Bonding: Forces Between Molecules

Question 1

Formation of Van der Waals Forces

Answer 1

• Van der Waals forces cause all atoms and molecules to be attracted to each other
• They are present between all molecules but they are weak
• Electrons in charge clouds are constantly moving
• So at any one instance, the electron cloud may be distributed unequally
• This causes partial charges to develop and this forms a temporary dipole within the molecule
• This dipole can induce (cause) another temporary dipole in a neighbouring atom
• There is an electrostatic force of attraction between the ẟ+ on one molecule and the ẟ‒ on the other molecule

Question 2

Where do van der Waals Forces Occur?

Answer 2

• Van der Waals forces occur in all molecules
• Molecules where the atoms in the molecule have the same or very similar electronegativity values only have this type of intermolecular forces present

Question 3

Factors Affecting the Strength of van der Waals Forces

Answer 3

• Size of molecules

• Larger molecules have more electrons so they have larger electron clouds, which means stronger van der Waals forces between the molecules
• The more electrons there are in a molecule the greater the opportunities for induced dipoles to be set up

• Shape of molecules

• Long, straight molecules can lie closer together than branched ones
• The closer together two molecules are, the stronger the forces between them

Question 4

Formation of Permanent Dipole-Dipole Forces

Answer 4

• These form in (polar) substances that have permanent dipoles
• This occurs when the atoms in a molecule have different electronegativity values
• Weak electrostatic forces of attraction form between the ẟ+ and ẟ- charges on neighbouring molecules
• These are called permanent dipole-dipole forces

Question 5

Formation of Hydrogen Bonds

Answer 5

• Hydrogen bonding is the strongest intermolecular force
• Only happens when hydrogen is covalently bonded to fluorine, nitrogen or oxygen
• Fluorine, nitrogen and oxygen are very electronegative
• Hydrogen has a high charge density
• The bond is so polarised, that hydrogen atoms form weak bonds with the lone pairs of electrons of fluorine, nitrogen or oxygen of other molecules

Question 6

Hydrogen Bonding in Ice

Answer 6

• Ice is less dense than liquid water
• This is unusual as substances are usually more dense as solids than as liquids
• As liquid water cools to form ice, the molecules make more hydrogen bonds and arrange themselves into a regular lattice structure
• Hydrogen bonds are relatively long so the average distance between H2O molecules is greater in ice than in liquid water - so ice is less dense