CHAPTER 6: Shapes of Molecules & Intermolecular Forces

Question 1

Give the 4 key points of Electron-pair repulsion theory.

Answer 1

1) Electron pairs surrounding a central atom determines the shape of the molecule.
2) The electron pairs repel and so are arranged as far as possible.
3) Arrangement minimises repulsion and holds the bonded atoms in a DEFINITE shape.
4) different number of electron pairs results in different shapes.

Question 2

What does a solid line, wedge and a dotted wedge represent with a 3D diagram?

Answer 2

solid line - bond in the plane of paper
wedge - coming out
dotted wedge - into the plane of paper

Question 3

Why do lone pairs repel more strongly?

Answer 3

Lone pairs are slightly closer to central atom so occupy more space.

Question 4

What affect does lone pairs have on a shape of a molecule?

Answer 4

Repels more strongly that bonded pairs so it causes the bonded pairs to move slightly closer together and therefore decreasing the bond angle.

Question 5

Name the shape and bond angle of a molecule with just 4 bonded pairs.

Answer 5

TETRAHEDRAL

109.5º

Question 6

Name the shape of a molecule with 3 bonded pairs and 1 lone pair.
Give the bond angle between the bonded pairs.

Answer 6

PYRAMIDAL

107º

Question 7

What shape does a water molecule have?

What is the bond angle between the bonded pairs?

Answer 7

NON- LINEAR

104.5º

Question 8

How do you determine the molecular shapes of molecules with multiple bonds?

Answer 8

Treat each multiple bond as a bonding region.

Question 9

What is the name of the shape of molecules with:
a) 2 bonding regions
b) 3 bonding regions
c) 6 bonding regions?
(give bonding angles)

Answer 9

a) linear - 180ª
b) trigonal planar - 120º
c) octahedral - 90º

Question 10

Why does a molecule with 6 bonded pairs have an OCTAHEDRAL shape?

Answer 10

When the 6 sides are joined up to form a 3D shape, it has 8 sides.

Question 11

Define electronegativity.

Answer 11

The attraction of a bonded atom for the pair of electrons within a covalent bond.

Question 12

What 3 factors changes the attraction for the electron pair within a bond?

Answer 12

1) the nuclear charges of the bonded atoms being different.
2) atoms may be different sizes
3) shared pair of electrons may be closer to one nucleus than the other.

Question 13

What 2 trends occur across a period?

Answer 13

1) nuclear charge increases

2) atomic radius decreases (electrons more attracted so brought closer)

Question 14

When might the bond be considered ionic regarding electronegativity?

Answer 14

When the electronegativity difference is large so an atom will have a greater attraction for the electron pair and consequently gained control of the electrons.

Question 15

When is a bond NON-POLAR?

Answer 15

When the bonded electron pair is shared equally because the bonded atoms are the same or have similar electronegativity.

Question 16

When is a bond POLAR?

Answer 16

When the bonded electron pair is not shared equally because the bonded atom are different have different electronegativity resulting in small partial charges and inducing a permanent dipole.

Question 17

When might a molecule be considered non-polar even though it consists of polar bonds?

Answer 17

When the dipoles cancel out to give an overall larger dipole of 0.

Question 18

What are the 3 types of intermolecular forces?

Answer 18

1) London forces
2) permanent dipole-dipole interactions
3) hydrogen bonding

Question 19

Which intermolecular force is the weakest and which is the strongest?

Answer 19

weakest - london forces

strongest - hydrogen bonding

Question 20

What are london forces?

Answer 20

• due to the random movement of electrons, produces a changing dipole in a molecule.
• the instantaneous dipole induces a dipole on a neighbouring molecule
• the induced dipole induces further dipoles on neighbouring molecules which then attract one another.

Question 21

What causes london forces to become stronger?

Answer 21

More electrons and more london forces

Question 22

What are permanent dipole-dipole interactions?

Answer 22

Between polar molecules the partially positive end of one molecule and a partially negative end of another.

Question 23

Describe how molecules are held in a simple molecular lattice?

Answer 23

Molecules held together with weak intermolecular forces.

Atoms held together with strong covalent bonds.

Question 24

How do simple molecular substances change state?

Answer 24

Breaking the intermolecular forces changes the state.

The covalent bonds are too strong to break.

Question 25

Are non-polar simple molecular substances soluble in non-polar solvents?

Answer 25

YES
intermolecular forces form between the molecules and the solvents.
The interactions successfully breaks the intermolecular forces and compound dissolves.

Question 26

Are non-polar simple molecular substances soluble in polar solvents?

Answer 26

NO
the intermolecular permanent dipole of the solvents are stronger than the london forces of non-polar substances so little interaction

Question 27

Are polar simple molecules soluble?

Answer 27

in polar solvents the polar solute molecules and polar solvent molecules can attract each other.

Question 28

Why is the solubility of polar simple molecules hard to predict?

Answer 28

Depends on the strength of the dipole.

Question 29

What are hydrogen bonds?

Answer 29

bond between a lone pair of an electronegative atom of one molecule and a hydrogen attached to a electronegative atom of another molecule.

Question 30

What anomalous properties of water are caused by hydrogen bonding?

Answer 30

• The solid (ice) is less dense than the liquid (water)
• Relatively high melting point and boiling point
• High surface tension and viscosity

Question 31

Whys is ice less dense than water?

Answer 31

Because each water molecule can form 4 hydrogen bonds which extend outwards holding water molecules slightly apart forming an open tetrahedral lattice full of holes which decrease the density of water when froze to form ice.

Question 32

Why does water have such a high melting point?

Answer 32

Has london forces and hydrogen bonds.

London forces require more energy to break than london forces