bonding 2 - intermolecular forces Flashcards
definition of electronegativity
ability of an atom to attract the bonding electrons towards itself in a covalent bond
non polar bonds
e.g H-H or Cl-Cl
both bonding atoms are the same
both have the same electronegativity
bonding electrons are evenly distributed between the bonded atoms
bond is non polar
polar covalent bonds e.g HCl
e.g H-CL
Cl atom is more electronegative
bonding electrons attracted closer to Cl atom
electron cloud is more desnse around Cl atom so there is a permanent dipole across the bond
bond is polar covalent bond as it has permanent dipole
define permeant dipole
small difference in charge across a bond that results from a difference in electronegativities of the bonded atoms
most electronegative element in the periodic table
flourine
name other strong electronegative elements
O, N and the halogens (group 7)
general trend in electronegativity towards top right of periodic table
increases
the greater the difference in ______ between the bonded atoms, the greater the ______ ______ and the more ______ the covalent bond
electronegativity, permanent dipole, polar
polar or non polar?
O=O
C-BR
C=O
F-F
C-H
B-F
polar
C-BR
C=O
B-F
non polar
O=O
F-F
C-H
which bond will have the greatest permanent dipole - why?
O=O
C-BR
C=O
F-F
C-H
B-F
B-F because there is the largest difference in electronegativity
does carbon and hydrogen form a polar covalent bond
no due to the very small difference in electronegativity between C and H
rules for if a molecule is symmetrical and must the molecule fit both rules or just one
-all atoms bonded to the central atom are the same element
-there are no lone pairs of electrons around the central atom
symmetrical molecules are….because….
non polar because the dipoles cancel out
CCl4 is a symmetrical molecule because…
each Cl-C bond is polar because chlorine is more electronegative than carbon
however because the CCl4 molecule is symmetrical the dipoles cancel out
therefore the CCl4 molecule is non polar as there is no net dipole across the molecule
unsymmetrical molecules which contain one or more polar bonds are….because….
polar because the dipoles do not cancel out
NH3 is an unsymmetrical molecule with polar N-H bonds because…
each N-H bond is polar because N is more electronegative that H
since the NH3 molecule is unsymmetrical the dipoles do not cancel
therefore the molecule is polar
rules for if a molecule is unsymmetrical
-lone pair around central atom
-central atom is bonded to atoms of different elements
-if there is only one covalent bond
define intermolecular forces
attractive forces between molecules - much weaker than ionic or covalent bonds. Only found in covalent structures
intermolecular forces can be…
hydrogen bonds
permanent dipole-dipole interactions
induced dipole-dipole interactions
define induced dipole-dipole interactions (londonforces)
very weak intermolecular forces between molecules. London forces exist between all molecules whether polar or not
how do London forces arise between molecules
-at any moment there may be an uneven distribution of electrons in a molecule (due to movement of electrons/ changing electron density)
-this causes a temporary dipole (or instantaneous dipole) to be present
-the instantaneous dipole in one molecule causes an induced dipole in a neighboring molecule
-the delta + of a dipole in one molecule attracts the delta - of a dipole in a neighboring molecule to produce a London force (induced dipole-dipole force)
how do London forces increases in strength
as the number of electrons increases so more electrons=stronger london force
define permanent dipole-dipole interactions
weak attractive forces between polar molecules
how do permanent dipole-dipole interactions arise between molecules e.g HCl
-the HCl bond is permanently polar because Cl is more electronegative than H and the molecule is not symmetrical so the dipoles do not cancel
-the delta + of one HCl molecule attracts the delta - of a neighboring HCl molecule to produce a permanent dipole-diple force of attraction between the molecules
what are Van der Waals forces
permanent dipole-dipole and induced dipole-dipole
define hydrogen bonds
strong dipole-dipole attraction between molecules containing O-H, N-H, F-H
a hydrogen bond exists between….
-a H delta + atom in one molecule and
-a lone pair on a highly electronegative atom (O delta-, N delta-, F delta-) on another molecule
list the order of the strongest type of intermolecular forces - starting with the weakest
induced dipole-dipole, permanent dipole-dipole, hydrogen bond
explain the hydrogen bonding between water molecules
the O-H bonds in H2O are polar because O is more electronegative than H and the molecule is not symmetrical so the dipoles do not cancel. Hydrogenbonds form between H2O molecules in the direction of the lone pair of electrons on the oxygen atom
explain the hydrogen bonding between ammonia molecules
the N-H bonds in NH3 are polar because N is more electronegative than H and th molecule is not symmetrical so the dipoles do not cancel. Hydrogen bonds form between the ammonia molecules in the direction of the lone pair of electrons on the N atom
anomalous properties of water
-ice is less dense than liquid water
-water has relatively high melting and boiling point
describe the property of why ice is less dense than liquid water
in the solid H2O molecules are held further apart by the hydrogen bonds
this gives the ice an open lattice structure
this makes solid ice less dense than liquid water
hydrogen bonds between H2O moleules are longer than the covalent bonds between O and H atoms in the H2O molecules
the H2O molecules are fixed in their positions in the 3dimensional ice crystal lattice by the hydrogen bonds
describe the property of why water has a relatively high melting and boiling point
water has a higher than expected melting and boiling point
hydrogen bonds are relatively strong and therefore stronger than other intermolecular forces so more energy is needed to break the hydrogen bonds
suggest why ice has a higher melting point than solid ammonia
-hydrogen bonds between H2O molecules are stronger than the hydrogen bonds between NH3 molecules
-O is more electronegative than N
-the dipole across the O-H bond is stronger than across the N-H bond
-there are also two hydrogen bonds between H2O molecules and only one between NH3 molecules
