M2 Shapes of Molecules and Intermolecular Forces

Question 1

What is the electron-pair repulsion theory?

Answer 1

Electron pairs surrounding a central atom determine the shape of the molecule or ion.
The electron pairs repel one another so they are arranged as far apart as possible.
The arrangement of electron pairs minimised repulsion, therefore holds bonded atoms in a definite shape.
Different numbers of electron pairs result in different shapes.

Question 2

What shape is formed with two bonded pairs of electrons and two lone pairs of electrons?

Answer 2

Non-linear

Question 3

What shape is formed with 3 bonded pairs of electrons and 0 lone pairs of electrons?

Answer 3

Trigonal planar

Question 4

What shape is formed with 4 bonded pairs of electrons and 1 lone pair of electrons?

Answer 4

Pyramidal

Question 5

What shape is formed from 4 bonded pairs of electrons and 0 lone pairs of electrons?

Answer 5

Tetrahedral

Question 6

What shape is formed from 6 bonded pairs of electrons and 0 lone pairs?

Answer 6

Otrahedral

Question 7

What shape is formed from 2 bonded pairs of electrons and 0 lone pairs of electrons?

Answer 7

Linear

Question 8

What bond angle is non-linear?

Answer 8

104.5°

Question 9

What bond angle is trigonal planer?

Answer 9

120°

Question 10

What bond angle is pyramidal?

Answer 10

107°

Question 11

What bond angle is tetrahedral?

Answer 11

109.5°

Question 12

What bond angle is otrahedral?

Answer 12

90°

Question 13

What bond angle is linear?

Answer 13

180°

Question 14

What creates a difference in electron pair repulsion?

Answer 14

Lone pair - lone pair > lone pair - bonded pair > bonded pair - bonded pair
A lone pair is closer to the central atom, and occupies more space than a bonded pair, therefore a lone pair repels more strongly than a bonding pair.

Question 15

Principles and of electron-pair repulsion

Answer 15

• Electron pairs around the central atom repel each other as far as possible
• The greater the number of electron pairs, the smaller the bond angle
• Lone pairs of electrons repel more strongly than bonded pairs of electrons

Question 16

Define electronegativity

Answer 16

Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond

Question 17

How is electronegativity measured?

Answer 17

• Using the Pauling scale, the Pauling electronegativity values depend on an elements position in the periodic table.
• Across the periodic table the nuclear charge increases and atomic radius decreases.
  0.0-0.4 (non polar)
  0.4-1.7 (polar)
  >1.7 (ionic)
• Non metals have the most electronegative atoms, group 1 metals have the least electronegative atoms.
• Electronegativity increases up and across the periodic table.

Question 18

Describe a non-polar bond

Answer 18

In a non-polar bond, the bonded electron pair is shared equally between bonded atoms:
- the bonded atoms are the same, or
- the bonded atoms have the same or similar electronegativity
In molecules of elements such as oxygen, hydrogen the bonded atoms come from the same element and the electron pair is shared equally - this bond is a pure covalent bond.

Question 19

Describe a polar bond

Answer 19

A difference in electronegativity between bonded atoms creates a polar bond.
In a polar bond, the bonded electron pair is shared unequally between the bonded atoms. A bond will be polar when the bonded atoms are different and have different electronegativity values, resulting in a polar covalent bond.
*the greater electronegative atom is has the negative charge

Question 20

Describe a polar molecule

Answer 20

HCl is a polar molecule as the H-Cl bond has a permanent dipole acting in the direction of the H-Cl bond.
For molecules with more than two atoms, there may be two or more polar bonds.
Dipoles may reinforce each other, or cancel out if acting in opposite directions.

*C-H is non polar

Question 21

Define intermolecular forces

Answer 21

Intermolecular forces are weak interactions between dipoles of different molecules:
- induced dipole-dipole interactions
- permanent dipole-dipole interactions
- hydrogen bonding

Question 22

Describe induced dipole-dipole interactions

Answer 22

• Induced dipole-dipole interactions are weak intermolecular forces that exist between all molecules, forming they act between induced dipoles in different molecules.
• Movement of electrons produces a changing dipole in a molecule. At any instant an instantaneous dipole will exist, but its position is constantly changing. The instantaneous dipole induces a dipole on a neighbouring molecule. The induced dipole induces further dipoles on neighbouring molecules, which then attract one other.
• Induced dipoles are only temporary.

