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Intermolecular Forces and Liquids and Solids


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Name six characteristics of gases?
– Constant, random motion of gas molecules
– Great distances between the molecules
– No appreciable interactions between molecules
– Gases are readily compressed
– Expand to fill the container they are in
– Low densities
What is the largest difference between liquids, solids,and gases?
distance between the molecules
What are liquids, solids, and gases called?
Liquids and solids are called condensed states
Gases are called gaseous states
Name seven characteristics of liquid?
– Very little space between molecules
– Much more dense
– Much less compressible
– Held together by attractive forces
– Definite volume
– Molecules do move freely past each other
– Assumes the shape of the container
Names seven characteristics of solids?
– Molecules rigidly held together
– No freedom of motion
– Molecules are held together in regular configurations in three dimensions
– Less empty space than liquids
– Almost incompressible
– Definite shape and volume
– Almost always more dense than liquid (exception: water)
Constant, random motion?
Molecules are held together in regular configurations in three dimensions
Much less compressible?
Held together by attractive forces?
Almost always more dense than liquid (exception: water)
No appreciable interactions between molecules
readily compressed
What are phases?
Different states of a substance.
A homogeneous part of the system in contact with other parts of the system but separated from them by well defined boundaries.
A glass of ice water contains both the solid phase (ice) and liquid phase (water).
What are intermolecular forces?
Attractive forces between molecules
What forces are responsible fr nonieal gas behavior?
When gases decrease in temperatue what happen to the KE?
The Kinetic energy of the moleucles decrease.
What happen when the energy get low enough?
When the energy gets low enough the molecules aggregate and form drops of liquid.
Called condensation
What is condensation?
Transition from the gaseous to the liquid phase is known as condensation.
What are intramolecular forces?
The forces within a molecule that hold atoms together in a molecule.
What do intramolecular forces do and how are they different from intermolecular?
Forces that help to stabilize molecules.
Intermolecular forces are responsible for the properties of matter.
Intermolecular forces are much weaker than intramolecular forces.
What are the types of Van der Waals forces?
Dipole-dipole interactions
Dipole-induced dipole interactions
Dispersion forces
What are ion-dipole forces?
Ions and dipoles arre attracted to one another by electrostatic forces.
What is hydrogen bonding?
Strong type of dipole-dipole interaction. Only a few elements can participate in hydrogen bond formation. More than one type of forces may be present.
What are Dipole-Dipole forces?
Attractive forces between polar molecules.
The origin of these forces is electrostatic interactions
The larger the dipole the greater the interaction. In liquids the molecules are not held as rigidly but they still align in a manner that maximizes the interactions.
What is Ion-Dipole forces?
Attract to an ion (either a cation or an anion) and a polar molecule to each other.
What does the strength of interacion depend on in an Ion-Dipole forces?
Strength of interaction depends on charge and size of ion as well as magnitude of the dipole and size of molecule.
Which charges are more concentrated?
The charges on cations are more concentrated than anions. Cations usually interaction more strongly with dipoles.
What is hydration?
Type of ion-dipole interaction
Results from the favorable interactions between ions and polar water molecules.
What are dispersion forces?
Attractive forces that arise from these temporary induced dipoles
At very low temperature the dispersion forces are capable of holding molecules together (liquids)
Dispersion forces are directly related to the polarizability of the atom
Helium has low polarizability and very small dispersion forces as evidenced by He having a boiling point of 4.2K
What is an induced dipole?
When an ion or polar molecule is next to a nonpolar molecule, a dipole is induced in the nonpolar molecule.
Induced dipole – separation of positive and negative charges in the nonpolar molecule is due to the proximity of the other ion or dipole.
What is the polarizability?
The likelihood that a dipole will be induced depends on the polarizability of the the molecule.
The ease with which the electron density of the atom can be distorted.
The larger the atom or molecule the more likely the molecule is to be polarized.
Polarizability allows gases of atoms and nonpolar molecules to?
What is the measured dipole?
