Chapter 6: Shapes of molecules and intermolecular forces (6.1, 6.2)

Question 1

what are the three types of intermolecular forces (weakest to strongest)

Answer 1

induced dipole-dipole interactions (London forces)
permanent dipole-dipole interactions
hydrogen bonding

Question 2

how do induced dipole-dipole interactions (London forces) come about

Answer 2

• movement of electrons movement of electrons produces a changing dipole in a molecule
• at any instant, an instantaneous dipole will exist, but its position is constantly shifting
• the instantaneous dipole induces a dipole on a neighbouring molecule (the positive charge attracts any electrons or the other way around)
• the induced dipole induces further dipoles on neighbouring molecules, which attract one another

Question 3

how permanent are induced dipoles

Answer 3

they are temporary, can appear and then disappear in an instant of time

Question 4

electron-pair repulsion theory

Answer 4

The electron pairs surrounding a central atom determine the shape of the molecule or ion
The electron pairs repel one another so that they are arranged as far apart as possible
The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape
Different numbers of electron pair result in different shapes
lone pairs of electrons repel more than bonded pairs of electrons
greater the number of electron pairs, the smaller the bond angle will be in a molecule

Question 5

4 bonded pairs, 0 lone pairs

Answer 5

tetrahedral

109.5 degree bond angle

Question 6

3 bonded pairs, 1 lone pair

Answer 6

pyramidal

107 degree bond angle

Question 7

2 bonded pairs, 2 lone pairs

Answer 7

non linear

104.5 degree bond angle

Question 8

2 bonding regions

Answer 8

linear

180 degree bond angle

Question 9

3 bonded pairs, no lone pairs

Answer 9

trigonal planar

120 degree bond angle

Question 10

6 bonded pairs, no lone pairs

Answer 10

octahedral

90 degree bond angle

Question 11

factors that effect electronegativity

Answer 11

different nuclear charges (high nuclear charge means high electronegativity)
atoms may be different sizes (low atomic radius means high electronegativity)
one shared pair of electrons may be closer to one nucleus than the other (closer means high electronegativity)

Question 12

electronegativity trends in the periodic table

Answer 12

electronegativity increases up and along (right) the periodic table

Question 13

non-polar bonds

Answer 13

the bonded electron pair is shared equally between the bonded atoms
a bond will be non-polar if the bonded atoms are the same or if the bonded atoms have similar electronegativity
non-polar solutes are soluble in non-polar solvents (e.g. hydrocarbon liquids like hexane)

Question 14

polar bonds

Answer 14

the bonded electron pair is attracted more to one of the bonded atoms than the other because it is more electronegative
polar solutes are soluble in polar solvents (e.g. water)

Question 15

What effects the strength of induced dipole-dipole interactions?

Answer 15

More electrons, larger instantaneous and induced dipoles, the greater the induced dipole, dipole interaction and the stronger the attractive forces between molecules.

Question 16

Why is the boiling point of a polar substance larger than a non polar substance with the same number of electrons.

Answer 16

1) Because non-polar molecules only have london forces
2) Polar substances have London forces and permanent dipole-dipole interactions between molecules
3) Extra energy is needed to break additional dipole-dipole interactions

Question 17

How do elements get their shapes?

Answer 17

The electron pairs repel one another so they are as far apart as possible to minimise repulsion giving them a definite shape.

Question 18

How much is the bond angle of a molecule reduced with each lone pair?

Answer 18

2.5 degrees

Question 19

What shape is a carbonate ion?

Answer 19

Trigonal planar

Question 20

What shape is a nitrate ion?

Answer 20

Trigonal planar

Question 21

What shape is a sulfate ion?

Answer 21

Tetrahedral

Question 22

What shape is an ammonium ion ?

Answer 22

Tetrahedral

Question 23

How are things charged in a polar molecule?

Answer 23

The more electronegative atom is negatively charged (d-) and the other atoms are positively charged (d+)

Question 24

How do you know if a molecule with more than two different atoms is polar?

Answer 24

It depends on the shape of the molecule, the dipoles may reinforce each other making a larger dipole or they may cancel each other out.

Question 25

Give an example of where two dipoles cancel each other out to make a non polar molecule?

Answer 25

Carbon dioxide

1) The two C=O have a permanent dipole
2) The two dipoles act in opposite directions because the shape is linear
3) Overall the dipole cancels out and the overall dipole is 0

Question 26

Explain how water breaks down a sodium chloride lattice (an ionic substance)?

Answer 26

1) Water attracts Na+ and Cl- ions
2) Na+ ions attracted towards oxygen of water molecules and Cl- ions are attracted towards the hydrogen in the water molecules
3) The ionic lattice breaks down and dissolves

Question 27

How do hydrogen bonds form?

Answer 27

Between molecules containing an electronegative atom with a lone pair of electrons and a hydrogen atom attaches to this electronegative atom.

Question 28

Why is solid Ice less dense than water?

Answer 28

Hydrogen bonds hold water molecules apart in an open lattice structure, the water molecules are held further apart in Ice than in water and this open lattice makes solid Ice less dense than liquid water and floats

Question 29

Why does water have a relatively high boiling point?

Answer 29

1) Water has london forces but also hydrogen bonds
2) A reasonably large amount of energy is needed to break the hydrogen bonds in water
3) This gives water a higher boiling point than what would be expected from just London forces.

Question 30

What are some other anomalous properties in water?

Answer 30

It has a relatively high surface tension this allows insects to walk on pond surfaces.

Question 31

bond enthalpy of London forces

Answer 31

1-10 kJ/mol

Question 32

bond enthalpy of permanent dipole-dipole interactions

Answer 32

3-25 kJ/mol

Question 33

bond enthalpy of hydrogen bonds

Answer 33

10-40 kJ/mol

Question 34

bond enthalpy of single covalent bonds

Answer 34

150-500 kJ/mol

Question 35

electronegativity difference in pure covalent bonds

Answer 35

0-0.4

Question 36

electronegativity difference in polar covalent bonds

Answer 36

0.4-1.8

Question 37

electronegativity difference in ionic bonds

Answer 37

greater than 1.8