Forces between molecules Flashcards
what are intermolecular forces?
Intermolecular forces are the attractive forces that exist between molecules. These forces are much weaker than the covalent bonds that hold atoms together within molecules.
What are the three main types of intermolecular force, listed in order of increasing strength?
1.Induced dipole-dipole forces (also known as London dispersion forces or Van der Waals’ forces).
-Permanent dipole-dipole forces.
-Hydrogen bonding.
what is the strength of induced dipole-dipole forces (Van der Waals)?
weak
what is the strength of permanent dipole-dipole force?
moderate
what is the strength of hydrogen bonding?
strong
where are Induced dipole-dipole forces found?
All molecules and noble gases
where are Permanent dipole-dipole forces found?
polar molecules
where are Hydrogen bonding found in?
Polar molecules with H-F, H-O or H-N bond (NOF)
How induced dipole-dipole forces arise (van der Waals)?
1.Electrons in atoms are constantly moving. At any instant, there may be more electrons on one side of the atom than the other, creating a temporary dipole.
2.This temporary dipole can induce an opposite dipole in a neighbouring atom, causing a weak electrostatic attraction between the atoms.
3.This induced dipole can then induce further dipoles in other nearby particles.
4.Although these dipoles are constantly forming and disappearing as the electrons move, the overall effect is a net attraction between the atoms or molecules.
Factors affecting the strength of induced dipole-dipole forces?
1.Size - Larger atoms and molecules have more electrons and a greater volume of electron density that can become polarised, creating stronger temporary dipoles.
2.Surface area - Molecules with a larger surface area also have stronger induced dipole-dipole forces as more of the electron cloud is exposed for interactions.
Consequently, substances with stronger induced dipole-dipole forces tend to have higher boiling points.
How permanent dipole-dipole forces arise?
Permanent dipole-dipole forces are electrostatic attractions between the partial positive end of one polar molecule and the partial negative end of another.
For hydrogen bonding to occur, two criteria must be met:
1.The molecule must contain a hydrogen atom covalently bonded to either fluorine (F), oxygen (O), or nitrogen (N).
2.There must be a lone pair of electrons on the F, O, or N atom of an adjacent molecule available to interact with the hydrogen.
How hydrogen bonds form?
The H-F, H-O, and H-N bonds are highly polar due to the large electronegativity differences between hydrogen and these elements. This leads to a significant partial positive charge (δ+) on the hydrogen atom and a partial negative charge (δ-) on the F, O, or N atom.
The small size of the hydrogen atom allows it to get close to the lone pair of electrons on an adjacent F, O, or N atom.
The lone pairs on F, O, and N atoms are regions of high electron density and therefore high partial negative charge.
The positively charged hydrogen is strongly attracted to the negatively charged lone pair, forming a hydrogen bond between the molecules.
