2.2 : Electrons, bonding and structure - intermolecular forces and hydrogen bonding

Question 1

Why do intermolecular forces occur?

Answer 1

Intermolecular forces occur due to constant random movements of the electrons within the shells of the atoms in molecules

Question 2

What is an intermolecular force?

Answer 2

An intermolecular force is an attractive force between neighbouring molecules

Question 3

What is a permanent dipole-dipole interaction?

Answer 3

A permanent dipole-dipole interaction is the weak attractive force between permanent dipoles and permanent/induced dipoles in neighbouring polar molecules

Question 4

How does an induced dipole form?

Answer 4

• if a molecule has a permanent dipole, it has a slightly negative and slightly positive end
• when near other non-polar molecules, it can cause electrons in the shells of the nearby molecule to shift slightly (by being repelled by the slightly negative end or attracted to the slightly positive side)
• this causes the non-polar molecule to become slightly polar and then an attraction occurs between the permanent dipole in one molecule and the induced dipole in the other

Question 5

What are London (dispersion) forces?

Answer 5

London (dispersion) forces are very weak attractive forces between induced dipoles in neighbouring molecules

Question 6

How do London forces occur?

Answer 6

• London forces are caused by the constant random movement of electrons in atoms’ shells. This movement causes their to be an uneven distribution of charge within the electron shells
• At any moment, there will be an instantaneous dipole across the molecule
• The instantaneous dipole induces a dipole in neighbouring molecules, which induce further dipoles on their neighbouring molecules
• The small induced dipoles attract one another, causing weak intermolecular forces known as London forces

Question 7

How does the number of electrons affect the size of the London forces?

Answer 7

The size of London forces increases with increasing number of electrons. The greater the number of electrons, the larger the induced dipoles and the greater the attractive forces between molecules

Question 8

How do London forces affect the boiling points of non-polar molecules?

Answer 8

• in non-polar molecules, there are only London forces to be overcome. London forces are very weak so these substances have very low boiling points
• the boiling point increased going down the group as the number of electrons increase, causing the strength of the intermolecular forces to increase and require more energy to overcome

Question 9

What is a hydrogen bond?

Answer 9

A hydrogen bond is a strong permanent dipole - permanent dipole attraction between:
~ an electron-deficient H atom on one molecule
~ and a lone pair of electrons on a highly electronegative atom (O, N or F) on a different molecule

Question 10

Why are 2 water molecules able to form hydrogen bonds?

Answer 10

O has lone pairs on it which can form a hydrogen bond with the H on the other water molecule, as it is electron deficient

Question 11

Why does ice float in water?

Answer 11

• when ice forms, water molecules become arranged in an orderly pattern in an open lattice
• hydrogen bonds form between water molecules which take up more space and hold the water molecules further apart
• so ice is less dense than water

Question 12

Why does water have an unusually high boiling point compared with other gr 16 hydrides?

Answer 12

Water is able to form hydrogen bonds which are much stronger than other intermolecular forces, so require more energy to break.

Question 13

What properties of water are caused by hydrogen bonds?

Answer 13

• ice = less dense than water
• unusually high boiling point
• high surface tension
• high viscosity