Chapter 6 - Shapes of molecules and intermolecular forces

Question 1

What is the electron pair repulsion theory used for?

Answer 1

Determining the shape of a molecule or ion

Question 2

Give the wedges used to visualise structures in 3 dimensions.

Answer 2

• Solid line represents a bond in the plane of the paper
• solid wedge comes out of the plane of the paper.
• Dotted wedge goes into the plane of paper

Question 3

Give the relative repulsions between lone pairs and bonding pairs.

Answer 3

Bonding pair/ bonding pair < bonding pair/ lone pair < lone pair/ lone pair

Question 4

What shape is a molecule with 4 bonding pairs? What is the bonding angle? Draw the shape.

Answer 4

Tetrahedral - 109.5 degrees

Question 5

What is the shape of a molecule with 3 bonding pairs and 1 lone pair? What is the bonding angle? Draw the shape.

Answer 5

Pyramidal - 107 degrees

Question 6

What shape is a molecule with 2 bonding pairs and 2 lone pairs? What is the bonding angle? Draw the shape.

Answer 6

Non linear - 104.5 degrees

Question 7

What shape is a molecule with 2 bonding pairs? What is the bonding angle? Draw the shape.

Answer 7

Linear - 180 degrees

Question 8

What shape is a molecule with 3 bonding pairs? What is the bonding angle? Draw the shape.

Answer 8

Triagonal planar - 120 degrees

Question 9

What shape is a molecule with 6 bonding pairs? What is the bonding angle? Draw the shape.

Answer 9

Octohedral - 90 degrees

Question 10

What is electronegativity?

Answer 10

The ability of an atom to attract shared electron pair in (covalent) bond

• measured on the pauling scale. Larger value = more electronegative
• Increases across a period, but decreases down a group

Question 11

Why are some bonds non-polar?

Answer 11

When the bonding atoms are the same or bonding atoms have similar electronegativity. This is due to electron pair being shared equally (pure covalent bond).

Question 12

What causes polar bonds?

Answer 12

• When a bonded electron pair is shared unequally between the bonded atoms (bonding atoms have different electronegativity).
• Small partial charges occur on each atom, with delta - on the atom with a higher electronegativity, and delta + on the atom with lower electronegativity.

Question 13

What is a dipole?

Answer 13

• separation of opposite charges (delta - and delta+) in a polar molecule

Question 14

What are intermolecular forces? And what are the 3 main categories.

Answer 14

Weak interactions between dipoles of different molecules.

• Induced dipole-dipole interactions (London forces)
• Permanent dipole-dipole interactions
• Hydrogen bonding

Question 15

What properties are intermolecular forces responsible for?

Answer 15

• Physical properties - such as melting and boiling point.
• covalent bonds determine identity + chemical reactions of molecules

Question 16

What are London forces?

Answer 16

Weak intermolecular forces that exist between all molecules, whether polar or non-polar. They act between induced dipoles in different molecules.

Question 17

How does an induced dipole form?

Answer 17

• Movement of electrons produces a changing dipole in a molecule. At any instant, an instantaneous dipole will exist, but its position is constantly changing.
• Instantaneous dipole induces a dipole on a neighbouring molecule.
• Induced dipole induces further dipoles on neighbouring molecules, which then attract one another.

Question 18

How does increasing electrons affect London forces?

Answer 18

• Larger instantaneous and induced dipoles. Greater induced dipole-dipole interactions the stronger the attractive forces between molecules.

Question 19

What are permanent dipole-dipole interactions?

Answer 19

• Act between permanent dipoles in different polar molecules. (permanent dipoles - small differences in charge between atoms bonded)

Question 20

What is hydrogen bonding?

Answer 20

• Permanent dipole-dipole interaction found between lone pair on a oxygen, nitrogen or fluorine atom and a hydrogen atom in different molecules.

Question 21

Why is ice less dense than water?

Answer 21

Hydrogen bonds in ice hold water molecules apart in open lattice structure.
hydrogen bonds are relatively long, so molecules on average will be further part in ice.

Question 22

Why does water have relatively high melting + boiling point?

Answer 22

• Hydrogen bonds are extra forces, in addition to London forces.
  Hydrogen bonds are strongest type of intermolecular bond. Lots of energy required to break bonds - and so higher temperature needed.

Question 23

Why might a compound not have an overall dipole, even if it has polar bonds?

Answer 23

Can be symmterical/ dipoles cancel

Question 24

When does hydrogen bonding strength increase?

Answer 24

Greater differences in electronegativity