Module 2 - Intermolecular Forces

Question 1

What are the 3 types of intermolecular force?

Answer 1

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

Question 2

Bond enthalpy of intermolecular forces:

Answer 2

Bond enthalpy/kJmol^-1
London forces = 1 - 10
Permanent dipole-dipole = 3 - 25
Hydrogen bonds = 10 - 40

Question 3

London forces (induced dipole):

Answer 3

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

Question 4

How does an induced dipole form?

Answer 4

As the electron cloud distorts, partial charges on the molecule are created due to the uneven distribution of electrons. This fluctuation in electron density creates an instantaneous dipole which induces a dipole in a neighbour molecule.

Question 5

Strength of induced dipole-dipole interactions (London forces):

Answer 5

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.

Question 6

London forces and boiling points:

Answer 6

-Larger numbers of electrons mean larger induced dipole.
-More energy is then needed to overcome the intermolecular forces, increasing the boiling point.
Examples…
He = 2e = -269C -lowest relative strength
Ne = 10e = -246C
Ar = 18e = -186*C -highest relative strength

Question 7

Permanent dipole-dipole interactions:

Answer 7

• Permanent dipole-dipole interactions act between permanent dipoles in different polar molecules.
• A permanent dipole has small charge differences that does not change across a bond (δ+ and δ- partial charges) due to the atoms in the molecule having different electronegativities.

Question 8

How does a permanent dipole form?

Answer 8

-Permanent dipoles are formed when there is a large difference in electronegativity between two atoms bonded together in a covalent bond. This causes the shared pair of electrons to be shared unequally.

Question 9

Comparing induced and permanent dipoles…

Answer 9

Molecule: Dipole: Induced: Permanent: No. e^-: b.p:
F-F none yes no 9x2=18 -220
H-Cl H(δ+)-Cl(δ-) yes yes 1+17=18 -85
=F molecules are non-polar and ONLY have London forces.
=HCl molecules are polar and have London forces AND permanent dipole-dipole interactions.
=Extra energy is needed to break the additional permanent dipole-dipoles between HCl molecules.
=Thus HCl b.p. is higher than F-F’s b.p.

Question 10

Simple molecular substances:

Answer 10

• form a regular structure called a simple molecular lattice.
• the molecules are held in place by weak intermolecular forces.
• the atoms within each molecule are bonded together strongly by covalent bonds.

Question 11

Properties: Low m.p. and b.p.

Answer 11

-in a simple molecular lattice, the weak intermolecular forces can be broken even by the energy present at low temps.
-simple molecular substances have low m.p. and b.p.
When a simple molecular lattice is broken apart during melting:
=only the weak intermolecular forces break.
=the covalent bonds are strong and do NOT break.

Question 12

Properties: Solubility 1 (NON-POLAR)

Answer 12

• when a simple molecular compound is added to a NON-POLAR SOLVENT such as hexane, 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.
• thus NON-POLAR simple molecular substances tend to be SOLUBLE in NON-POLAR SOLVENTS.

Question 13

Properties: Solubility 2 (NON-POLAR)

Answer 13

• 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.
• thus NON-POLAR simple molecular substances tend to be INSOLUBLE in POLAR SOLVENTS.

Question 14

Properties: Solubility 1 (POLAR)

Answer 14

• POLAR covalent substances may dissolve in POLAR SOLVENTS as the polar solute molecules and the polar solvent molecules can attract each other. The process is similar to dissolving of an ionic compound.
• The solubility depends on the strength of the dipole and can be hard to predict. Some compounds such as ethanol, C2H5OH, contain both polar (O-H) and non-polar (carbon chain) in their structure and can dissolve in both polar and non-polar solvents.
• Some biological molecules have hydrophobic and hydrophilic parts. Hydrophilic = polar, electronegative atoms. Hydrophobic = non-polar, carbon chains.

Question 15

Properties: Electrical conductivity

Answer 15

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