6) Shapes of molecules and intermolecular forces

Question 1

Describe electron-pair repulsion theory (4)

Answer 1

• 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 pairs result in different shapes

Question 2

What are wedges used for?

Answer 2

to help visualise 3D structures

Question 3

A lone pair of electrons is _ and _ than a bonded pair. This results in a lone pair _ more strongly than a bonding pair.

Answer 3

slightly closer to the central atom
occupies more space
repelling

Question 4

How much is the bond angle reduced per lone pair?

Answer 4

about 2.5 degrees

Question 5

Define bond angle

Answer 5

the angle between two bonds at an atom

Question 6

Give the name of the shape, its bond angle and an example for a molecule with 3 bonded pairs and 1 lone pair

Answer 6

pyramidal
107 degrees
NH3

Question 7

Give the name of the shape, its bond angle and an example for a molecule with 2 bonded pairs and 2 lone pairs

Answer 7

non-linear
104.5 degrees
H2O

Question 8

Give the shape, its bond angle and an example for a molecule with 2 electron pairs / regions

Answer 8

linear
180 degrees
CO2

Question 9

Give the name of the shape, its bond angle and an example for a molecule with 3 electron pairs / regions

Answer 9

trigonal planar
120 degrees
BF3

Question 10

Give the name of the shape, its bond angle and an example for a molecule with 4 electron pairs / regions

Answer 10

tetrahedral
109.5 degrees
CH4

Question 11

Give the name of the shape, its bond angle and an example for a molecule with 6 electron pairs / regions

Answer 11

octahedral
90 degrees
SF6

Question 12

Define electronegativity

Answer 12

a measure of the attraction of a bonded atom fro the pair of electrons in a covalent bond

Question 13

Define Pauling electronegativity value

Answer 13

a value assigned as a measure of the relative attraction of a bonded atom for the pair of electrons in a covalent bond

Question 14

What does Pauling electronegativity value depend upon?

Answer 14

an element’s position in the periodic table

as you go up and right electronegativity increases

Question 15

What will happen if the electronegativity difference is large?

Answer 15

one bonded atom will have a much greater attraction for the shared pair of electrons than the other bonded atom
the more electronegative atom will have gained control of the e- and the bond will now be ionic rather than covalent

Question 16

Give the electronegativity differences for the following types of bonding:
covalent
polar covalent
ionic

Answer 16

0
0 - 1.8
> 1.8

Question 17

Describe a non-polar bond

Answer 17

the bonded pair of electrons is shared equally between bonded atoms that are the same / have the same or similar electronegativity values

Question 18

Describe a pure covalent bond

Answer 18

when bonded atoms come from the same element and the electron pair is shared equally

Question 19

Describe a polar bond

Answer 19

the bonded electron pair is shared unequally between the bonded atoms that are different and have different electronegativity values

Question 20

Define dipole

Answer 20

a separation in electrical charge so that one atom of a polar covalent bond, or one end of a polar molecule, has a small positive charge and the other has a small negative charge

Question 21

The atom with the larger electronegativity value has the _ charge. The atom with the smaller electronegativity value has the _ charge.

Answer 21

partially negative

partially positive

Question 22

Define permanent dipole

Answer 22

a small charge difference that does not change across a bond, with positive and negative partial charges on the bonded atoms: the result of the bonded atoms have different electronegativities

Question 23

Depending on the shape of a molecule dipoles may _ or _?

Answer 23

reinforce one another to produce a larger dipole over the whole molecule
cancel out if dipoles act in opposite directions

Question 24

Define intermolecular force

Answer 24

an attractive force between molecules

Question 25

Define induced dipole-dipole interactions (London forces)

Answer 25

attractive forces between induced dipoles in different molecules

Question 26

Describe London forces (3)

Answer 26

only temporary exist between all molecules (polar and non-polar) the more electrons in each molecule, the stronger the force

Question 27

How do induced dipole-dipole interactions occur?

Answer 27

1. movement of e- produces a changing dipole in a molecule 2. at any instant, an instantaneous dipole will exist, but its position is constantly shifting 3. the instantaneous dipole induces a dipole on a neighbouring molecule 4. the induced dipole induces further dipoles on neighbouring molecules which then attract each other

Question 28

Define permanent dipole-dipole interactions

Answer 28

an attractive force between permanent dipoles in neighbouring polar molecules

Question 29

Define hydrogen bonding

Answer 29

a strong dipole-dipole attraction between an electron-deficient hydrogen atom of -NH, -OH or HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule

Question 30

Name one significant instance where hydrogen bonding occurs (related to biology)

Answer 30

occurs between purine and pyrimidine bases in DNA to form the double helix structure

Question 31

Give 3 anomalous properties of water

Answer 31

solid ice is less dense than liquid water relatively high melting point and boiling point relatively high surface tension and viscosity

Question 32

Explain how solid ice is less dense than liquid water

Answer 32

hydrogen bonds hold water molecules apart in an open lattice structure, so that they are further apart in ice than water - making ice less dense and able to float

Question 33

What is a simple molecular substance made up of?

Answer 33

simple molecules

Question 34

Define simple molecular lattice

Answer 34

a 3D structure of molecules, bonded together by weak intermolecular forces

Question 35

Give 3 properties of simple molecular lattice

Answer 35

* low melting point and boiling point * non-polar dissociates in non-polar solvents and polar in polar depending on the strength of dipole * non-conductors of electricity

Question 36

What should you note about the melting and boiling of simple molecular lattices?

Answer 36

strong covalent bonds do not break on changing state, only the weak intermolecular interactions

Question 37

Explain why NF3 has a permanent dipole

Answer 37

F is more electronegative than N so contains polar bonds; the molecule is non-symmetrical so dipoles do not cancel out