The measured dipole is an average of the dipoles over time.
Instantaneous dipoles
The electrons moving around a He atom at any point in time may have an instantaneous dipole.
These instantaneous dipoles can induce dipoles in surrounding molecules.
Where do dispersion forces exist?
Dispersion forces exist in all types of molecules
Dispersion forces are in many cases combined with other types of attractive forces to hold molecules together
What types of intermolecular forces would you expect in the following:
What is the general trend in boiling points?
As a general trend the boiling points of molecules increases with increasing molar mass
What is a hydrogen Bonds?
A speical type of dipole-dipole interaction between the hydrogen atom in a polar bond such as N-H, O-H, or F-H, and an electrongegative O,N, or F atom.
How is the interaction written in a hydrogen bond?
A-H⬦B or A-H⬦A
The strength of a hydrogen bond is determined by?
The strength of the bond is determined by the interaction of the H nucleus with the electron pair of the electronegative atom
The more electronegative an atom is?
The more electronegative the atom the stronger the bond.
So it is expected that the H bonds in HF would be stronger than H bonds in water.
Which of the following molecules would be capable of hydrogen bonding among themselves
What forces exist between intermolecular forces?
All the forces talked about have been attractive forces.
There repulsive forces that exist between the molecules as well.
What happens as the molecules get closer together?
As molecules get closer together the repulsive forces between the electrons and the nulcei of atoms become more important
What are some of liquids properties that are due to the intermolecular forces?
A couple of these properties are surface tension and viscosity
What pulls molecules within a liquid in all directions?
What is the exception to this?
Intermolecular forces
The exception to this is the molecules on the surface of the liquid. Molecules on the surface get pulled down toward the liquid.
This is the principal behind waxing a car or applying Rain-X to windows.
What is surface tension?
Measure of the elastic force in the surface of a liquid.
Amount of energy required to stretch or increase the surface of a liquid by one unit of area.
As the intermolecular forces become greater what happens to the surface tension?
The greater the surface tension. Hydrogen bonding in water greatly increases the intermolecular forces.
Capillary action is brought about by what two types of forces?
Cohesion and Adhesion
What is Cohesion?
Cohesion – intermolecular attraction between like molecules (such as water)
What is Adhesion?
Adhesion – attraction between unlike molecules (water and the tube)
How is cohesion and adhesion related?
Adhesion > cohesion liquid goes up the tube and vice versa
What is viscosity?
Measure of the ability of a liquid to flow.
Greater viscosity means that the liquid flows more slowly.
Liquids with strong intermolecular forces have what kind of viscosities?
high viscosities
What are some charactistics of water?
Involved in all of life’s processes
Water is an excellent solvent for many molecules
Water has a very high specific heat
Most striking property is that the solid form is less dense than the liquid form
The electronic characteristics of water give some insights into the unique nature of water
Two lone pair on the oxygen
The two lone pair and the two covalent bonds are unique to oxygen
Water can form 3-D structures that cannot be duplicated in other molecule
Why do lakes freeze from the top down?
Ice is less dense that water.
As water decreases in temperature the colder water descends to the bottom of the lake.
Water is most dense at 4ºC.
Below this temperature the cold water stays on top.
Once the ice freezes on top of the water it acts as an insulator to the water below.
What are two types of structures in solids?
Crystalline and Amorphous
What is Crystalline?
Crystalline - possess rigid and long-range order; its atoms, molecules, or ions occupy specific positions
What is Amorphous?
Amorphous – lack the well defined arrangement and long-range order seen in crystalline solids (glass)
What is a unit cell?
Unit cell – basic repeating unit of a crystalline solid
What is a lattice point?
Lattice point – the points of the unit cell of a crystalline solid.
The lattice points usually contain a molecule or atom but not always
What is more accurate way of thinking of unit cells?
Packing Spheres
Each atoms takes up space and can be thought of as a sphere.
What is a coordination number?
Coordination number – number of atoms surrounding an atom in a crystal structure.
