Ch 11 States of Matter and Intermolecular Forces
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Transcript of Ch 11 States of Matter and Intermolecular Forces
Ch 11 States of Matter and Intermolecular Forces
Intramolecular forces (bonds) govern molecular properties.
Intermolecular forces are between molecules and determine the macroscopic physical properties of liquids and solids.
This chapter: describes changes from one state of matter to
another. explores the types of intermolecular forces that
underlie these and other physical properties of substances.
Chapter 11 Preview
Review:Intramolecular Forces
Ionic Covalent Metallic
Ionic bond- electron donated and accepted
Covalent- sharing electrons
Metallic-sea of electrons
Ionic Bonds as“Intermolecular” Forces
There are no molecules in an ionic solid, and therefore there can’t be any intermolecular forces.
These forces increase: as the charges on the ions increase. as the ionic radii (size) decrease.
Interionic Forces of Attraction
Melting point of NaCl is about 801 oC.
Mg2+ and O2– have much stronger forces of attraction for one another than do Na+ and Cl–. Melting point of MgO is about 2800 oC.
Molecular Forces Compared
States of Matter ComparedIntermolecular
forces are of little significance; why?
Intermolecular forces must be
considered.
Intermolecular forces are very
important.
Intermolecular Forces
Hydrogen bonding Dipole-dipole London Forces
Cohesion
Attraction for each other Water Mercury
Boiling point varies based on cohesion
Adhesion
A liquids attraction for solid particles Water’s attraction for glass etc.
WATER MERCURY
Meniscus Formation
Water wets the glass (adhesive forces) and its
attraction for glass forms a concave-
up surface.
What conclusion can we draw about the cohesive forces
in mercury?
Plant root- capillary action
Surface Tension The forces present in liquids A molecule inside the liquid experiences
cohesive forces with other molecules in all directions.
A molecule at the surface experiences only downward cohesive forces
Surface TensionThere is no force above the molecule on the top of the
surface.
Pool ball floating on mercury
Adhesive and Cohesive Forces
The liquid spreads, because adhesive forces
are comparable in strength to cohesive
forces.
The liquid “beads up.” Which forces are
stronger, adhesive or cohesive?
Cohesive vs. Adhesive Water on plastic Water on metal Water on glass
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Intermolecular Forces
Hydrogen bonding Dipole-dipole London Forces
Hydrogen bonding A hydrogen is attracted to a highly
electronegative atom like O, N, or Cl.
Hydrogen Bonding in Water
Hydrogen Bonding in Ice
Hydrogen bonding arranges the water molecules into an
open hexagonal pattern.
“Hexagonal” is reflected in the crystal structure. “Open”
means reduced density of the solid (vs. liquid).
Hydrogen Bonding in Acetic Acid
Hydrogen bonding occurs between
molecules.
Intermolecular Hydrogen Bonds
Intermolecular hydrogen bonds give proteins their secondary shape, forcing
the protein molecules into particular orientations, like a folded sheet …
Intramolecular Hydrogen Bonds
… while intramolecular
hydrogen bonds can cause proteins to
take a helical shape.
In which of these substances is hydrogen bonding an important intermolecular force: N2, HI, HF, CH3CHO, and CH3OH? Explain.
In which of these substances is hydrogen bonding an important intermolecular force: N2, HI, CH3CHO, and CH3OH? Explain.
CH3CHO and CH3OH because of the attraction between the H of one molecule and the O of another.
HI would not have hydrogen bonding b/c iodine is not highly electronegative.
Dipole–Dipole Forces
A polar molecule has a positively charged “end” (δ+) and a negatively charged “end” (δ–).
When molecules come close to one another, repulsions occur between like-charged regions of dipoles. Opposite charges tend to attract one another.
Dipole Forces
The more polar a molecule, the more pronounced is the effect of dipole–dipole forces on physical properties.
Dipole–Dipole Interactions
Opposites attract!
London Forces-aka Dispersion Forces
At first no dipole, like Argon. But the electrons are mobile, and at any one
instant they might find themselves towards one end of the molecule, making that end -. The other end will be temporarily short of electrons and so becomes +.
An instant later the electrons may have moved up to the other end, reversing the polarity of the molecule.
Induced London ForcesWhat would happen if we mixed HCl with the
element argon, which has no dipole? The electrons on an argon atom are distributed
homogeneously around the nucleus of the atom. But these electrons are in constant motion. When an argon atom comes close to a polar HCl molecule, the electrons can shift to one side of the nucleus to produce a very small dipole moment that lasts for only an instant.
Dispersion Forces Illustrated (1)At a given instant, electron density, even in a nonpolar
molecule like this one, is not perfectly uniform.
Dispersion Forces Illustrated (2)
The region of (momentary) higher
electron density attains a small (–) charge …
When another nonpolar molecule
approaches …
… the other end of the molecule is
slightly (+).
Dispersion Forces Illustrated (3)… this molecule induces a tiny
dipole moment …
… in this molecule.
Opposite charges ________.
Types of forces
Click here for an Animation of the forces
Molecular Shape and PolarizabilityLong skinny molecule …
… can have greater separation of charge along its length. Stronger forces of attraction, meaning …… higher
boiling point.
In the compact isomer, less possible separation of charge …
… giving weaker dispersion forces and a lower boiling point.
Arrange the following substances in the expected order of increasing boiling point: Carbon tetrabromide, CBr4; Butane, CH3CH2CH2CH3; Fluorine, F2;Acetaldehyde, CH3CHO.
Arrange the following substances in the expected order of increasing boiling point: Carbon tetrabromide, CBr4; Butane, CH3CH2CH2CH3; Fluorine, F2;Acetaldehyde, CH3CHO.
Answer: F2, CBr4, CH3CH2CH2CH3, CH3CHO
Vapor Pressure
The vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with the liquid at a constant temperature.
vaporizationLiquid Vapor
condensation
Liquid–Vapor Equilibrium
More vapor forms; rate of condensation of that
vapor increases …
… until equilibrium is attained.
Phase DiagramsA phase diagram is a graphical representation of the conditions of temperature and pressure under which a substance exists as a solid, liquid, a gas, or some combination of these in equilibrium.
A—B, solid-vapor equilibrium.
A—D, solid-liquid equilibrium.
A—C, liquid-vapor equilibrium.
Triple point
Phase Diagram for CO2
Note that at 1 atm, only the solid and
vapor phases of CO2 exist.
Phase Diagram for H2O
Supercritical Fluid
Above the critical temperature and pressure, only one phase exists…a combination of liquid and gas.
Properties are in between those of liquids and gases.
They act as solvents and dissolve well. They diffuse well like gases. CO2 and H2O- environmentally friendly
The Critical PointAt room temperature there is relatively little vapor, and
its density is low.
At higher temperature, there is more vapor, and its
density increases …
… while the density of the liquid decreases; molecular
motion increases.
At Tc, the densities of liquid and vapor are equal; a single
phase.