define intermolecular bonds
weak forces of attraction between molecules
what are the three types of intermolecular bonds
-instantaneous-induced dipole forces
-permanent dipole-permanent dipole forces
-hydrogen bonding
define a dipole
a molecule with a positive end and a negative end, it is polarised.
define instantaneous-induced dipole forces
intermolecular forces between temporary dipoles of molecules
1) how is an instantaneous dipole in a molecule formed
electrons in molecules are randomly moving. At any moment they can become unevenly distributed which forms an instantaneous dipole(temporary)
2) how is an instantaneous-induced dipole formed
The δ⁺ end of one molecule is attracted to the δ⁻ end of a neighbouring molecule, inducing a dipole (could be the other way around)
what are three characteristics of instantaneous-induced dipole forces
-they occur in all molecules
-they are the weakest type of intermolecular force
-they are the only intermolecular bond in non-polar molecules
what two factors affect the strength of instantaneous-induced dipole forces
-surface area
-mr
what do we mean by surface area in this case
The area of a molecule that is in close contact with neighbouring molecules
how does surface area affect the strength of ID-ID forces(straight v branched)
Straight-chain molecules have stronger ID-ID forces than branched molecules because they can pack closer together, increasing the molecular surface contact between neighbouring molecules and allowing more interactions between their electrons
how does surface area affect the strength of ID-ID forces(long v short)
long molecules have stronger ID-ID forces than short molecules because they have a larger molecular surface contact with neighbouring molecules which allows more interactions between their electrons
how does Mr affect the strength of ID-ID forces
a higher Mr value means a molecule has more electrons which means there are more electron interactions between neighbouring molecules
define permanent dipole-permanent dipole forces
Intermolecular forces between the permanent dipoles of polar molecules
1) how is a permanent dipole formed
two atoms have different electronegativities, the more electronegative atom pulls the bonding electrons closer to itself, making the bond polar
2) how is a PD-PD formed
The δ⁺ end of one molecule is attracted to the δ⁻ end of a neighbouring molecule
PD-PD forces only occur in _____ molecules
polar
what can cause repulsion in a substance with PD-PD forces
in a liquid state, molecular dipoles are constantly moving and sometimes δ⁺ or δ- end of molecules are next to one another, causing repulsion
1) what do we put next to a jet of liquid
an electrostatically charged rod(-ve in this case)
2) how do we know if the liquid is polar
the jet is deflected because the delta positive part of the polar molecules in the liquid is attracted to the rod
define hydrogen bonding
-A δ⁺ hydrogen atom is covalently bonded to a highly electronegative atom (O, N, or F)
-it forms a hydrogen bond with the lone pair on an electronegative atom (O, N, or F) on another molecule
the strength of hydrogen bonds depends on ______ ______
bond polarity, when the delta positive is larger on the H atom it forms stronger bonds
what two examples of molecules is hydrogen bonding found in
-water
-ammonia
explain the trend in hydrogen bonds in hydrogen halides
as you go down the group, hydrogen bonds become weaker because the electronegativity decreases which lowers bond polarity since there is less of a difference in electronegativity between atoms
explain why H2O has stronger hydrogen bonds than HF
H2O -> the oxygen atom has two lone pairs and each molecule has two hydrogen atoms so the maximum amt of H bonds can form
HF -> the F atom has three lone pairs but each molecule has one H atom so only 1/3 of all lone pairs are used in H bonds
