Module 2 - C6 - Shapes of molecules and intermolecular forces

Question 1

What is the electron-pair repulsion theory?

Answer 1

• Electrons have a negative charge so pairs repel each other
• Therefore the electron pairs/bonded regions are arranged as far from each other as far as possible
• This minimises repulsion and holds the atoms in a definite shape

Question 2

What are lone-pairs?

Answer 2

Lone pairs are pairs of electrons that haven’t been bonded to another atom:

• Closer to the central atom and occupy more space
• Therefore repel more strongly than a bonded pair
• Bond angle is reduced by 2.5 degrees for each lone pair

Question 3

What are the different angles and shapes to remember?

Answer 3

• 4 bonded pairs, 0 lone pairs, tetrahedral 109.5
• 3 bonded pairs, 1 lone pair, pyramidal 107
• 2 bonded pairs, 2 lone pairs, non-linear 104.5
• 3 bonded pairs/regions, 0 lone pairs, trigonal planar 120
• 2 bonded pairs/regions, linear 180
• 6 bonded pairs/regions, octahedral 90

Question 4

Explain the shape of a CH4 molecule:

Answer 4

• Tetrahedral
• This is because there are 4 bonded pairs and 0 lone pairs around the central carbon atom
• These repel each other as far as possible producing a tetrahedral shape with a bond angle of 109.5

Question 5

Define electronegativity:

Answer 5

The attraction of the nucleus of a bonded atom to the pair of electrons within a covalent bond

Question 6

What factors affect electronegativity?

Answer 6

• bigger the nuclear charge the more electronegative

- smaller the atomic radius the more electronegative

Question 7

What is the Pauling scale?

Answer 7

Used to find the electronegativity of atoms, as you go across and up the electronegativity value gets higher

Question 8

What is a covalent bond?

Answer 8

The strong electrostatic force of attraction between the nucleus and the shared pair of electrons

Question 10

What is the difference between a polar and non polar covalent bond?

Answer 10

• In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms. It will be non polar when the electronegativity is the similar or the bonded atoms are the same(pure covalent)
• In a polar bond, the bonded electron pair is shared unequally between the bonded atoms, being attracted more towards the atom with a higher electronegativity. This means there will be a partial negative charge on the higher electronegativity atom and a partial positive charge on the other creating a permanent dipole

Question 11

Describe 2 anomalous properties of water?

Answer 11

Solid water(ice) is less dense than water, this is because the molecules of water in ice are held apart in an open lattice by hydrogen bonds which reduces the density.

Water has a RELATIVELY high boiling point because it contains hydrogen bonds which are relatively strong and require a lot of energy to overcome

Question 12

Why is H2O a polar molecule?

Answer 12

• The two O-H bonds have a permanent dipole
• The molecule is asymmetrical therefore the dipoles act in different directions but do not directly oppose each other
• Overall the oxygen end has a partial negative charge and the hydrogen end a partial positive charge meaning there is an overall dipole

Question 13

Describe and explain if CO2 is a polar or non-polar molecule?

Answer 13

It is non-polar

• The two C—O bonds have a permanent dipole
• However these dipoles act in opposite directions and as the molecule is symmetrical they directly oppose each other
• Therefore overall the dipoles cancel each other out so there is no overall dipole and the molecule is non-polar

Question 14

How do London forces arise between molecules?

Answer 14

• The constant movement of electrons produces a changing dipole within a molecule
• At any instant an instantaneous dipole will exist as there could be more electrons to one side of the molecule giving it a partial negative charge and the other side a partial positive charge
• This instantaneous dipole induces a dipole on a neighbouring molecule
• This induced dipole induces further dipoles on neighbouring molecules which then attract each other

Question 15

What are permanent dipole-dipole interactions?

Answer 15

These interactions act between the oppositely charged permanent dipoles in polar molecules such as HCl

Question 16

What are the requirements for hydrogen bonding to occur?

Answer 16

• an electronegative atom with an active lone pair of electrons
• a hydrogen atom attached to an electronegative atom

Question 17

What can polar and non-polar substances be dissolved in?

Answer 17

Polar substances can be dissolved in polar solvents

Non-polar substances can be dissolved in non-polar substances

Question 18

What are the two requirements for hydrogen bonding?

Answer 18

• An electronegative atom with an active lone pair

- A hydrogen atom

Question 19

Why does NH3 have a higher boiling point than PH3?

Answer 19

• NH3 contains hydrogen bonds as nitrogen is an electronegative atom with an active lone pair and hydrogen atoms
• These are relatively strong and require more energy to break therefore increasing the boiling point