intermolecular interactions Flashcards
how does standard enthalpy of formation of water change going from solid, liquid to gas
decreases
what is the electrostatic potential
point charges interact with each other as a function of distance Coulombs law
how to calculate the number of electrostatic interaction between all atoms in the system
(N)(N-1) / 2
what is the unit of dipole moment?
what is the formula for the dipole moment?
Debyes (D) which is equal to 3.34 e-30 Cm
mu = Qr
how to work out the dipole moment of all three coordinates
mures = (mux2 + muy2 + muz2)^0.5
what are the distance dependences and typical energy of ion-ion, ion-dipole, dipole-dipole and London interactions
ion-ion - 1/r 250 kJ/mol
ion-dipole - 1/r2 15 kJ/mol
dipole-dipole - 1/r3 2kJ/mol (stat)
dipole-dipole - 1/r6 0.3 kJ/mol (rot)
london 1/r6 2 kJ/mol
explain the interaction energy differences in the enthalpy of formation of water in the different phases
solid - stationary molecules orientates to maximise favourable dipole-dipole interactions, 1/r3 dependence of 2kJ/mol
liquid - tumbling molecules 1/r6 dependence and small compared to solid interaction
gas - low density molecules far apart and very small d-d interactions.
how do non-polar molecules liquify / solidify
form a temporary dipoles moments mu* electric field is generated by nearby ion or polar molecule distorts electron distribution of molecule.
molecule is polarisable and magnitude of induced dipole moment is proportional to electric field strength E
alpha is the proportionality constant and is the polarisability
mu = alpha E
how do London forces arise
instantaneous fluctuations of electron position cause instantaneous dipole moment to arise in molecule 1, mu1 induces an instantaneous dipole in molecule 2.
the two dipoles attract each other and so potential energy is lowered.
the first molecule then changes the shape and direction of its dipole second molecule follows it and the two dipoles are correlated.
what is the Lennard-Jones Potential
u(r) = (C12/r12) - (C6/r6)
interaction between atoms and molecules
first term is repulsive
second term is attractive - van der waal term
how is the second term of lennard-jones potential calculated
van der waal term is the sum of dipole-dipole, induced dipole-dipole and the dispersion interactions
how can the Lennard-Jones potential also be written
what is epsilon a measure of
what is sigma a measure of
4epsilon[(sigma/r)^12 - (sigma/r)^6]
epsilon is the depth of the potential well, is a measure of how strongly the molecules attract each other
sigma is the distance at which u(r) =0 a measure of size of the molecules
well minimum at r = 2^1/6 sigma
