4.4 Intermolecular forces Flashcards
what is another name for intermolecular forces?
van der waals forces
what are intermolecular forces?
the attractive forces present between covalent molecules
what are the general properties of intermolecular forces?
- weak
- broken when melting or boiling a substance
what are the 3 main types of intermolecular forces?
- london dispersion force
- dipole dipole forces
- hydrogen bonds
describe the strength of london dispersion forces
- weakest type of intermolecular forces
- they form between all covalent molecules
describe the movement of electrons when there is the presence of london dispersion forces
- electrons are constantly moving
- so at any point in time the electrons within an atom will be present in a higher concentration on one side than the other
- creates a dipole
define instantaneous dipole
a dipole created by having a high concentration of electrons in a certain area at any point in time, found in london dispersion forces
what is an induced dipole-dipole attraction and how does it link to london dispersion forces?
a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles
why does the dipole constantly appear and disappear in london dispersion forces?
- this is due to electron movement
- but maintains a continual attraction between atoms of different molecules
how does the increase in molecular mass affect london dispersion forces?
- the molecule has more electrons
- increase the chance of having an uneven distribution
- this distribution will be of greater strength
- london force strength increases as molecular mass increases
how does the shape/ structure of a molecule affect the strength of the london dispersion force?
- as chain is longer, ldf increases
- longer chain has more contact points so more ldf can develop
- making the overall effect of the force stronger
what is the difference between an induced dipole and a dipole-dipole force?
induced dipoles are temporary
dipole dipole forces are permanent
define dipole-dipole force
the attraction between oppositely charged dipoles of 2 polar molecules
describe the strength of dipole-dipole forces
- the second weakest form of intermolecular forces
- forms purely between polar molecules
how are molecules attracted to each other in dipole-dipole forces?
- polar molecules have a dipole
- the positive end of a molecule is attracted to the negative end of another molecule
describe the strength of hydrogen bonds
- the strongest type of intermolecular forces
- form when a negative lone pair of a nitrogen, oxygen or fluorine atom is attracted towards a positive hydrogen atom
explain the existence of a hydrogen bond
- h bonds have a directional component towards the hydrogen atom
- the electronegative atom withdraws electron density, polarizing the bond
rank the 3 intermolecular forces from weakest to strongest
ldf < d-d force < h bonds
how does the strngth of the intermolecular forces have an affect on the substances’ melting and boiling point?
substances containing only ldf have the lowest mp and bp, whereas substances containing h bonds have the highest mp and bp
* note that this only applies when molecular mass or area if roughly similar
what can change the default ranking of the 3 intermolecular forces?
if a non-polar substance has a large molecular mass or area, the subsequent increases in ldf can change this ranking
what is the affect on solubility as alcohol chain length increases?
- as alcohol chain increases, solubility decreases
- number of contact point increases, increasing ldf
- the energy input to break the ldf and dissolve the substance thus increases whilst the energy output remains the same
- energy output is less than the energy inout, the substance will not dissolve in a solvent
