Chapter 6

Question 1

What is the VSEPR theory?

Answer 1

Valence Shell Electron Pair Repulsion Theory

You can predict the shape of a molecular based on electrons
Electron pairs repel each other so they are as far apart as possible
It’s based on the central atom
The arrangement of electron pairs minimise the repulsion and holds the bonded atoms in a definite shape

Question 2

How do you draw a bond going towards and away from you?

Answer 2

Wedge/ triangle shape = toward
Dashed line = away

Question 3

Which pair of electrons repel more and why?

Answer 3

Lone pair electrons
This is because they are closer to the central atom and occupy more space than a bonded pair so they repel more strongly than bonded pairs

Question 4

How do you find the number for bonded pair of electrons and lone pairs in a compound?

Answer 4

Draw the compound covalently bonded and find the number of bonded and lone pairs of electrons

Question 5

What is the name of the shape and bond angle for a compound with 4 bonds and 0 lone pairs of electrons? Draw it:

Answer 5

Tetrahedral shape
109.5 degrees
E.g. methane
Check drawing

Question 6

What is the name of the shape and bond angle for a compound with 3 bonds and 1 lone pairs of electrons? Draw it:

Answer 6

Pyramidal shape
107 degrees
E.g. ammonia
Check drawing

Question 7

What is the name of the shape and bond angle for a compound with 2 bonds and 2 lone pairs of electrons?

Answer 7

Non - linear shape
104.5 degrees
E.g. water
Check drawing

Question 8

Why do compounds with more lone pairs have smaller bond angles?

Answer 8

As lone pairs repel more strongly than bonded pairs. Therefore lone pairs repel the bonded pairs slightly closer together, decreasing the bond angle

Question 9

What is the name of the shape and bond angle for a compound with 2 bonds and 0 lone pairs of electrons? Draw it:

Answer 9

Linear shape
180 degrees
E.g. carbon dioxide
Check drawing

Question 10

What is the name of the shape and bond angle for a compound with 3 bonds and 0 lone pairs of electrons? Draw it:

Answer 10

Trigonal planar shape
120 degrees
E.g. boron trifluoride
Check drawing

Question 11

What is the name of the shape and bond angle for a compound with 6 bonds and 0 lone pairs of electrons? Draw it:

Answer 11

Octahedral shape
90 degrees
E.g. sulfur hexafluoride
Check drawing

Question 12

How are multiple bonds regarded?

Answer 12

Each multiple bond is treated as a bonding region
Double bind = 1 “bonded pair”

Question 13

What is electronegativity?

Answer 13

Power of an atom to attract the electron density in a covalent bond towards itself

Question 14

How is electronegativity measured?

Answer 14

Pauling system
0-4, 4 being the highest

Question 15

What factors affect electronegativity?

Answer 15

Atomic radius (larger radius = lower)
Nuclear charge (higher nuclear charge = greater)
Electron shielding (more shielding = lower)
Same reasons as affecting ionisation energy

Question 16

What is the trend of electronegativity down the group?

Answer 16

Decrease of electronegativity:
- greater electron shielding
- atomic radius increases

Question 17

What is the trend of electronegativity across a period?

Answer 17

Increase of electronegativity:
- nuclear charge increases
- smaller atomic radius
So nuclear attraction to shared pair of electrons increases

Question 18

How can electronegativity be used to estimate type of bonding?

Answer 18

No difference in electronegativity between elements = covalent (Cl2)
0 - 1.8 difference in electronegativity = polar covalent (HCl)
1.8< = ionic (NaCl)

Question 19

What are pure covalent bonds?

Answer 19

Covalent bonds where the electrons are shared evenly as the bonded atoms are either the same or have the same electronegativity

Question 20

What are polar covalent bonds?

Answer 20

Shared pair of electrons where the electron pair is not shared equally between the bonded atoms

Question 21

How are polar covalent bonds shown?

Answer 21

Partial charges (lower case delta)

Question 22

What is a dipole?

Answer 22

The separation of opposite charges

Question 23

Which molecules contain intermolecular forces and what properties do they have?

Answer 23

Simple molecules
- LOW MP/BP
- don’t conduct electricity

Question 24

What are the 3 types of intermolecular forces?

Answer 24

1) induced dipole-dipole (London forces) - occurs in ALL atoms and molecules
2) permanent dipole-dipole forces - only for polar molecules
3) hydrogen bonding - certain molecules

Question 25

How do induced dipole-dipole interactions occur?

Answer 25

Due to an instantaneous dipole = movement of electrons which produces a temporary changing dipole that is constantly changing its position
This induces a dipole on a neighbouring molecule
The induced dipole induces further dipoles which then attract
This attraction is the intermolecular force

Question 26

When are London forces stronger in a molecule?

Answer 26

In a larger molecule there are more electrons which means the larger the instantaneous and up induced dipoles
Which means the greater the induced dipole-dipole interaction
So more energy is needed to overcome these interactions = higher BP

Question 27

When do permanent dipole-dipole interactions occur?

Answer 27

Polar molecule
Between 2 different molecules
The polar molecules will have partial charges which attract to other partial charges which causes the intermolecular attraction
E.g. H-Cl

Question 28

What is hydrogen bonding?

Answer 28

Strongest type of intermolecular force
Type of permanent dipole-dipole interaction

Question 29

Which molecule does hydrogen bonding occur for?

Answer 29

Hydrogen atom attached to F, O or N

Question 30

Where do hydrogen bonds act?

Answer 30

Acts between a lone pair of electrons on the electronegative atom on one molecule and a hydrogen atom in a different molecule

Question 31

What shape does hydrogen bonding make?

Answer 31

Linear

Question 32

What is the solubility of non-polar structures?

Answer 32

Dissolve in non polar solvents:
Intermolecular forces form between the simple molecular compound and the solvent molecules causing the simple molecules to break apart form on another and dissolve
Insoluble in polar solvents:
Little interaction between the simple molecules and solvent molecules as the intermolecular forces in polar molecules are too strong to be broken by interactions with non polar molecules

Question 33

What is the solubility of polar simple molecules?

Answer 33

Tend to dissolve in polar solvents:
Solvent molecules form interaction with the polar molecules causing them to break sort from one another

Question 34

What substances can dissolve in both non-polar and polar solvent?

Answer 34

Substances which have both non-polar and polar parts

Question 35

Why is water more dense than ice?

Answer 35

When water freezes the hydrogen bonds hold the water molecules in an open lattice structure
Water molecules are less closely packed in ice than water so ice is less dense and floats in water

Question 36

Why does water have a higher boiling point than expected?

Answer 36

As water also forms hydrogen bonds which are very string and require lots of energy to break these strong intermolecular forces
Therefore MP/BP are much higher than other group 16 hydrides