6.3 Intermolecular forces

Question 1

What are intermolecular forces

Answer 1

Weak interactions between dipoles of DIFFERENT molecules

Question 2

What are the three main categories of intermolecular forces

Answer 2

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

Question 3

What are intermolecular forces responsible for

Answer 3

Physical properties such as melting and boiling points

Question 4

What are covalent forces responsible for

Answer 4

The identity and chemical reactions of molecules

Question 5

Bond enthalpy/kJmol^-1 of London forces

Answer 5

1 to 10

Question 6

Bond enthalpy/kJmol^-1 of permanent dipole-dipole interactions

Answer 6

3 to 25

Question 7

Bond enthalpy/kJmol^-1 of hydrogen bonds

Answer 7

10-40

Question 8

Bond enthalpy/kJmol^-1 of single covalent bonds

Answer 8

150 to 500

Question 9

What are London forces

Answer 9

Weak intermolecular forces that exist between ALL molecules whether polar or non-polar
Act between induced dipoles in different molecules

Question 10

Explanation of the origin of induced dipoles

Answer 10

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 neighbouring molecule
The induced dipole induces further dipoles on neighbouring molecules, which then attract one another

Question 11

Induced dipoles are only…

Answer 11

… temporary

Question 12

The more electrons in each molecule:

Answer 12

The larger the instantaneous and induced dipoles
The greater the induced dipole-dipole interactions
The stronger the attractive forces between molecules
Larger numbers of electrons mean larger induced dipoles
More energy is then needed to overcome the intermolecular forces, increasing the boiling point

Question 13

What was the term van der Walls, forces, used to describe

Answer 13

Sometimes used to describe induced dipole-dipole interactions
The IUPAC recommends it also be used for permanent dipole-dipole interactions so the term is ambiguous

Question 14

Where do permanent dipole-dipole interactions act

Answer 14

Between the permanent dipoles in different polar molecules

Question 15

Are fluorine molecules polar

Answer 15

No

Question 16

Do fluorine molecules have London forces

Answer 16

Yes, they ONLY have London forces

Question 17

Are hydrogen chloride molecules polar

Answer 17

Yes

Question 18

Do hydrogen chloride molecules have London forces

Answer 18

Yes, and ALSO permanent dipole-dipole interactions between molecules
Extra energy is needed to break the additional permanent interactions so the boiling point is higher than fluorine

Question 19

What is a simple molecular substance

Answer 19

A substance made up of simple molecules

Question 20

What is a simple molecule

Answer 20

Small units containing a definite number of atoms with a definite molecular formula

Question 21

What do simple molecules form in the solid state

Answer 21

A simple molecular lattice

Question 22

In a simple molecular lattice:

Answer 22

The molecules are held in place by weak intermolecular forces
The atoms within each molecule are bonded together strongly by covalent bonds

Question 23

All simple molecular substances can be…

Answer 23

…solidified into simple molecular lattices by reducing the temperature

Question 24

Are the forces easily broken in a simple molecular lattice

Answer 24

Yes, they can be broken even by energy present at low temperatures

Question 25

Properties of simple molecular substances

Answer 25

They have low melting and boiling points

Question 26

When a simple molecular lattice is broken apart during melting:

Answer 26

Only the weak intermolecular forces break

The covalent bonds are strong and do NOT break

Question 27

What happens when a simple molecular compound is added to a non-polar solvent

Answer 27

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

Question 28

Non-polar simple molecular substances tend to be _____ in _____ solvents

Answer 28

Soluble

Non-polar

Question 29

What happens when a simple molecular substance is added to a polar solvent

Answer 29

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

Question 30

Simple molecular substances tend to be _____ in _____ substances

Answer 30

Insoluble

Polar

Question 31

What does the solubility of polar simple molecular substances depend on

Answer 31

The strength of the dipole and so can be hard to predict

Question 32

Some biological molecules have hydrophobic and hydrophilic parts, what is each part

Answer 32

The hydrophobic part will be non-polar and comprised of a carbon chain
The hydrophilic part will be polar and contain electronegative atoms (usually oxygen) that can interact with water

Question 33

Electrical conductivity in simple molecular substances

Answer 33

There are no mobile charged particles in simple molecular structures
With no charged particles that can move, there is nothing to complete an electrical circuit
THEREFORE THEY ARE NON-CONDUCTORS OF ELECTRICITY