London dispersion forces: result of the Coulombic attractions between temporary, fluctuating dipoles
often the strongest net intermolecular force between large molecules
dispersion forces increase with:
increasing contact area between molecules
increasing polarizability of the molecules
polarizability increases with:
increasing number of electrons in the molecule
size of the electron cloud
not synonymous with van der Waals forces
the dipole moment of a polar molecule leads to additional interactions with other chemical species
dipole-induced dipole interactions: between polar and nonpolar molecules
always attractive
strength increases with:
magnitude of the dipole of the polar molecule
polarizability of the nonpolar molecule
dipole-dipole interactions: between polar molecules
strength depends on:
magnitude of dipoles
relative orientation
interactions between polar molecule are typically greater than those between nonpolar molecules of comparable size (interactions act in addition to London dispersion forces)
ion-dipole interactions: between ions and polar molecules
stronger than dipole-dipole forces
relative strength and orientation dependence of dipole-dipole and ion-dipole forces can be understood qualitatively
sign of the partial charges responsible for the molecular dipole moment
how those partial charges interact with an ion or adjacent dipole
hydrogen bonding: hydrogen atoms covalently bonded to the highly electronegative atoms (N, O, F) are attracted to the negative end of a dipole formed by the electronegative atom (N, O, F)
in large biomolecules, noncovalent interactions may occur between different molecules or between different regions of the same large biomolecule
properties of solids
many properties of liquids and solids are determined by the strengths and types of intermolecular forces present
vapor pressure
boiling point
melting point
particulate-level representations can show how intermolecular interactions help to establish macroscopic properties
ionic solids: have strong interactions between ions
low vapor pressures
high melting points
high boiling points
brittle (repulsion of like charges when one layer slides over another)
conduct electricity when ions are mobile
melted (i.e. in molten state)
dissolved in solvent (e.g. water)
covalent network solids: atoms are covalently bonded together into a three-dimensional network (e.g. diamond) or layers of two-dimensional networks (e.g. graphite)
layers of two-dimensional: adjacent layers can slide past each other relatively easily
soft
metallic solid
good conductors of electricity and heat (free valence electrons)
malleable and ductile (metal cores can easily rearrange structure)
alloys retain sea of mobile electrons and remain conducting
interstitial alloys are less malleable and ductile (more rigid)
large biomolecules/polymers: noncovalent interactions may occur
functionality and properties depend on shape
solids, liquids, and gases
ideal gas law
P: pressure (atm)
V: volume (L)
n: amount of particles (mol)
R: gas constant
T: temperature (K)
partial pressure of a gas within a mixture is proportional to its mole fraction
mole fraction (X): moles gas/total moles
total pressure of a sample is the sum of its partial pressures
kinetic molecular theory (KMT): relates the macroscopic properties of gases to motions of the particles in the gas
gas particles are so small that they have no volume
particles are always randomly moving; collisions cause pressure
particles never react with each other
average kinetic energy in the gas is directly proportional to the temperature (K)
Maxwell-Boltzmann distribution: graphical representation of the kinetic energies (energies/velocities) of particles at a given temperature
real gas behavior deviates from the ideal gas law
interparticle attractions among gas molecules
particle volumes
real gases approximate ideal gases at high temperatures and low pressures
solution/homogenous mixture: macroscopic properties do not vary throughout the sample
can be solid, liquid, or gas
solution composition (molarity)
heterogenous mixture: macroscopic properties depend on location in the mixture
particulate representations of solutions communicate the structure and properties of solutions
relative concentrations of the components
interactions among the components
separation of liquid solutions/mixtures
takes advantage of difference in the intermolecular interactions of the components
chromatography (paper, thin-layer, column)
mobile phase: components of solution
stationary phase: surface components interact with mobile phase
chromogram can be used to infer the relative polarities of components in a mixture
distillation
evaporation
uses different vapor pressures of components
solubility: substances with similar intermolecular interactions tend to be miscible/soluble in one another (e.g. polar molecules)
spectroscopy and the electromagnetic spectrum
differences in absorption or emission of photons in different spectral regions are related to the different types of molecular motion or electronic transition
microwave radiation: transitions in molecular rotational levels
infrared radiation: transitions in molecular vibrational levels
ultraviolet/visible radiation: transitions in electronic energy levels
properties of photons
when a photon is absorbed/emitted by an atom or molecule, the energy of the species is increased/decreased by an amount equal to the energy of the photon
c: speed of light
λ: wavelength
ν: frequency
E: energy of photon
h: Planck’s constant
ν: frequency
Beer-Lambert law: absorption of light by a solution
A: absorptionof light
ε: molar absorptivity
how intensely a chemical species absorbs light of a specific wavelength
b: path length
proportional to number of light-absorbing particles in light path
c: concentration
proportional to number of light-absorbing particles in light path
in most experiments, path length and wavelength of light are held constant
absorbance is proportional only to the concentration of absorbing molecules/ions
spectrophotometer is set to wavelength of maximum absorbance (optimum wavelength) to ensure the maximum sensitivity of measurement
