[CH 102] - Midterm Exam Guide - Comprehensive Notes for the exam (43 pages long!)

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CH 102
MIDTERM EXAM
STUDY GUIDE
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Intermolecular Forces:
Intermolecular forces- the attractive forces that exist between all molecules and atoms
o A molecule's structure determines the strength of its intermolecular forces
o They hold many liquids and solids together and are responsible for the very existence of
condensed states (in molecular compounds)
o The states/phases of matter depend on the magnitude of these intermolecular forces
between the constituent particles relative to the amount of thermal energy
When thermal energy is high relative to intermolecular forces, matter tends to be
gaseous
When thermal energy is low relative to intermolecular forces, matter tends to be
liquid or solid
The densities of ice and liquid water are much larger than the density of steam
o The densities and molar volumes of ice and liquid water are much closer to each other than
to steam
o The density of ice is lower than the density of water
This is not the norm; most solids have a greater density than their liquids
Definite- matter keeps its shape when placed in a container
Indefinite- matter takes the shape of the container
The strength of the attractions between the particles of a substance determines its state
o Solid
o Liquid
o Gas
At room temperature, moderate to strong attractive forces result in materials being solids or
liquids
The stronger the attractive forces are, the higher will be the boiling point of the liquid and the
melting point of the solid
o Other factors also influence the melting point
Intermolecular attractions are due to attractive forces between opposite charges
o (+) ion to (-) ion
o (+) end of polar molecule to (-) end of polar molecule
o Even nonpolar molecules will have temporary charges
Larger charge = stronger attraction
Longer distance = weaker attraction
Intermolecular attractive forces are small relative to the bonding forces between atoms
(intramolecular)
o Generally smaller charges
o Generally over much larger distances
The stronger the attractions between the atoms or molecules, the more energy it will take to
separate them
Boiling a liquid requires adding enough energy to overcome all the attractions between the
particles
o However, it does not require breaking the covalent bonds
o The higher the normal boiling point of the liquid, the stronger the intermolecular attractive
forces
o HF, H2O, and NH3 have hydrogen bonds
Therefore, they have higher boiling points than would be expected from the general
trends
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o For nonpolar molecules, such as the hydrides of Group 4, the intermolecular attractions are
due to dispersion forces
Therefore, they increases down the column, causing the boiling point to increase
o Polar molecules, such as the hydrides of Groups 5-7, have both dispersion forces and dipole-
dipole attractions
Therefore, they have higher boiling points than the corresponding Group 4 molecules
Dispersion forces- temporary polarity in the molecules due to unequal electron distribution
o Fluctuations in the electron distribution in atoms and molecules result in a temporary dipole
Region with excess electron density has partial (-) charge
Region with depleted electron density has partial (+) charge
o Also known as London forces or van der Waals forces
o As a temporary dipole is established in one molecule, it induces a dipole in all the
surrounding molecules
o All molecules will have dispersion forces
o The noble gases are all nonpolar atomic elements
o The stronger the attractive forces between the molecules, the higher the boiling point will
be
As the molar mass increases, the number of electrons increases
Therefore, the strength of the dispersion forces increases
o The weakest of the intermolecular attractions
Dipole-dipole attractions- permanent polarity in the molecules due to their structure
o Polar molecules have a permanent dipole
Bond polarity and molecular geometry (shape) determine the substance's polarity
Dipole moment
The permanent dipole adds to the attractive forces between the molecules, raising
the boiling and melting points relative to nonpolar molecules of similar size and shape
o Hydrogen bonds (H-bonding)- An especially strong dipole-dipole attraction results when H is
attached to an extremely electronegative atom
When a very electronegative atom is bonded to hydrogen, it strongly pulls the
bonding electrons toward it
O--H, N--H, or F--H
Because hydrogen has no other electrons, when its electron is pulled away, the
nucleus becomes deshielded, exposing the H proton
The exposed proton acts as a very strong center of positive charge, attracting all
the electron clouds from neighboring molecules
Hydrogen bonds are very strong intermolecular attractive forces
Stronger than dipole-dipole or dispersion forces
Substances that can hydrogen bond will have higher boiling points and melting points
than similar substances that cannot
But hydrogen bonds are not nearly as strong as chemical bonds
2-5% the strength of covalent bonds
Liquids:
The particles in a liquid are closely packed, but they have some ability to move around
o The close packing results in liquids being incompressible
The ability of the particles to move allows liquids to take the shape of their container and to flow
o However, they don't have enough freedom to escape or expand to fill the container
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