.
• This leads to SURFACE TENSION — the energy reqired to break the surface.
Surface Tension
SURFACE TENSION also leads to spherical
liquid droplets.
Surface Tension
IMF’s also lead to CAPILLARY action and to the
existence of a concave meniscus for a water column.
concave
meniscus
ATTRACTIVE FORCES
between water
and glass
COHESIVE FORCES
between water
molecules
H2O in
glass
tube
Capillary Action
Movement of water up a piece of paper is a
result of H-bonds between H2O and the OH
groups of the cellulose in the paper.
Capillary Action
• Ice, H2O(s) floats because it is less dense than
water, H2O(l).
• The H–bonds allow the molecules in the liquid phase
to to approach closer than normal for non H–bonding
liquids.
• This is why water has its maximum density at 4 °C.
Ice has open lattice-like structure.
Ice density is < liquid and so solid floats on water.
One of the VERY few
substances where
solid is LESS DENSE
than the liquid.
Hydrogen Bonding in H2O
The Consequences of Hydrogen Bonding
Boiling Points of Simple Hydrogen-Containing Compounds
18 g/mol
20 g/mol
17 g/mol
16 g/mol
Notice that water has an
unusually high bp for its Mwt...
This is a result of
hydrogen bonding!
H-bonding leads to
abnormally high
boiling point of water.
Hydrogen Bonding
Dipole-induced
dipole
How can non-polar molecules such as O2 and
I2 dissolve in water?
The water dipole INDUCES a dipole in the O2
electron cloud.
Forces Involving Induced Dipoles
How can non-polar molecules such as O2 and I2
dissolve in water?
The water dipole INDUCES a dipole in the O2
electron cloud.
Once polarized, the O2 is attracted to additional
water molecules.
Induced Dipole Forces
The degree to which electron cloud of an atom or molecule can
be distorted is measured by its polarizability.
The larger the molecule, the more easily it is polarized.
As the electrons in a molecule become more loosely held and
more spread out, the greater the degree of polarizibility in
the molecule.
The explains the trend we see in solubility.
Induced Dipole Forces
Formation of a dipole in two nonpolar I2 molecules.
Induced dipole-
induced dipole
Forces Involving Induced Dipoles
London dispersion forces exist between all
molecules.
London dispersion forces are a function of molecular
polarizability.
The Polarizability of a molecule is measured by the
ease with which an electron cloud can be distorted.
The larger the molecule (the greater the number of
electrons) the greater polarizability. The greater the
surface area available for contact, the greater the
dispersion forces.
London dispersion forces therefore increase as
molecular weight increases.
London Dispersion Forces
For molecules with the same relative polarizability, the forces
scale with molar mass:
CH4
C2H6
C3H8
C4H10
Note the linear relation between bp and molar mass.
Higher Mwt. = larger induced dipoles.
Molecule BP (oC)
CH4 (methane) - 161.5
C2H6 (ethane) - 88.6
C3H8 (propane) - 42.1
C4H10 (butane) - 0.5
London Dispersion Forces
The induced forces between I2 molecules are very weak, so solid I2 sublimes (goes from a solid to gaseous molecules).
Forces Involving Induced Dipoles
Intermolecular Forces Summary
Intermolecular Forces
• Of the three states of matter, liquids are the
most difficult to describe precisely.
• Under ideal conditions the molecules in a gas
are far apart and are considered to be
independent of one another.
• The structures of solids can be described easily
because the particles that make up solids are
usually in an orderly arrangement.
• The particles of a liquid interact with their
neighbors, like the particles in a solid, but, unlike
in solids, there is little long-range order.
Properties of Liquids
Liquids
• Particles are in constant
motion.
• Particles are in close
contact.
• Liquids are almost
incompressible
• Liquids do not fill the
container.
• Intermolecular forces are
relevant.
Properties of Liquids
Breaking IM forces requires energy.
The process of vaporization is
therefore endothermic.
In order for a
liquid to vaporize,
sufficient energy
must be available
to overcome the
intermolecular
forces.
Liquids: Vaporization
The HEAT OF VAPORIZATION is the heat required to
vaporize the liquid at constant P.
vapH
Liquid + energy = Vapor
Notice how the types of forces greatly affects the Hvap and boiling point.
Compound IMF ∆vapH (kJ/mol) BP
H2O
SO2
Xe
H-bonds
Dipole
London
40.7
26.8
12.6
100 °C
47 °C
107 °C
Liquids: Enthalpy of Vaporization
When molecules of liquid are in the vapor state, they
exert a VAPOR PRESSURE.
The EQUILIBRIUM VAPOR
PRESSURE is the pressure
exerted by a vapor over a liquid
in a closed container.
rate of evaporation = the rate of condensation.
At equilibrium,
Liquids: Enthalpy of Vaporization
When molecules of liquid
are in the vapor state, they
exert a VAPOR PRESSURE
EQUILIBRIUM VAPOR
PRESSURE is the pressure
exerted by a vapor over a
liquid in a closed container
when the rate of evaporation
= the rate of condensation.
Vapor Pressure
Recall from kinetic molecular theory…
As Temp increases, so does the average KE of the
particles.
This means that there are more particles that can
escape into the gas phase!
Vapor Pressure
Liquid boil when Pvap = Patm
(Vapor pressure equals
atmospheric pressure.
Boiling Point
As the external pressure is lowered, the vapor pressure equals the external pressure at a lower temperature. Boiling therefore occurs at a reduced temperature.
Boiling Point at Reduced Pressure
When can cools, vapor pressure of water drops. Pressure inside of the can is less than that of atmosphere, which collapses the can.
Consequences of Vapor Pressure Changes
The vapor pressure of a liquid is seen to increase
exponentially with temperature.
Equilibrium Vapor Pressure
Liquid in flask evaporates and exerts pressure on
manometer.
Measuring Equilibrium Vapor Pressure
A plot of lnPvap vs. yields a
slope of:
∆vapH° is related to T and P by
the Clausius-Clapeyron
equation
vap
vap
HlnP C
RT
1
T
vap
slope :
H
R
D-
y-intercept
= C
The Temperature Dependence of Vapor Pressure Goes As:
Molecules in the Liquid
State vapH Volatility
Equilibrium Vapor
Pressure
Boiling Point
Strong IMF’s
More Endothermic
Low Low High
Weak IMF’s
Less Endothermic
High High Low
Liquids: IMF’s Summary