Measures how closely the crystal is packed.
Higher the coordination number the greater the packing.
How do the atoms pack in order to get the most efficient packing in a molecule?
The body-centered model has less empty space than the simple cubic.
If we have an atom at the center of a hexagon, it would be in contact with six atoms in that leve.l
The next layer packs the atoms into the depressions between the atoms in the first layer.
What is the most efficient ways to pack atoms?
Hexagonal close-packed and cubic close-packed represent the most efficient ways to pack atoms
The conformation that atoms take in the solid state depends on what?
The conformation that atoms take in the solid state depends on the stability of the particular atom in a particular structure.
There is no periodic or group relationship to determine how the atoms pack
What is X-ray diffraction?
The primary method of gathering data from crystals.
The scattering of X-rays by the units of a crystalline solid.
What does the scattering produce?
The scattering produces a pattern that can be used to determine the positions of molecules.
How can X-rays be diffracted by crystals?
The wavelength of the X ray is about the same as the distance between the lattice points in a crystal
Bragg equation?
In order for the waves to reinforce each other the BC
+ CD must?
In order for the waves to reinforce each other the BC + CD must equal an integer multiple of the wavelength
When the waves reinforce each other a dark spot on the photographic film develops
X rays of wavelength 0.154 nm are diffracted from a crystal at an angle of 14.17º. Assuming that n = 1, calculate the distance (in pm) between layers of the crystal.
X-ray diffraction is?
Most accurate method of determining bond lengths and bond angles
X-rays are diffracted by? What does the patterns given from the X-ray diffraction allow?
The patterns given from the X-ray diffraction allow an electron density map to be drawn.
This allows for the determination of the positions of the atoms in a crystal.
The structure and properties of crystals are determined by?
The types of forces that hold the crystals together
What are the four types of crystals?
What are ionic crystals two important characteristics?
1. They are composed of charged species
2. Anions and cations are generally quite different in size.
Knowing the approximate atomic radii is helpful to understanding the structure and stability.
Copper crystallizes in a face-centered cubic lattice (the Cu atoms are at lattice points only). If the density of the metal is 8.96 g/cm3, what is the unit cell length in pm?
Ionic cystals have weak or strong cohesive forces? What is lattice energy?
Have strong cohesive forces as shown by the high melting points of ionic crystals.
Lattice energy is a measure of the stability of ionic crystals.
What are Covalent cystals?
The atoms are held together in an extensive 3-D network held together by covalent bonds
Carbon – graphite or diamonds
What are Molecular crystals?
Molecules located at lattice points held together by van der Waal forces and hydrogen bonds.
Sulfur dioxide – dipole-dipole interactions
Ice – entirely hydrogen bonds
Molecular crystals?
Molecules are packed as close as possible in the structure.
Much more easily broken apart than ionic and covalent crystals
What are metallic cystals?
At each lattice point of the crystal is an atom of the same element.
Usually very dense crystals
Bonds are delocalized over the entire crystal?
Bonds are delocalized over the entire crystal.
Array of positive ions in a sea of delocalized valence electrons.
Delocalization of electrons make for good conductors.
What is amorphous solids?
Solid that is formed quickly and its atoms (molecules) do not have time to align properly
Lack a regular 3-D structure
What is glass?
An optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing.
Major component is silicon dioxide with other compounds added for color or other properties.
Glass behaves more like a liquid than solid in some respects.
Lacks long range periodic order.
What are phase changes?
Transition from one phase to another.
When do phase changes occur?
Occur when energy (usually heat) is added or removed from a system
Phase changes are characterized by?
Changes in molecular order.
Solids have the greatest order and gases have the greatest randomness.
Molecules in a liquids are not fixed in a?
Rigid lattice and lack total randomness. Are in constant motion. Because liquids are denser than gases, the collsion rate among moleules is much higher in the liquid phase than in the gas phase.
What Evaporation (vaporization)?
Evaporation (vaporization) – the process in which a liquid is transformed into a gas.

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