6.3 Intermolecular forces

Question 1

What are intermolecular forces?

Answer 1

weak interactions between dipoles of different molecules. 3 different types:

Dipole: a bond or molecule whose ends have opposite charges

-induced dipole -dipole interactions (London forces)
-permanent dipole-dipole interactions
-hydrogen bonding

Question 2

What type of property are intermolecular forces responsible for?

Answer 2

physical properties such as melting and boiling points

Question 3

Strengths of all the intermolecular bonds (bond enthalpy

Answer 3

London forces: 1-10 KJmol-1
Permanent dipole-dipole interactions: 3-25 KJmol-1
Hydrogen bonds: 10-40 KJmol-1

Covalent: 150-500 KJmol-1

Question 4

What are London forces?

induced dipoles are only…

Answer 4

weak intermolecular forces that exist between all molecules, whether non polar or polar

-induced dipoles are only temporary and can disappear immediately

Question 5

What causes induced dipole-dipole interactions?

Answer 5

-Electrons moving around randomly cause instantaneous dipoles at any instant

-An instantaneous dipole may induce a dipole on a neighbouring molecule

-induced dipole induces further dipoles on neighbouring molecules, which then attract each other

Question 6

What is the strength of London forces linked to?

Answer 6

the number of electrons in each molecule

• the more electrons, the larger the instantaneous & induced dipole
  -greater attractive forces between molecules
  -the more energy needed to overcome the bonds=higher boiling point

Question 7

What are Permanent dipole-dipole interactions

Answer 7

bonds that act between the permanent dipoles in different polar molecules

• takes much more energy to break the additional permanent interactions
  -results in a higher boiling point

Question 8

What are simple molecular substances?

Structure?

Answer 8

made up of molecules which have a definite number of atoms with a definite molecular formula

they form a regular structure: simple molecular lattice, where:
- molecules are held together by weak intermolecular forces
- atoms within each molecule are bonded together strongly by covalent bonds

Question 9

Melting & boiling point of simple molecular substances

Answer 9

Weak intermolecular forces can be broken by low temperature. thus resulting in low melting and boiling points

When melting, only intermolecular forces break, not covalent bonds

Question 10

Solubility of non-polar simple molecules ( non-polar solvent)

Answer 10

simple molecular compound + non-polar solvent= intermolecular forces between the two
- interactions weaken the intermolecular forces in the lattice, breaking them and dissolving the compound

Non-polar simple molecules tend to be soluble in non-polar solvents

Question 11

Solubility of non-polar simple molecules (polar solvent)

Answer 11

simple molecular compound + polar solvent= little interaction between molecules in the lattice and solvent
- intermolecular bonding within polar solvent are too strong to be broken

Non-polar simple molecules ten to be insoluble in polar solvents

Question 12

Solubility of polar simple molecular substances

Answer 12

Polar covalent substances may dissolve in polar solvents as the polar solute molecules and polar solvent molecules can attract each other.

-solubility depends on the strength of the dipole

Question 13

Exceptions to solubility rules

Answer 13

Some compounds e.g. ethanol contain both polar and non polar parts
- can dissolve in both polar /non-polar solvents

Some biological molecules have hydrophobic and hydrophilic parts
- hydrophilic= polar with electronegative atoms
- hydrophobic part will be non-polar and comprised of a carbon chain

Question 14

Electrical conductivity of simple molecular structure s

Answer 14

• no mobile charged particles
• no charged particles moving= no complete electrical circuit

they are non-conductors of electricity