Question 23

Describe the strength of induced dipole-dipole interactions

Answer 23

The more electrons in each molecule:
- the larger the instantaneous and induced dipoles
- the greater the induced dipole-dipole interactions
- the stronger the attractive forces between molecules (higher boiling + melting points)

Question 24

Describe a permanent dipole-dipole interactions

Answer 24

• Permanent dipole-dipole interactions act between the permanent dipoles in different polar molecules.
• Occur when a permanent dipole exists in a polar molecule (electronegativity 0.4 - 1.7)

Question 25

Define a simple molecular substance

Answer 25

• A simple molecular substance is made up of simple molecules - small units containing a number of atoms with a definite molecular formula.
• In a solid state, simple molecules form a regular structure called a simple molecular lattice: molecules are held in place by weak intermolecular forces and atoms within each molecule are bonded more strongly by covalent bonds.

Question 26

Describe the melting and boiling points of simple molecular substances

Answer 26

• All simple molecular substances are covalently bonded
• In a simple molecular lattice, the weak intermolecular forces can be broken by energy present at low temperatures, so they have low melting and boiling points
• This is because only the weak intermolecular forces are broken, the covalent bonds are strong and do not break

Question 27

Describe the solubility of non-polar covalent substances

Answer 27

• Covalent substances with simple molecular structures are polar and non-polar
• Non-polar: intermolecular forces form between the molecules and the solvent, the interactions weaken the intermolecular forces in the simple molecular lattice, the intermolecular forces break and the compound dissolves, therefore non-polar simple molecular forces are soluble in non-polar solvents.
• When a simple molecular substance is added to a polar solvent, there is little interaction between the molecules in the lattice and the solvent molecules. The intermolecular bonding within the polar solvent is too strong to be broken. Therefore simple molecular substances are insoluble in polar solvents.

Question 28

Describe the solubility of polar covalent substances

Answer 28

• Polar covalent substances may dissolve in polar solvents as the polar solute molecules and the polar solvent molecules can attract each other.
• The solubility depends on the strength of the dipole
• Some biological molecules contain hydrophobic and hydrophilic parts, the hydrophilic part is polar and contains electronegative atoms that can interact with water. The hydrophobic part will be non-polar and comprised of a carbon chain.

Question 29

Describe the electrical conductivity of simple molecular substances

Answer 29

• There are no free charges particles in simple molecular structures, with no charged particles that can move there is nothing to complete an electrical circuit.
• Therefore simple molecular structures are non-conductors of electricity.

Question 30

Describe a hydrogen bond

Answer 30

A hydrogen bond is a permanent dipole-dipole interaction found between molecules containing:
- an electronegative atoms with a lone pair of electrons: oxygen, nitrogen or fluorine
- a hydrogen atom attached to an electronegative atom (H-O, H-N, or H-F)

The hydrogen bond acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in another molecule, they are the strongest type of intermolecular attraction.

Question 31

Describe the density of water

Answer 31

Ice is less dense than liquid water:
- Hydrogen bonds hold water molecules apart in an open lattice structure
- The water molecules in ice are further apart than in water
- Solid ice is less dense than liquid water and floats

Ice floats in lakes and ponds, forming an insulating layer and preventing water from freezing solid. With two lone pairs in the oxygen atom, and two on the hydrogen atom, each water molecule can form four hydrogen bonds, forming an open tetrahedral lattice full of holes which decrease the density of water.

Question 32

Describe the melting point and boiling point

Answer 32

• Hydrogen bonds are extra forces as well as the London forces
• Therefore more energy is needed to break the hydrogen bonds in water, so water has a much higher melting and boiling than just London forces
• When the ice lattice breaks, the rigid arrangement of hydrogen bonds in ice is broken. When water boils, the hydrogen bonds break completely.