Chapter 6 - Shapes of molecules and intermolecular forces

Question 1

What is electron pair repulsion theory?

Answer 1

Electrons have a negative charge, therefore repel each other, so are arranged as far apart as possible

Question 2

What is the difference in repulsions between a lone pair and bonded pair?

Answer 2

A lone pair is slightly closer to the central atom and occupies more space than a bonded pair - so it repels more strongly than a bonding pair.

Question 3

What is the molecular shape and bond angle for a molecule with 4 bonded pairs?

Answer 3

Tetrahedral, 109.5°

Question 4

What is the molecular shape and bond angle for a molecule with 3 bonded pairs and one lone pair?

Answer 4

Pyramidal, 107°

Question 5

What is the molecular shape and bond angle for a molecule with 2 bonded pairs and 2 lone pairs?

Answer 5

Non linear, 104.5°

Question 6

What is the molecular shape and bond angle of a molecule with 2 electron pairs/regions?

Answer 6

Linear, 180°

Question 7

What is the molecular shape and bond angle for a molecule with 3 electron pairs/regions?

Answer 7

Trigonal planar, 120°

Question 8

What is the molecular shape and bond angle for a molecule with 6 electron pairs/regions?

Answer 8

Octahedral, 90°

Question 9

What is electronegativity?

Answer 9

The ability of an atom to attract a bonding pair of electrons

Question 10

How does electronegativity change across the periodic table?

Answer 10

Electronegativity increases along each period and decreases down each group (due to nuclear charge increasing and atomic radius decreasing)

Question 11

What are the most electronegative atoms?

Answer 11

Nitrogen, oxygen, fluorine and chlorine

Question 12

What is a non polar bond, and where does it occur?

Answer 12

A bond where the bonded electron pair is shared equally between the bonded atoms. It occurs when the bonded atoms are the same or the bonded atoms have similar/the same electronegativity

Question 13

What is a polar bond and where does it occur?

Answer 13

A bond where the bonded electron pair is shared unequally between the bonded atoms. Bonds are polar when the bonded atoms are different and have different electronegativity values

Question 14

What is a dipole?

Answer 14

The separation of charges in a molecule

Question 15

What are the three types of intermolecular forces?

Answer 15

Induced dipole-dipole interactions, permanent dipole-dipole interactions, hydrogen bonding

Question 16

How do induced dipole-dipole interactions work?

Answer 16

Movement of electrons in a molecule produces a changing dipole. At any time an instaneous dipole exists, but it is constantly shifting. Instantaneous dipole induces a dipole on neighbouring molecule, which induces further dipoles on other molecules, which then attract one another

Question 17

Where do induced dipole-dipole interactions occur?

Answer 17

Exist between all molecules, whether polar or non polar

Question 18

What increases the strength of induced dipole-dipole interactions?

Answer 18

Larger number of electrons mean larger induced dipoles (larger instantaneous and induced dipoles so stronger attractive forces)

Question 19

How do permanent dipole-dipole interactions work?

Answer 19

Permanent dipoles in polar molecules have attractive forces between them

Question 20

What structure are simple molecular substances, and how is this held together?

Answer 20

Simple molecular lattice, molecules held in place by weak intermolecular forces - but atoms within each molecule are bonded together strongly by covalent bonds

Question 21

Why do simple molecular structures have low melting and boiling points?

Answer 21

Only have to break weak intermolecular forces to break lattice, so little energy is required

Question 22

What is the solubility of non-polar simple molecular structures?

Answer 22

Soluble in non polar solvents as intermolecular forces form between molecules and solvent, breaking lattice
Not soluble in polar solvent as intermolecular bonding in polar solvent is too strong to be broken

Question 23

What is the solubility of polar simple molecular substances?

Answer 23

May dissolve in polar solvents as polar solute molecules and polar solvent molecules can attract each other.
Solubility depends on strength of dipole

Question 24

What is the electrical conductivity of simple molecular substances?

Answer 24

There are no mobile charged particles in simple molecular structures. With no charged particles to move, there is nothing to complete and electrical circuit

Question 25

What is a hydrogen bond?

Answer 25

A type of permanent dipole-dipole interaction found between molecules containing:

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

Question 26

What are the strongest of the three intermolecular forces?

Answer 26

Hydrogen bonds

Question 27

What are the anomalous properties of water?

Answer 27

The solid (ice) is less dense than the liquid (water), and it has a relatively high melting and boiling point

Question 28

Why is ice less dense than water?

Answer 28

Hydrogen bonds hold water molecules apart in an open lattice structure, so the water molecules in ice are further apart than in water, it is therefore less dense