bonding 2 - intermolecular forces

Question 1

definition of electronegativity

Answer 1

ability of an atom to attract the bonding electrons towards itself in a covalent bond

Question 2

non polar bonds

Answer 2

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

Question 3

polar covalent bonds e.g HCl

Answer 3

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

Question 4

define permeant dipole

Answer 4

small difference in charge across a bond that results from a difference in electronegativities of the bonded atoms

Question 5

most electronegative element in the periodic table

Answer 5

flourine

Question 6

name other strong electronegative elements

Answer 6

O, N and the halogens (group 7)

Question 7

general trend in electronegativity towards top right of periodic table

Answer 7

increases

Question 8

the greater the difference in ______ between the bonded atoms, the greater the ______ ______ and the more ______ the covalent bond

Answer 8

electronegativity, permanent dipole, polar

Question 9

polar or non polar?
O=O
C-BR
C=O
F-F
C-H
B-F

Answer 9

polar
C-BR
C=O
B-F
non polar
O=O
F-F
C-H

Question 10

which bond will have the greatest permanent dipole - why?
O=O
C-BR
C=O
F-F
C-H
B-F

Answer 10

B-F because there is the largest difference in electronegativity

Question 11

does carbon and hydrogen form a polar covalent bond

Answer 11

no due to the very small difference in electronegativity between C and H

Question 12

rules for if a molecule is symmetrical and must the molecule fit both rules or just one

Answer 12

-all atoms bonded to the central atom are the same element
-there are no lone pairs of electrons around the central atom

Question 13

symmetrical molecules are….because….

Answer 13

non polar because the dipoles cancel out

Question 14

CCl4 is a symmetrical molecule because…

Answer 14

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

Question 15

unsymmetrical molecules which contain one or more polar bonds are….because….

Answer 15

polar because the dipoles do not cancel out

Question 16

NH3 is an unsymmetrical molecule with polar N-H bonds because…

Answer 16

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

Question 17

rules for if a molecule is unsymmetrical

Answer 17

-lone pair around central atom
-central atom is bonded to atoms of different elements
-if there is only one covalent bond

Question 18

define intermolecular forces

Answer 18

attractive forces between molecules - much weaker than ionic or covalent bonds. Only found in covalent structures

Question 19

intermolecular forces can be…

Answer 19

hydrogen bonds
permanent dipole-dipole interactions
induced dipole-dipole interactions

Question 20

define induced dipole-dipole interactions (londonforces)

Answer 20

very weak intermolecular forces between molecules. London forces exist between all molecules whether polar or not

Question 21

how do London forces arise between molecules

Answer 21

-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)

Question 22

how do London forces increases in strength

Answer 22

as the number of electrons increases so more electrons=stronger london force

Question 23

define permanent dipole-dipole interactions

Answer 23

weak attractive forces between polar molecules

Question 24

how do permanent dipole-dipole interactions arise between molecules e.g HCl

Answer 24

-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

Question 25

what are Van der Waals forces

Answer 25

permanent dipole-dipole and induced dipole-dipole

Question 26

define hydrogen bonds

Answer 26

strong dipole-dipole attraction between molecules containing O-H, N-H, F-H

Question 27

a hydrogen bond exists between….

Answer 27

-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

Question 28

list the order of the strongest type of intermolecular forces - starting with the weakest

Answer 28

induced dipole-dipole, permanent dipole-dipole, hydrogen bond

Question 29

explain the hydrogen bonding between water molecules

Answer 29

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

Question 30

explain the hydrogen bonding between ammonia molecules

Answer 30

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

Question 31

anomalous properties of water

Answer 31

-ice is less dense than liquid water
-water has relatively high melting and boiling point

Question 32

describe the property of why ice is less dense than liquid water

Answer 32

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

Question 33

describe the property of why water has a relatively high melting and boiling point

Answer 33

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

Question 34

suggest why ice has a higher melting point than solid ammonia

Answer 34

-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