2.5 - Shapes of molecules and intermolecular forces

Question 1

What are the two types of electron pairs molecules and ions posses ?

Answer 1

• Bonding pairs ( the two shared electrons in a covalent bond )
• Lone pairs ( two electrons in a pair not involved in bonding = non-bonding pairs )

Question 2

What is the shape of molecules and ions determined by ?

Answer 2

The electron pairs surrounding the central atom of a molecule/ ion determine the shape of its molecule/ ions

Question 3

What is ‘repulsion theory‘ ?

Answer 3

Repulsion theory is a model used in chemistry used for explaining and predicting shapes of molecules and poly atomic ions

Question 4

Explain how electron pairs surrounding the central atom determine the shape of a molecule/ ion ?

Answer 4

• Electron pairs repel each other as they have alike charges
• This means electron pairs are arranged as far as possible
• Arrangement of electron pairs minimises repulsion and therefore holds bonded atoms in a definite shape
• Different number of electron pairs result in different shapes

Question 5

Why do lone pairs repel more than bonding pairs ?

Answer 5

Lone pairs are more compact than bonding pairs and therefore lone pairs repel more than bonding pairs

Question 6

What is the effect of lone pairs being more compact than bonding pairs on the bond angle ?

Answer 6

Since Lone pairs are more compact than bonding pairs this reduces the bond angle to a small extent

Question 7

What is the name, shape and bond angle of molecules with two electron pairs ?

Answer 7

Question 8

What is the name, shape and bond angle of molecules with three electron pairs ?

Answer 8

Question 9

What is the name, shape and bond angle of molecules with four electron pairs ?

Answer 9

Question 10

What is the name, shape and bond angle of molecules with five electron pairs ?

Answer 10

trigonal bipyramidal

Question 11

What is the name, shape and bond angle of molecules with six electron pairs ?

Answer 11

Question 12

What is the name, shape and bond angle of molecules with three bonding pairs and one lone pair ?

Answer 12

pyramidal

Question 13

What is the name, shape and bond angle of molecules with two bonding pairs and two lone pairs ?

Answer 13

Question 14

What is the name, shape and bond angle of molecules with four bonding pairs and two lone pairs ?

Answer 14

Question 15

What is the name, shape and bond angle of molecules with two double bonds ?

Answer 15

linear, 180o

Question 16

What is the name, shape and bond angle of molecules with one triple and one single bond ?

Answer 16

linear, 180o

Question 17

What is the name, shape and bond angle of molecules with one double bond and four single bonds ?

Answer 17

planar, 118o

Question 18

What is the method to figure out the shape of molecules/ ions ?

Answer 18

• Work out how many outer shell electrons are on the central atom (accounting for any charge on the species)
• Work out how many atoms are bonded to the central atom (= the number of bonding pairs)
• Work out how many lone pairs there are (which is half the difference between (a) and (b))

Question 19

Do you know how to explain the shape of molecules and ions such as linear, non-linear, trigonal planar, pyramidal, tetrahedral and octahedral using electron pair repulsion ?

Answer 19

Yes

eg. It is … shape because there are … bonding pairs and … lone pairs ( and lone pairs repel more than bonding pairs )

Question 20

What are the bonding angles for the following : CHv4, NHv3 and Hv2O ?

Answer 20

CHv4 = 109.5o
NHv3 = 107o
Hv2O = 104.5o

Question 21

What is a polar bond ?

Answer 21

A bond formed when there is a difference in the electronegativity of two covalently bonded atoms ( the bond will have a dipole )

Question 22

What is a permanent dipole ?

Answer 22

Partial charges

eg. H ( δ+ ) - Cl ( δ- )

Cl is more electronegative than H, so has a greater share of bonding electrons

Question 23

What is a polar molecule ?

Answer 23

A molecule where the dipoles of the molecule do not cancel out resulting in the molecule being polar. These dipoles are permanent.

Question 24

What is a non-polar molecule ?

Answer 24

A molecule where the dipoles of the molecule cancel out meaning it is non-polar. Therefore these dipole are not permanent.

Question 25

When are molecules non-polar ?

