SHAPES OF MOLECULES AND INTERMOLECULAR FORCES

Question 1

The electron pair repulsion theory

Answer 1

Electron pairs surrounding central atom determines the molecule / ion shape

Pairs repel each other so they are arranged as far as possible

Arrangement minimises repulsion, so hold atoms in definite shape

Different number of electron pairs = different shape

Question 2

What 2 factors determines the shape of a molecule / ion

Answer 2

Bonding regions

Lone pairs

Question 3

Lone pair

Answer 3

Lone pair slightly closer to central atom

Occupies more space than a bonded atom because they repel more strongly than a bonding pair

Question 4

How does a lone pair affect the bond angle?

Answer 4

Reduces the bond angle by 2.5 per lone pair

Question 5

What is the bond angle of a tetrahedral molecule

Answer 5

109. 5

e. g. CH4

Question 6

What is the bond angle of a pyramidal molecule

Answer 6

107

e.g. NH3

Question 7

What is the bond angle of a non linear molecule

Answer 7

104.5

Non-linear

Question 8

A molecule with 2 bonding regions

Answer 8

Bond angle : 180
linear

e.g. CO2

Question 9

A molecule with 3 bonding regions

Answer 9

Bond angle: 120
Trigonal planar

e.g. BF3

Question 10

A molecule with 4 bonding regions

Answer 10

Bond angle: 109.5
Tetrahedral

e.g. CH4

Question 11

A molecule with 6 bonding regions

Answer 11

Bond angle: 90
Octahedral

e.g. SF6

Question 12

Examples of tetrahedral molecules

Answer 12

CH4
Ammonium ions
SO4^2-

Question 13

Examples of trigonal planar molecules

Answer 13

CO3 ^2-

NO3 ^-

Question 14

What is meant by electronegativity

Answer 14

The attarcted of a bonded atom for the pair of electrons in a covalent bond

Question 15

What scale is used to compare electrongetavitivties

Answer 15

Pauling scale

Question 16

What is the most electronegative element?

Answer 16

Fluorine

Question 17

Electronegativity trend

Answer 17

Electronegativity increases upwards and across towards fluorine

Group 1 is the least electronegative elements

Question 18

What kind of bond is formed where there is a large difference in electronegativities?

Answer 18

Ionic rather than Covalent

Question 19

Non polar bond

Answer 19

Bonded electron pair shared equally between bonded atoms

When atoms are the same element / have similar electrongetiavity

Question 20

Pure covalent bond

Answer 20

Bonded atoms are the same element

Question 21

Polar covalent bond

Answer 21

Bonded electron pair shared unequally

Different atoms with different electronegativity.
More electronegative has greater attraction for bonded pair electrons

Question 22

Polar bonds are polarised with…

Answer 22

Dipoles

Question 23

Permanent dipole

Answer 23

Dipole in a polar covalent bond which doesn’t change

Question 24

What can polar bonds do?

H2O and CO2

Answer 24

Polar bonds may reinforce one another to produce larger dipole over whole molecule or cancel out

H2O:
Polar
OH bonds have permanent dipoles
Act in opposite directions but don’t exactly oppose each other
Overall O end is delta neg and H end is delta positive

CO2:
Non-polar
C=O have permanent dipole
Act in opposite directions and exactly oppose each other
Overall dipoles cancel

Question 25

Ionic lattices dissolving in polar solvents

Answer 25

Water molecules attract +/- ions Ionic lattice breaks down as dissolves Water molecules surround ions + ions attracted towards delta negative oxygen of water - ions attracted to delta positive hydrogen of water

Question 26

What are intermolecular forces

Answer 26

Weak interactions between dipoles of different molcules

Question 27

Induced dipole-dipole interactions (London forces)

Answer 27

Exist between all molecules, only temporary Movement electrons produces changing dipole in any molecule, at any instant an instantaneous dipole will exist but position shifts constantly Instantaneous dipole induces a dipole on neighbouring molecule, which induces dipoles on further molecules, they then attract one another. More electrons in each molecule Larger instantaneous and induced dipoles Greater induced dipole - dipole interactions Stronger attractive forces (Explains increased bp of noble gases)

Question 28

Permanent dipole - dipole interactions

Answer 28

Act between permanent dipoles in polar molecules Mean boiling point of polar molecules is much greater: Have both london and permanent dipole-dipole interactions Extra energy needed to break additional permanent interactions So bp is higher

Question 29

Why is the mean boiling point of polar molecules high

Answer 29

Mean boiling point of polar molecules is much greater: Have both london and permanent dipole-dipole interactions Extra energy needed to break additional permanent interactions So bp is higher

Question 30

What are simple molecules?

Answer 30

Simple molecules contain only a few atoms held together by covalent bonds. e.g CO2

Question 31

What do simple molecules form in a solid state?

Answer 31

Simple molecular lattices Held together by weak intermolecular forces But atoms within molecules bonded strongly with covalent bonds

Question 32

The bp and mp of of simple molecular substances

Answer 32

Low mp/bp Only weak intermolecular forces break Not strong covalent bonds

Question 33

Nonpolar simple molecules are soluble in...

Answer 33

Nonpolar solvents

Question 34

Why are nonpolar simple molecules soluble in nonpolar solvents

Answer 34

Nonpolar simple molecules tend to be soluble in non-polar solvents (hexane) because intermolecular forces form between molecules and solvent, weakening intermolecular forces in simple lattice, so they break and the compound dissolves

Question 35

Simple molecules are insoluble in...

Answer 35

Polar solvents

Question 36

Why are simple molecules insoluble in polar solvents?

Answer 36

Tend to be insoluble in polar solvents (water) because of little interaction between molecules in lattice and solvent molecules, intermolecular bonding in solvent too strong to be broken

Question 37

What determines the solubility of polar simple molecular substances?

Answer 37

Depends on strength of dipole May dissolve because polar solute / solvent molecules can attract each other Some with part polar and part non-polar dissolve

Question 38

Do simple molecular structures conduct electricity?

Answer 38

No No mobile charged particles within structure So nothing to complete an electrical circuit

Question 39

How many lone pairs does Nitrogen, Oxygen and Fluorine have?

Answer 39

Nitrogen: Three bonds and 1 lone pair Oxygen Two bonds and 2 lone pairs Fluorine (and all halogens): One bond and 3 lone pairs

Question 40

Where is hydrogen bonding found?

Answer 40

Found in molecules containing Electronegative atom with lone pair electrons (O,N,F) attached to a hydrogen atom

Question 41

Why does ice float?

Answer 41

Solid is less dense than liquid water Hydrogen bonds hold water molecules apart in open lattice structure, water molecules in ice is held further apart than in water, so ice is less dense that liquid water and floats 2 lone pairs on oxygen and 2 hydrogens, so each molecule can form 4 bonds = open tetrahedral lattice full of holes