5. polarity and intermolecular forces

Question 1

electronegativity is

Answer 1

the atoms ability to attract the electron pair in a covalent bond

Question 2

what is the most electronegative element

Answer 2

fluorine

Question 3

what are the 3 most electronegative elements

Answer 3

Fluorine
Oxygen
Nitrogen

Question 4

how does electronegativity affect covalent bonds

Answer 4

one atom more electronegative than another = polarise’s the bond

Question 5

a permanent dipole occurs when

Answer 5

great difference between electronegativity of atoms causing a shift in electron density in the bond

Question 6

if polar bonds are arranged symmetrically

e.g. CO2

Answer 6

the dipoles cancel each other out

molecule has no overall dipole and is non polar

Question 7

if polar bonds aren’t arranged symmetrically

e.g. H2O

Answer 7

dipoles dont cancel eachother out
charge arranged unevenly across molecule
overall dipole and is polar

Question 8

to work out if a molecule is polar you need to

Answer 8

draw molecule out in 3D
label partial charges
see if they cancel each other out

Question 9

the higher the difference in electronegativity

Answer 9

the more ionic in character the bonding becomes

Question 10

3 types of intermolecular forces

Answer 10

1. induced dipole-dipole or London (dispersion) forces
2. permanent dipole-dipole interactions
3. hydrogen bonding

Question 11

intermolecular forces are strong/weak?

Answer 11

very weak

Question 12

what is the strongest type of intermolecular force

Answer 12

hydrogen bonding

Question 13

london dispersion forces are found between

Answer 13

all atoms and molecules

Question 14

how do london dispersion forces work

Answer 14

1. electrons in charge clouds moving quickly.
2. at any one moment electrons more likely to be more concentrated on one side than the other = temporary dipole.
3. temporary dipole can cause another temporary (induced) dipole on a neighbouring atom = 2 dipoles then attracted to each other
4. 2nd dipole can induce a dipole in 3rd atom and so on.
5. constantly moving = dipoles created and destroyed all the time.
   overall effect = atoms attracted to each other

Question 15

what affects strength of london dispersion forces

Answer 15

1. larger molecules = larger electron clouds = stronger ldf

2. greater surface area = bigger exposed electron cloud = stronger ldf

Question 16

stronger LDF mean

Answer 16

higher bpt/mpt

need more energy to overcome stronger intermolecular forces

Question 17

LDF can hold molecules in a

Answer 17

lattice

Question 18

permanent dipole dipole interactions happen between

Answer 18

polar molecules

Question 19

why do permenant dipole dipole interactions happen

Answer 19

the delta +/- charges on polar molecules cause weak electrostatic forces of attraction between molecules

Question 20

when does H bonding happen

Answer 20

when H is covalently bonded to Fluorine Nitrogen or oxygen

H has a high charge density because it is so small. F O N are very electronegative

Question 21

how does H bonding happen

Answer 21

bond is so polarised between a H and F/O/N

causes weak bond to form between the H of one molecule and a lone pair of electrons on FON on another molecule

Question 22

what effect does H bonding have on the properties of substances

Answer 22

soluble in water

higher boiling and freezing points than molecules of a similar size that aren’t able to form H bonds

Question 23

how is ice less dense than water

unusual most substances get denser when freeze.

Answer 23

ice = molecules held together in lattice by long H bonds 
water = less H bonds

Question 24

in general, main factor that determines bpt of a substance

Answer 24

strength of LDF (unless molecules can form H bonds)

Question 25

why do simple covalent compounds have low mpt/bpt

Answer 25

weak intermolecular forces
don’t need much energy to break
often liquid/gas at room temp

Question 26

why are polar molecules soluble in water

Answer 26

water is polar tends to dissolve other polar substances
compounds with h bonds can form h bonds with water molecules and will be soluble
molecules with only ldf e.g. methane=insoluble

Question 27

when talking about mpt/bpt of simple covalent compounds you refer to

Answer 27

the intermolecular forces between simple covalent compounds not the actual covalent bond.