Answer 25

When the molecule is symmetrical

Question 26

What are the three types of intermolecular forces ?

Answer 26

- Hydrogen bonding - Permanent dipole-dipole - London force/ Induced dipole-dipole

Question 27

What is the order of strength of the different intermolecular forces ?

Answer 27

1 ) Hydrogen bonding 2 ) Permanent dipole-dipole 3 ) London force/ Induced dipole-dipole

Question 28

Explain what is London force and how it comes about ?

Answer 28

- London force is the weakest type of intermolecular force - This occurs due to constant movement of electrons which causes uneven distribution of charge which causes temporary dipoles - This induces a temporary dipole in the adjacent molecule which produces an induced dipole-dipole attraction

Question 29

What is an induced dipole-dipole interaction also known as ?

Answer 29

London forces

Question 30

What factors affect London force ?

Answer 30

- Number of electrons - Shape of molecule

Question 31

Explain how the number of electrons affects London force ?

Answer 31

- The greater the number of electrons , the greater the size of the temporary and induced dipoles - This means the London force is stronger between molecules

Question 32

Explain how the shape of the molecule affects London force ?

Answer 32

- Unbranched molecules have a greater surface area and greater surface contact between molecules than branched molecules - This means the London force is greater between molecules

Question 33

Explain what is permanent dipole-dipole interaction and how it comes about ?

Answer 33

- Permanent dipole-dipole interaction only happens between polar molecules - It is the attraction between the permanent dipoles of adjacent polar molecules

Question 34

What factors affect permanent dipole-dipole interaction ?

Answer 34

Magnitude of electronegativity difference

Question 35

Explain how the magnitude of the difference in electronegativity affects permanent dipole-dipole interaction ?

Answer 35

- The greater the difference in electronegativity between two bonded atoms, the greater the permanent dipole - The greater the permanent dipole on a molecule, the stronger the permanent dipole-dipole interaction between molecules

Question 36

What is a hydrogen bond ?

Answer 36

- A special type of permanent dipole-dipole interaction - It acts between a hydrogen atom in a molecule and a lone pair of electrons on an electronegative atom

Question 37

Where does a hydrogen bond occur ?

Answer 37

It occurs between a very electronegative atom ( ie. oxygen, nitrogen or fluorine ) with a lone pair of electrons and a hydrogen atom bonded to a very electronegative atom

Question 38

What should you include the drawing hydrogen bonds ?

Answer 38

- Dipoles - Lone pairs - Hydrogen bonds

Question 39

Do you know how to draw hydrogen bonds ?

Answer 39

Yes

Question 40

What are the anomalous characteristics of water due to hydrogen bonding ?

Answer 40

- Ice is less dense than water - Water has a relatively high melting and boiling point

Question 41

Explain why ice is less dense than water ?

Answer 41

- Hydrogen bonds hold water molecules apart in an open lattice structure - Therefore the water molecules are farther part in ice than in water and it floats

Question 42

Explain why water has a relatively high melting and boiling point ?

Answer 42

- Water contains London forces between molecules - Water contains hydrogen bonds between water molecules - This means that more energy is required to break hydrogen bonds between water molecules causing water to have a higher melting and boiling point

Question 43

Explain the structure of simple molecules ?

Answer 43

- Atoms are chemically joined together by strong covalent bonds - Molecules are joined together by weak intermolecular forces - This forms a simple molecular lattice structure

Question 44

What are the features of the simple molecular lattice structure ?

Answer 44

- High melting / boiling point - Cannot conduct electricity - They are insoluble in polar solvents

Question 45

Explain why simple molecules have a low melting/ boiling point ?

Answer 45

- They have strong covalent bonds between atoms but weak intermolecular forces between molecules - Therefore, not a lot of energy is required to break weak intermolecular forces between molecules and simple molecules have a low melting / boiling point

Question 46

Explain why simple molecules cannot conduct electricity ?

Answer 46

- They have no mobile ions and therefore cannot conduct electricity

Question 47

Explain why simple molecules are insoluble in polar solvents ?

Answer 47

- They are typically non-polar molecules - This means they are not soluble and are not dissolved by a polar solvent