C6.3 Intermolecular Forces Flashcards
3 types of intermolecular forces
induced dipole-dipole interactions (London forces)
permanent dipole-dipole interactions
hydrogen bonding
intermolecular forces definition
weak interactions between dipoles of different molecules
what are london forces?
- weak intermolecular forces that exist between all molecules (polar or non polar)
- act between induced dipoles in all different directions
- result from interactions of electrons between molecules
origins of induced dipole-dipole interactions
how they move
- movement of electrons produces a changing dipole in a molecule
- any instant, instantaneous dipole exists, with position constantly moving
- instantaneous dipole induces dipole on neighbouring molecule
- induced dipole induces further dipoles, attract each other, due to opposing dipoles
how long do induced dipoles last?
they are temporary, in next instant may disappear, only for whole process to take place amongst other molecules
how strong are london forces/ what affects their strength?
more electrons in each molecule = larger the instantaneous and induced dipoles, greater the london forces, stronger sttractive forces between molecules
- more electrons = larger induced dipoles = more energy to overcome intermolecular forces = higher boiling point
van der waals forces
term for permanent and induced dipole-dipole interactions, people thought it was for just induced dipole-dipole interactions
permanent dipole-dipole interactions
act between the permanent dipoles in different polar molecules
polar molecules have induced and permanent dipole-dipole interactions
- extra energy needed to break permanent dipole-dipole interactions = higher b.p
-stronger than induced dipole-dipole interactions
what are simple molecular substances?
made up of simple molecules
- which are small units containing a definite number of atoms with a definite molecular formula (Ne, H2, H20,CO2 etc..)
simple molecules form a structure of?
in solid state, they form a simple molecular lattice, where:
- molecules held in place by weak intermolecular forces
- atoms in each molecule bonded by strong covalnet bonds
what are the properties of simple molecular substances?
only polar and mp and bp
low mp and bp ( all are covalently bonded and can be solidified into simple molecular lattices by reducing their temps = in simple molec. lattice, weak intermolecular forces can be broken even at low temps)
- when a simple molec. lattice broken during melting, only intermolecular forces break nt covalent
-** covalent simple mol. structures can be polar or non polar** non-polar = easier to predict = when simple molec. compound added to non polar, intermolec. forces form between molecules and solvent, interactions weaken interm. forces between simple molec. lattice, intermolec. forces break and compound dissolves
solubility of non polar simple molecular substances and non polar solvents
when simple molec. substance added to polar solvent, little interaction bteween molecules in lattice and solvent, therefore intermolecular forces are too strong to be broken down
= non polar simple molecular substances normally dissolve in non polar solvents
solubility of polar simple molecular substances
basic on what the solubility is
polar covalent substances can dissolve in polar solvent molecules, as the polar solute and solvent molecules attract each other, similar process to dissolving ionic compounds
eg. sugar issolving in water, sugar has many O-H bonds, attracting and bonding with the water molecules
extends to gas and liquids too
what does the solubility of the polar molecules depend on?
depends on the strength of dipole and can be hard to predict
some may contain polar and non polar parts in their structure (ethanol) and dissolve in polar and non polar solvents
some biological molecules have hydrophilic and hydrophobic heads
hydrophillic = polar = contain electronegative atoms (usually o2) = interacts with wtaer
hydrophobic = non polar = comprised of carbon usually = doesnt interact with water
electrical conductivity in simple molecular substances
no mobile charged atoms = nothing to complete electrical circuit = **non-conductors of electricity **
